France Collard
email: francecollard16@gmail.com

Johnny Gasperi

Geir W Gabrielsen

Bruno Tassin

Plastic particle ingestion by wild freshwater fish: a critical review

Plastic pollution, especially microplastics (MP) pollution, is a hot topic in both mainstream media and scientific literature. Although rivers are potentially the major transport pathway of this pollution to the sea, plastic contamination in freshwater bodies is comparatively understudied. Microplastic pollution in freshwater fish is of growing interest, and while few studies exist, discrepancies do occur in the sampling, extraction, and identification of MP and in the expression of the results. Even though those differences hamper comparisons between some studies, a comparative work has been performed to identify the factors influencing MP ingestion by fish and consequently to target potential ecological traits that can be used to monitor species. Monitoring plastic ingested by fish will give relevant ecological information on MP pollution. This review focuses on MP ingestion by wild freshwater and estuarine fish.

In addition to providing an overview of the existing data concerning contamination levels in wild freshwater fish, we aimed to (1) propose several overall recommendations on the methodologies applicable to all biota, (2) compare MP contamination levels in fish and in their environment, and (3) determine which parameters could help to define fish species for monitoring.

Introduction

Due to their exponentially increasing production since the 1950s, [START_REF] Ryan | A Brief History of Marine Litter Research[END_REF] plastic materials are polluting all types of environments: marine surface waters, [START_REF] Law | Distribution of Surface Plastic Debris in the Eastern Pacific Ocean from an 11-Year Data Set[END_REF][START_REF] Gago | Synthetic Microfibers in the Marine Environment: A Review on Their Occurrence in Seawater and Sediments[END_REF] deep-sea sediments, [START_REF] Woodall | The Deep Sea Is a Major Sink for Microplastic Debris[END_REF][START_REF] Tekman | Marine Litter on Deep Arctic Seafloor Continues to Increase and Spreads to the North at the HAUSGARTEN Observatory[END_REF] arctic sea ice, [START_REF] Obbard | Global Warming Releases Microplastic Legacy Frozen in Arctic Sea Ice[END_REF][START_REF] Peeken | Arctic Sea Ice Is an Important Temporal Sink and Means of Transport for Microplastic[END_REF] soils, [START_REF] Liu | Microplastic and Mesoplastic Pollution in Farmland Soils in Suburbs of Shanghai, China[END_REF] and even air. [START_REF] Dris | Synthetic Fibers in Atmospheric Fallout: A Source of Microplastics in the Environment?[END_REF][START_REF] Cai | Characteristic of Microplastics in the Atmospheric Fallout from Dongguan City, China: Preliminary Research and First Evidence[END_REF] As an expected and direct consequence, the number of species or taxa exposed to plastic pollution is alarming. 11 Plastic pieces of all sizes are found in the environment, and thus are ingested by many different organisms: marine mammals, [START_REF] Lusher | Microplastic and Macroplastic Ingestion by a Deep Diving, Oceanic Cetacean: The True's Beaked Whale Mesoplodon Mirus[END_REF][START_REF] Nelms | Investigating Microplastic Trophic Transfer in Marine Top Predators[END_REF][START_REF] Zhu | Cetaceans and Microplastics: First Report of Microplastic Ingestion by a Coastal Delphinid, Sousa Chinensis[END_REF] marine and terrestrial birds, [START_REF] Trevail | Elevated Levels of Ingested Plastic in a High Arctic Seabird, the Northern Fulmar (Fulmarus Glacialis)[END_REF][START_REF] Zhao | Microscopic Anthropogenic Litter in Terrestrial Birds from Shanghai, China: Not Only Plastics but Also Natural Fibers[END_REF][START_REF] Reynolds | Micro-Plastic Ingestion by Waterbirds from Contaminated Wetlands in South Africa[END_REF] crustaceans, [START_REF] Devriese | Microplastic Contamination in Brown Shrimp (Crangon Crangon, Linnaeus 1758) from Coastal Waters of the Southern North Sea and Channel Area[END_REF][START_REF] Iannilli | Microplastics in Talitrus Saltator (Crustacea, Amphipoda): New Evidence of Ingestion from Natural Contexts[END_REF] worms, [START_REF] Hurley | Ingestion of Microplastics by Freshwater Tubifex Worms[END_REF] and fish. [START_REF] Bråte | Plastic Ingestion by Atlantic Cod ( Gadus Morhua ) from the Norwegian Coast[END_REF][START_REF] Rummel | Plastic Ingestion by Pelagic and Demersal Fish from the North Sea and Baltic Sea[END_REF][START_REF] Collicutt | Microplastics in Juvenile Chinook Salmon and Their Nearshore Environments on the East Coast of Vancouver Island[END_REF] Scientific research is currently more focused on the marine environment. The reasons for this focus are probably due the high economic value of marine resources and abundant funding allocated to marine research and monitoring. In particular, microplastics (MP), which are plastic pieces with sizes less than 5 mm, [START_REF] Arthur | Proceedings of the International Research Workshop on the Occurrence, Effects and Fate of Microplastic Marine Debris[END_REF] are well known to pollute seas and oceans. Numerous laboratory studies have shown that this exposure leads to plastic ingestion by various organisms and is associated with some negative impacts, [START_REF] Anbumani | Ecotoxicological Effects of Microplastics on Biota: A Review[END_REF] such as neurotoxicity, [START_REF] Barboza | Marine Microplastic Debris: An Emerging Issue for Food Security, Food Safety and Human Health[END_REF] a change in swimming behavior [START_REF] Choi | Toxicological Effects of Irregularly Shaped and Spherical Microplastics in a Marine Teleost, the Sheepshead Minnow (Cyprinodon Variegatus)[END_REF] and reduction of predatory performances. [START_REF] De Sá | Effects of Microplastics on Juveniles of the Common Goby (Pomatoschistus Microps): Confusion with Prey, Reduction of the Predatory Performance and Efficiency, and Possible Influence of Developmental Conditions[END_REF] Human populations are closely linked to water from both marine and freshwater sources. A considerable fraction of the human population lives in the near-coastal zone. [START_REF] Small | A Global Analysis of Human Settlement in Coastal Zones[END_REF] Approximately half of the world's population lives within 3 km of a freshwater body and only 10% of the population lives farther than 10 km away. [START_REF] Kummu | How Close Do We Live to Water? A Global Analysis of Population Distance to Freshwater Bodies[END_REF] However, given that proximity, between 1.15 and 2.41 million tons of plastics are released into the oceans via rivers each year. [START_REF] Lebreton | River Plastic Emissions to the World's Oceans[END_REF] Conventional wastewater treatment plants may act as an MP source to rivers, [START_REF] Carr | Transport and Fate of Microplastic Particles in Wastewater Treatment Plants[END_REF][START_REF] Mason | Microplastic Pollution Is Widely Detected in US Municipal Wastewater Treatment Plant Effluent[END_REF] and inappropriate waste management [START_REF] Free | High-Levels of Microplastic Pollution in a Large, Remote, Mountain Lake[END_REF] combined with a high population density [START_REF] Baldwin | Plastic Debris in 29 Great Lakes Tributaries: Relations to Watershed Attributes and Hydrology[END_REF] may both be positively correlated with riverine plastic loads.

Increasing number of studies have revealed that freshwaters are contaminated, sometimes largely by plastic, regardless of its size. [START_REF] Eriksen | Plastic Pollution in the South Pacific Subtropical Gyre[END_REF][START_REF] Gasperi | Assessment of Floating Plastic Debris in Surface Water along the Seine River[END_REF][START_REF] Lechner | The Danube so Colourful: A Potpourri of Plastic Litter Outnumbers Fish Larvae in Europe's Second Largest River[END_REF][START_REF] Dris | Beyond the Ocean: Contamination of Freshwater Ecosystems with (Micro-)Plastic Particles[END_REF] The number of studies about MP pollution and its impacts is growing (in March 2019, searching the Web of Science for "plastic marine pollution" returned 3 results in 2000 and 351 results in 2018), but a small number of these studies deal with a freshwater environment (0 out of the 3 results found in 2000 and 42 out of the 351 results found in 2018). The same observation can be made for fish; searching the Web of Science for the keywords "fish plastic pollution" gave 4 results in 2000 and 99 in 2018, in which 0 and 10 results concern freshwaters, respectively. Despite the large amounts of plastic debris input into seas and oceans by rivers, the interactions between these debris and the biota of these ecosystems are poorly studied. Nevertheless, some discrepancies in performed protocols do occur. Among others, the sampling size is frequently too small, and MP identification often relies on visual sorting. In addition, the results are reported in one or two units while at least three are used in publications. Those discrepancies lead to difficulties in comparing data.

Fish are the main taxa studied with regard to MP ingestion in freshwater environments [START_REF] O'connor | Microplastics in Freshwater Biota: A Critical Review of Isolation, Characterization, and Assessment Methods[END_REF] and constitute an economically, ecologically and highly diversified group. [START_REF] Powers | Fish as Model Systems[END_REF][START_REF] Nelson | Fishes of the World[END_REF] Microplastic ingestion by freshwater fish has been studied worldwide, but these studies have although focused on limited regions (Fig. 1) in comparison with the studies of plastic ingested by wild marine fish. [START_REF] Markic | Plastic Ingestion by Marine Fish in the Wild[END_REF] In total, the studies focusing on freshwater fish, while few, have described MP ingestion by more than 200 species. This large number indicates that some studies have collected few samples. Few published papers have studied the interactions between MP and freshwater biota compared to the number of marine studies, although scientific interest in MP ingestion by freshwater fish is rising. We thus believe that a summary of recommendations for the methods and expression of results and factors leading to MP ingestion by freshwater fish would be useful for the scientific community, especially regarding monitoring. Three main factors have been investigated: the MP levels in abiotic compartments, the living habitat and the feeding strategy. Other factors could have been reviewed, but there are too few studies to allow for a reliable discussion. Exposure through abiotic compartments could be directly correlated with the ingestion of MP. As further discussed in this review, MP ingestion does not always depend on abiotic compartment exposure. Additionally, there is not a clear relationship between MP ingestion and the living habitat and feeding strategy. Some studies have found that benthic or demersal fish ingested more MP, [START_REF] Jabeen | Microplastics and Mesoplastics in Fish from Coastal and Fresh Waters of China[END_REF][START_REF] Murphy | The Uptake of Macroplastic & Microplastic by Demersal & Pelagic Fish in the Northeast Atlantic around Scotland[END_REF] while others reported the contrary. [START_REF] Rummel | Plastic Ingestion by Pelagic and Demersal Fish from the North Sea and Baltic Sea[END_REF] Monitoring and assessment are essential steps towards addressing specific questions about marine litter, including MP. Monitoring and assessment are needed to assess the state or level of pollution and provide objective information to design mitigation measures as well as assess their effectiveness and promote adaptive management. [START_REF] Gesamp | Guidelines or the Monitoring and Assessment of Plastic Litter and Microplastics in the Ocean[END_REF] Recommendations for defining indicator fish species are very few but crucial to improve the comparability between studies. In this critical review, we aim to (1) propose several recommendations for the harmonization of methodologies, which is applicable not only to MP contamination in freshwater fish studies but also to all biota, (2) compare MP contamination levels in fish and their environment, and subsequently (3) determine which parameters could help to define fish species for monitoring.

Our work is based on the occurrence of MP in the guts of fish. The translocation of plastic particles to other organs is beyond the scope of this review but should be considered in further studies.

Methodologies for the study of MP contamination in fish guts

Globally, studies on MP ingestion by fish, both marine and freshwater, follow a general pattern regarding their methodology, which include sampling, gut content (GC) extraction, MP extraction (visual sorting or chemical treatment, which is not always performed), and identification (including spectroscopic analyses which are not always performed). No specific method has been defined as the method to be used in fish GC analyses. In addition, this general pattern is not specific to studies about fish, as it is also used for other organisms or even for water or sediment matrices. In the two latter cases, density separation is often chosen to carry out the MP extraction. Whatever the matrix, discrepancies in methodologies occur, including in the study of MP in the GCs of wild freshwater fish. There are variations in all the steps used to extract and identify MP. In the next sections, we highlight the main issues of the key protocol steps used to extract MP from GCs (Fig. 2) and propose some recommendations to increase the comparability of studies for future research. We will discuss the following main steps: the sampling, extraction, and identification of MP and the expression of the results. Within those steps, we highlight some parameters that we think are of great importance including the sampling size, method for digesting the GCs, definition of the size threshold for the extracted particles, spectroscopic analyses, and the way results are provided. In addition, some recommendations for preventing and evaluating the contamination will be proposed. We selected the most important criteria, as other reviews have already focused on methodologies for the sampling, extraction and analyses of MP in biota or abiotic compartments. [START_REF] O'connor | Microplastics in Freshwater Biota: A Critical Review of Isolation, Characterization, and Assessment Methods[END_REF][START_REF] Lusher | Sampling, Isolating and Identifying Microplastics Ingested by Fish and Invertebrates[END_REF][START_REF] Hermsen | Quality Criteria for the Analysis of Microplastic in Biota Samples: A Critical Review[END_REF] 2.1. Sampling: Sample size Among all the species studied, more than twenty have been analyzed at least twice, and most of these species have been studied thanks to studies conducted in South America. [START_REF] Dantas | The Seasonal and Spatial Patterns of Ingestion of Polyfilament Nylon Fragments by Estuarine Drums (Sciaenidae)[END_REF][START_REF] Ramos | Ingestion of Nylon Threads by Gerreidae While Using a Tropical Estuary as Foraging Grounds[END_REF][START_REF] Vendel | Widespread Microplastic Ingestion by Fish Assemblages in Tropical Estuaries Subjected to Anthropogenic Pressures[END_REF][START_REF] Pegado | First Evidence of Microplastic Ingestion by Fishes from the Amazon River Estuary[END_REF] In addition, when considering a sampling of at least 10 individuals per species, only seven out of twenty species can be spatially compared. A review focusing on wild marine fish found that the detection of plastic ingestion was positively correlated with an increased sample size (up to n=10). [START_REF] Markic | Plastic Ingestion by Marine Fish in the Wild[END_REF] In many studies reviewed here, the number of individuals representing each species was low (n<10), which is obviously too small to define solid trends in MP contamination. In several publications, data were given for one or two individuals, limiting the representativeness.

Given that a single dataset is available and the number of fish sampled is usually low for most studied species, very local one-time events may influence the results and impair the global understanding of MP ingestion by fish. A large sample size is highly important since it will provide enough data to perform reliable statistical analyses. Previously, a threshold of 50 individuals has been defined as sufficiently reliable regarding to achieve statistical power, [START_REF] Hermsen | Quality Criteria for the Analysis of Microplastic in Biota Samples: A Critical Review[END_REF][START_REF] Galgani | Guidance on Monitoring of Marine Litter in European Seas[END_REF]54 while 40 specimens were considered to be an adequate sample size for the monitoring of plastic ingestion by northern fulmars. [START_REF] Van Franeker | Marine Litter Monitoring by Northern Fulmars; a Pilot Study[END_REF] Fig. 3 shows that most studies did not sample more than 50 specimens per species. This chart highlights the need for a better sampling design to statistically support the results. shown in Table 1.

Extraction of MP: Digestion of fish gut contents

The digestion of organic matter is not mandatory but recommended, [START_REF] Gesamp | Guidelines or the Monitoring and Assessment of Plastic Litter and Microplastics in the Ocean[END_REF] especially when aiming to isolate MP from biota. Several methods exist regarding GCs, which are indiscriminate for marine and freshwater fish species. Fig. 4 shows that when present, the most common agents are oxidizers (e.g., H 2 O 2 and NaClO) [START_REF] Jabeen | Microplastics and Mesoplastics in Fish from Coastal and Fresh Waters of China[END_REF][START_REF] Pazos | Microplastics in Gut Contents of Coastal Freshwater Fish from Río de La Plata Estuary[END_REF][START_REF] Collard | Anthropogenic Particles in the Stomach Contents and Liver of the Freshwater Fish Squalius Cephalus[END_REF] and hydroxides (e.g., KOH). [START_REF] Dehaut | Microplastics in Seafood: Benchmark Protocol for Their Extraction and Characterization[END_REF][START_REF] Zhang | Occurrence and Characteristics of Microplastic Pollution in Xiangxi Bay of Three Gorges Reservoir, China[END_REF] No studies reported acids as the main digesting agent, which is most likely because acids alter some polymers, such as polyethylene terephthalate (PET), high density polyethylene (HDPE) and polyamide (PA).

Acids can also lead to suboptimal digestion. [START_REF] Dehaut | Microplastics in Seafood: Benchmark Protocol for Their Extraction and Characterization[END_REF][START_REF] Claessens | New Techniques for the Detection of Microplastics in Sediments and Field Collected Organisms[END_REF][START_REF] Avio | Experimental Development of a New Protocol for Extraction and Characterization of Microplastics in Fish Tissues: First Observations in Commercial Species from Adriatic Sea[END_REF][START_REF] Catarino | Development and Optimization of a Standard Method for Extraction of Microplastics in Mussels by Enzyme Digestion of Soft Tissues[END_REF] Remarkably, to our knowledge, no studies have used enzymes as digesting agents yet. While the efficiency of such agents has been assessed, [START_REF] Catarino | Development and Optimization of a Standard Method for Extraction of Microplastics in Mussels by Enzyme Digestion of Soft Tissues[END_REF][START_REF] Cole | Isolation of Microplastics in Biota-Rich Seawater Samples and Marine Organisms[END_REF][START_REF] Railo | Application of an Enzyme Digestion Method Reveals Microlitter in Mytilus Trossulus at a Wastewater Discharge Area[END_REF] the cost and duration of this treatment might discourage researchers. Recently, von Friesen et al. [START_REF] Von Friesen | An Efficient and Gentle Enzymatic Digestion Protocol for the Extraction of Microplastics from Bivalve Tissue[END_REF] proposed a protocol more efficient than that using KOH with the convenient advantages of being less time-consuming and commercially available at a low price. This protocol has been thoroughly tested using bivalves but suits a broader range of organisms according to the authors. No studies on fish GCs have used Fenton's agent either, which is probably due to the efficiency of the previously cited methods. This agent does not affect plastic polymers and reduces the sample preparation time. [START_REF] Tagg | s Reagent for the Rapid and Efficient Isolation of Microplastics from Wastewater[END_REF] Both enzymes and Fenton's agent could be a solution to achieving the efficient, rapid and inexpensive digestion of the organic matter in the GCs of freshwater fish but could probably also be used in other biological matrices such as the liver. Some GCs can be very fatty and challenging to digest. No study has identified a means of treating those samples properly. Finding an easy way to process fatty GCs would be useful for further research on fish gut contamination. Only a couple of studies have reported data on all types of anthropogenic particles, i.e., small pieces created or handled by humans, such as plastics, dyed particles or textile fibers. [START_REF] Collard | Anthropogenic Particles in the Stomach Contents and Liver of the Freshwater Fish Squalius Cephalus[END_REF][START_REF] Horton | The Influence of Exposure and Physiology on Microplastic Ingestion by the Freshwater Fish Rutilus Rutilus (Roach) in the River Thames, UK[END_REF] Indeed, studies have only focused on plastic materials and have developed protocols for that purpose. To our knowledge, few studies have tested the protocols used to extract MP from biota on natural or modified natural materials such as cellulose or cellulose acetate. [START_REF] Dehaut | Microplastics in Seafood: Benchmark Protocol for Their Extraction and Characterization[END_REF][START_REF] Collard | Detection of Anthropogenic Particles in Fish Stomachs: An Isolation Method Adapted to Identification by Raman Spectroscopy[END_REF][START_REF] Kühn | The Use of Potassium Hydroxide (KOH) Solution as a Suitable Approach to Isolate Plastics Ingested by Marine Organisms[END_REF] Dehaut et al. [START_REF] Dehaut | Microplastics in Seafood: Benchmark Protocol for Their Extraction and Characterization[END_REF] tested three extraction protocols on cellulose acetate materials, in which two contained hydroxides (NaOH & KOH) and one contained a combination of a hydroxide and an oxidizer (NaOH & K 2 S 2 O 8 ). All the protocols led to a mass reduction and a size and shape modification of the cellulose acetate while two of them allowed for identification by Raman spectroscopy.

On the other hand, cellulose fibers, along with plastic polymers, were tested with an oxidizing agent and a combination of an oxidizer and an acid (NaClO & HNO 3 ). [START_REF] Collard | Detection of Anthropogenic Particles in Fish Stomachs: An Isolation Method Adapted to Identification by Raman Spectroscopy[END_REF] Cellulose fibers were left unaffected by both treatments and Raman spectroscopy could successfully be performed.

A potassium hydroxide-based protocol has also been tested on several natural materials. [START_REF] Kühn | The Use of Potassium Hydroxide (KOH) Solution as a Suitable Approach to Isolate Plastics Ingested by Marine Organisms[END_REF] Cellulose acetate has been found to lose a significant part of its mass after treatment, which is similar to the study by Dehaut et al. [START_REF] Dehaut | Microplastics in Seafood: Benchmark Protocol for Their Extraction and Characterization[END_REF] Some natural materials, such as sheep wool, were dissolved, while others showed a resistance to KOH. The extraction method should then be carefully chosen when textile fibers are targeted.

Data on anthropogenic particles made from natural materials such as wool and cotton should be included in further studies. Laboratory studies have mainly focused on plastic materials.

Textile fibers, independent of their composition, are found in high quantities in the environment [START_REF] Gago | Synthetic Microfibers in the Marine Environment: A Review on Their Occurrence in Seawater and Sediments[END_REF][START_REF] Barrows | Marine Environment Microfiber Contamination: Global Patterns and the Diversity of Microparticle Origins[END_REF] but their impacts are unknown. These particles contain additives and dyes [START_REF] Zhao | Microscopic Anthropogenic Litter in Terrestrial Birds from Shanghai, China: Not Only Plastics but Also Natural Fibers[END_REF][START_REF] Barrows | Marine Environment Microfiber Contamination: Global Patterns and the Diversity of Microparticle Origins[END_REF][START_REF] Darbra | Global Risk-Based Management of Chemical Additives I. Production, usage, and environmental occurrence[END_REF] and may constitute a threat to the environment [START_REF] Robinson | Remediation of Dyes in Textile Effluent: A Critical Review on Current Treatment Technologies with a Proposed Alternative[END_REF][START_REF] Pokharia | Toxicological Effect of Textile Dyes and Their Metabolites: A Review[END_REF] , aquatic organisms and human health. [START_REF] Darbra | Global Risk-Based Management of Chemical Additives I. Production, usage, and environmental occurrence[END_REF][START_REF] Remy | When Microplastic Is Not Plastic: The Ingestion of Artificial Cellulose Fibers by Macrofauna Living in Seagrass Macrophytodetritus[END_REF] All anthropogenic particles, regardless of their size, should therefore be studied in both in situ and in vitro studies. 1.

Extraction of MP: Filtration and targeted MP sizes

The minimum size of the targeted particles is not always mentioned (Fig. 5). Generally, the particle isolation process includes a filtration/sieving step, in which a precise mesh size practically defines the threshold for particles to be further analyzed. In several studies, that mesh size is close to the micrometer scale (Fig. 5). Although this mesh size perfectly fits the microplastic size definition, from 5 mm down to a few microns, [START_REF] Andrady | Microplastics in the Marine Environment[END_REF] the limiting factor is then the spectroscopic analyses. Depending on the technique and equipment used, the size threshold allowing for the analysis of a particle varies. For instance, when dealing with particles between 1 and 20 µm in length, Raman spectroscopy is recommended. [START_REF] Renner | Analytical Methodologies for Monitoring Micro(Nano)Plastics: Which Are Fit for Purpose?[END_REF] However, if the aim of the study is to investigate direct ingestion by fish, it might not be worth studying such small MP. Adult fish are unlikely to ingest micron-sized particles because these particles will probably be ejected into the surrounding water from the branchial system, which is not adapted to retain such small particles. [START_REF] Collard | Morphology of the Filtration Apparatus of Three Planktivorous Fishes and Relation with Ingested Anthropogenic Particles[END_REF] The filtration step could then be made easier by using larger mesh sizes depending on the studied species, mainly if no digestion step is performed. However, if the transfer of MP from prey is considered in the study, looking at smaller particles might be of interest. Regarding filtering fish, their filtration system defines the size limit of particles that are retained and further swallowed. For those species, correlating the filtration-limited size to the size of MP ingested might be a way to discriminate MP coming from prey (indirect ingestion) and MP ingested through feeding (direct ingestion). This assumption has to be assessed by future studies. A size of 5 µm could be the lowest limit of investigated MP sizes, as it almost perfectly covers the MP size definition, which comprises direct and indirect MP ingestion and allows for reliable spectroscopic analyses. We encourage authors to provide at least the size classes of the ingested particles, as well as the size distribution. This information will allow for the comparison of those size classes according to the minimal size targeted and will provide baseline data for studies further dealing with the uptake of ingested MP by other organs and tissues.

Identification of MP: Spectroscopic analyses

Half of the studies on plastics in freshwater fish used spectroscopic analyses to confirm the plastic composition of the particles found, and most of these studies are the oldest (Table 1).

When considering other matrices such as sediment or water, such techniques are often set aside. [START_REF] Renner | Analytical Methodologies for Monitoring Micro(Nano)Plastics: Which Are Fit for Purpose?[END_REF] Thus, these techniques only rely on visual observation, which may bias the results. [START_REF] Löder | Focal Plane Array Detector-Based Micro-Fourier-Transform Infrared Imaging for the Analysis of Microplastics in Environmental Samples[END_REF][START_REF] Song | A Comparison of Microscopic and Spectroscopic Identification Methods for Analysis of Microplastics in Environmental Samples[END_REF][START_REF] Gesamp | Fate and Effects of Microplastics in the Marine Environment: Part Two of a Global Assessment[END_REF] Visual observations with a stereomicroscope tend to underestimate the amount of microplastic fragments and, conversely, tend to overestimate the amount of MP fibers compared to the results obtained by Fourier transform infrared (FTIR) analyses. [START_REF] Song | A Comparison of Microscopic and Spectroscopic Identification Methods for Analysis of Microplastics in Environmental Samples[END_REF] Markic et al. [START_REF] Markic | Plastic Ingestion by Marine Fish in the Wild[END_REF] found that plastic was detected more often when a chemical digestion-coupled spectroscopic analyses was performed compared to visual methods alone. The smaller the particles are, the more difficult it is to distinguish them from biological tissues and to assess their synthetic composition with the naked eye. The 'hot point-test' is also sometimes used to determine the plastic composition of particles. [START_REF] Devriese | Microplastic Contamination in Brown Shrimp (Crangon Crangon, Linnaeus 1758) from Coastal Waters of the Southern North Sea and Channel Area[END_REF][START_REF] Roch | A Systematic Study of the Microplastic Burden in Freshwater Fishes of South-Western Germany -Are We Searching at the Right Scale?[END_REF] This test consists of touching a particle with a hot needle and observing whether the needle leaves a mark on the particle. [START_REF] Hermsen | Quality Criteria for the Analysis of Microplastic in Biota Samples: A Critical Review[END_REF][START_REF] Vandermeersch | A Critical View on Microplastic Quantification in Aquatic Organisms[END_REF] Although convenient and cheap, this technique should not be solely used for identification but used as a complement to visual sorting prior to spectroscopic analyses. [START_REF] Hermsen | Quality Criteria for the Analysis of Microplastic in Biota Samples: A Critical Review[END_REF] In a report, the GESAMP group of experts 40 stated that spectroscopy is recommended for particles smaller than 1 mm. Consequently, using either Raman or FTIR spectroscopy when working with microplastic pollution is highly recommended. [START_REF] Gesamp | Guidelines or the Monitoring and Assessment of Plastic Litter and Microplastics in the Ocean[END_REF][START_REF] Gesamp | Fate and Effects of Microplastics in the Marine Environment: Part Two of a Global Assessment[END_REF] Both techniques usually allow for precisely identify the particle composition to be precisely identified and have the strong advantage of being nondestructive in contrast with other techniques such as pyrolysis-gas chromatography coupled with mass spectrometry (py-GC/MS) or thermogravimetric analysis coupled with solid-phase extraction (TGA-SPE). [START_REF] Dümichen | Analysis of Polyethylene Microplastics in Environmental Samples, Using a Thermal Decomposition Method[END_REF][START_REF] Käppler | Analysis of Environmental Microplastics by Vibrational Microspectroscopy: FTIR, Raman or Both? Anal[END_REF][START_REF] Araujo | Identification of Microplastics Using Raman Spectroscopy: Latest Developments and Future Prospects[END_REF] These techniques are complementary, as they give different information but using two or three of these techniques is obviously expensive and time-consuming. Therefore, depending on the information needed or the type of particle, researchers have to choose the best technique. Raman spectroscopy constitutes a better choice if results about additives are needed. Indeed Raman spectroscopy can provide additional information on dyes, for example, and will identify the composition of small particles (< 20 µm) that FTIR will miss. [START_REF] Song | A Comparison of Microscopic and Spectroscopic Identification Methods for Analysis of Microplastics in Environmental Samples[END_REF][START_REF] Käppler | Analysis of Environmental Microplastics by Vibrational Microspectroscopy: FTIR, Raman or Both? Anal[END_REF][START_REF] Araujo | Identification of Microplastics Using Raman Spectroscopy: Latest Developments and Future Prospects[END_REF] On the other hand, FTIR spectroscopy is less time-consuming and may allow for the identification of black particles that lead to fluorescence issues in Raman spectroscopy. [START_REF] Käppler | Analysis of Environmental Microplastics by Vibrational Microspectroscopy: FTIR, Raman or Both? Anal[END_REF] All these techniques are expensive and time consuming; thus, a random subsample is often collected prior to the analyses, and the results are extrapolated to the total number particles found. [START_REF] Cai | Characteristic of Microplastics in the Atmospheric Fallout from Dongguan City, China: Preliminary Research and First Evidence[END_REF][START_REF] Jabeen | Microplastics and Mesoplastics in Fish from Coastal and Fresh Waters of China[END_REF][START_REF] Steer | Microplastic Ingestion in Fish Larvae in the Western English Channel[END_REF][START_REF] Bessa | Occurrence of Microplastics in Commercial Fish from a Natural Estuarine Environment[END_REF][START_REF] Wieczorek | Frequency of Microplastics in Mesopelagic Fishes from the Northwest Atlantic[END_REF] This random subsample is usually poorly described and might result from subjective choices, especially regarding the sizes and colors. As selecting the particles may lead to bias, we instead recommend selecting a zone covering both the center and the edge of the filter (a half or a quarter) used during MP extraction.

Spectroscopic analyses provide useful and reliable data on particles. However, part of this data is often set aside because studies focus only on plastic polymers (Table 1). As explained in a previous section, we encourage researchers to provide all information, even on natural and semisynthetic materials.

Expression of the results

We recorded three different units of MP contamination in GCs, i.e., the percentage of contaminated individuals, the number of MP per individual and the number of MP in one gram of the GCs. Additionally, units of contamination differ between studies making comparisons impossible. All three units are easy to calculate and, although the most common (Table 2), the percentage of occurrence is not the most representative unit. This unit gives an overview of the number of fish having ingested at least one piece of plastic in a given time but does not give any valuable quantitative information. The number of MP per gram of GCs could be considered to be the most accurate unit but greatly depends on the fullness of the stomach or gut. As it is not clear whether MP have a greater retention time than food in the digestive tract, [START_REF] Watts | Uptake and Retention of Microplastics by the Shore Crab Carcinus Maenas[END_REF][START_REF] Grigorakis | Determination of the Gut Retention of Plastic Microbeads and Microfibers in Goldfish (Carassius Auratus)[END_REF] expressing the quantity of MP per mass of gut contents is not the most consistent option. It is instead preferable to express the results as the number of MP per individual and for comparison purposes, we recommend authors and researchers to report the results with the three units described in this review.

Procedural blanks

Procedural blanks (PBs), also termed controls or negative controls, [START_REF] Lusher | Sampling, Isolating and Identifying Microplastics Ingested by Fish and Invertebrates[END_REF][START_REF] Mcneish | Microplastic in Riverine Fish Is Connected to Species Traits[END_REF] are used in all procedure steps with the reagents only, and the sample in itself is excluded. Few studies that we have reviewed made PBs although most of the recent studies did. Even if contamination is prevented as best as possible, including PBs is recommended to evaluate possible contamination of the whole treatment protocol [START_REF] Gesamp | Guidelines or the Monitoring and Assessment of Plastic Litter and Microplastics in the Ocean[END_REF][START_REF] Hermsen | Quality Criteria for the Analysis of Microplastic in Biota Samples: A Critical Review[END_REF] and, if needed, to adapt the results according to those blank data. [START_REF] Devriese | Microplastic Contamination in Brown Shrimp (Crangon Crangon, Linnaeus 1758) from Coastal Waters of the Southern North Sea and Channel Area[END_REF][START_REF] Vandermeersch | A Critical View on Microplastic Quantification in Aquatic Organisms[END_REF][START_REF] Mcneish | Microplastic in Riverine Fish Is Connected to Species Traits[END_REF] Plastic contamination levels depend on many parameters, including the type of equipment used for sampling, the extraction and the analyses. [START_REF] Wesch | Assuring Quality in Microplastic Monitoring: About the Value of Clean-Air Devices as Essentials for Verified Data[END_REF] Given that microplastic fibers are also found in the air, 9,10 tap water [START_REF] Kosuth | Anthropogenic Contamination of Tap Water, Beer, and Sea Salt[END_REF] and table salt, [START_REF] Yang | Microplastic Pollution in Table Salts from China[END_REF][START_REF] Iñiguez | Microplastics in Spanish Table Salt[END_REF] in which the latter is used in laboratories to perform density separation, PBs are necessary to evaluate the contamination levels in environmental samples. Moreover, we recommend including some PBs before starting the sample treatment. This will give an overview about the contamination level in the working environment, which can be then compared to the expected levels in the samples. This step could be critical if minimally contaminated samples are to be processed. In this case, PB levels would easily exceed what may be found in the samples, leading to an impossible interpretation of the results. Depending on the studies and equipment used, PB contamination can be important: 50 times higher than plastics within the samples, [START_REF] Campbell | Microplastics in the Gastrointestinal Tracts of Fish and the Water from an Urban Prairie Creek[END_REF] or low: less than 5% of all plastics found in the samples. [START_REF] Jabeen | Microplastics and Mesoplastics in Fish from Coastal and Fresh Waters of China[END_REF] However, if PB levels are assessed first, precautions and decisions can be made to prevent the samples from being highly contaminated during the treatment.

China R - X > 1.2 X* X - - - X - - Zhang et al. 59 60 France R X X > 5 - - - - X X - X Collard et al. 57 64 England R X X > 1.2 X - - - - X - - Horton et al. 67 46 1-16 Brazil E - X ? X - - - - - X - Pegado et al. 52 4-17 Iran E - X ? X X X - X - - X ┼ Abbasi et al. 106 11 1-17 U.S.A. R - X > 0.45 X* - - - X - X X McNeish et al. 91 10 China L - X > 200 X X - - - X - X Xiong et al. 107 16 1-63 Brazil R - X ? X - - - - - X - Andrade et al. 108 78 Belgium R - X > 8 X - - - X X X X Slootmaekers et al. 109 11 China L - X > 8 X X - - X X - X Yuan et al. 110 14 8-9 China E - X > 20 X - - - X - X X Su et al. 111 2 265 & 184 Brazil E - X ? X - - - - - - - Ferreira et al. 112 1 20 Argentina E - X > 8 X - - - X - - - Arias et al. 113 22 - Germany R & L - X > 20 X X X - - - - X Roch et al. 81 1 180 Australia W - X > 20 X X - - - - X X Su et al. 114
No.=number, sp.=species, R=river, L=lake, E=estuary, W=wetland, FR=fragments, FI=fibers, AP=anthropogenic particles excluding plastic particles, PL=plastics, Hyd.=hydroxides, Enz.=enzymes, Ox.=oxidizers, Rn=Raman spectroscopy, FTIR=Fourier transform infrared spectroscopy.

*Visual examination after the digestion process. In this table, when the minimum size of the MP was not mentioned in the study, the porosity of the filters was considered. ┼ No results for those blanks were provided. [START_REF] Collard | Anthropogenic Particles in the Stomach Contents and Liver of the Freshwater Fish Squalius Cephalus[END_REF] , which concerned all anthropogenic particles. When the global mean is not provided, we gave the total range.

Factors influencing MP ingestion by freshwater fish

Here, we aimed to discuss factors that are likely to influence MP ingestion by freshwater fish foraging for prey. Those factors include specific traits, such as the feeding strategy, and abiotic parameters, such as the environmental MP levels. However, the results in the discussed in situ studies are also influenced by the retention time of MP inside the gut, which is not well understood. The results might be different depending on the size of plastic pieces or the species. Also, the gut content reflects only the instantaneous diet, that is why repeated studies should be performed.

Levels in abiotic compartments

According to studies, several factors are at play when plastic is ingested by fish. The first factor to consider is probably the exposure. Very few studies have investigated both the abiotic contamination and MP ingestion by freshwater fish. Therefore, this section will give an overview of the contamination levels in those matrices as a first step in determining what induces or prevents MP ingestion.

Many different units are used for both water and sediment concentrations (Table S1), which is also the case for marine environments. 115,116 Regarding water contamination, the results are often given in items per cubic meter of water, and the format "items/volume" is the most commonly used. Regarding the sediment, the results are usually expressed in items per kilogram of sediment (dry weight).

Remarkably, all except one study chose to express their results per dry weight. We thus encourage researchers to further express sediment concentration per dry weight too. In addition, some authors calculated the mean concentration while others chose to only show a range. Some authors also excluded values equal to zero from the mean calculation, and others did not give any concentration mean or range. While a global mean value may be very restrictive and not representative of all the parameters such as the different rivers sampled or difference in the sampling locations (close or far from an outlet, close or far from urban activities, etc.), we recommend showing as many values as possible. This will allow further studies to choose the most convenient and adapted value to be compared to.

When considering studies that performed analyses in both water and sediment samples obtained from the same place, several trends and questions arise. Regarding the abundances of MP, some studies found different patterns for the two compartments. [START_REF] Zhang | Occurrence and Characteristics of Microplastic Pollution in Xiangxi Bay of Three Gorges Reservoir, China[END_REF]110,117,118 The authors of these studies discussed those results by mentioning potential factors that could influence MP distribution. MP levels in lake sediments are related to their distance from MP sources. 119,120 Yuan et al. 110 confirmed this fact. The northern and middle regions of Poyang Lake are subject to anthropogenic activities and are the most contaminated parts with regards to the sediment and surface waters, respectively. Sediment and water obtained from the south of Poyang Lake, where less human activities occur, are less contaminated than those of the northern and middle regions. Furthermore, the presence of a wastewater treatment plant in the vicinity of a sampling station might induce higher concentrations of polyethylene terephthalate (PET) fibers. 117,121 The topography of the environment may also be a factor of the concentration or dilution of MP in sediment. 122 In addition, the hydrodynamics, which are influenced by the shoreline morphology, may favor or prevent the deposition of plastics in the sediment; a low velocity environment, such as a harbor will enable plastics to settle. [START_REF] Zhang | Occurrence and Characteristics of Microplastic Pollution in Xiangxi Bay of Three Gorges Reservoir, China[END_REF][START_REF] Possatto | Plastic Debris Ingestion by Marine Catfish: An Unexpected Fisheries Impact[END_REF]120,123 Surprisingly, MP contamination in fish shows different patterns than MP contamination in water or sediment for at least one characteristic (Table S2). Sometimes, a correlation between the water or sediment contamination and fish contamination exists for only one or two parameter(s) (e.g., the shape or the size range) or at one sampling location. [START_REF] Mcneish | Microplastic in Riverine Fish Is Connected to Species Traits[END_REF] Therefore, this shows evidences that exposure is not the only factor involved, [START_REF] Horton | The Influence of Exposure and Physiology on Microplastic Ingestion by the Freshwater Fish Rutilus Rutilus (Roach) in the River Thames, UK[END_REF] and perhaps, that exposure is not a factor at all. 124 Of course, MP ingestion cannot occur if there is not any plastic in the environment, but Kim et al. 124 found that the food concentration is surprisingly more important than the MP concentration in influencing MP ingestion. Kim et al. 124 exposed zebrafish to various mixtures of MP and food and concluded that MP ingestion increased with increasing concentrations of food. However, in that study, the authors exposed fish to virgin plastic. It is now known that animals react differently to virgin or biofouled plastic. The copepods Calanus finmarchicus and female copepods Acartia longiremis ingested significantly more biofouled PS beads than virgin PS beads. 125 Similarly, procellariiform seabirds can be fooled by the dimethyl sulfide signature present on marine plastic particles. 126 In 2017, it has also been shown that Engraulis mordax anchovies responded to medium and high concentrations of biofouled plastic odors with foraging behaviors. 127 Those behaviors did not appear when these anchovies were exposed to clean plastic particles.

Indirectly, surface runoff and seasons also indirectly act on the exposure level. 112,128 Surface runoff increases the exposure to plastic debris. 112 Consequently, the exposure plays a role, but the ingestion also depends on other parameters, such as the age of the plastic particle or the time of the year.

Living habitat

Sediment is a sink for microplastics, [START_REF] Woodall | The Deep Sea Is a Major Sink for Microplastic Debris[END_REF]129 and therefore, demersal and benthic fish could be exposed to more MP than pelagic species. Some studies have focused on both pelagic and benthic or demersal fish with varying results. In China, demersal species ingested more plastics than pelagic species. [START_REF] Jabeen | Microplastics and Mesoplastics in Fish from Coastal and Fresh Waters of China[END_REF] The same assumption was made by McGoran et al. 104 ; a demersal species, Platichthys flesus flounder had ingested far more MP than a pelagic species, Osmerus eperlanus smelt. McGoran et al. 104 assumed that P. flesus had ingested more MP, because it ingested sediment, likely leading to the unintended ingestion of MP. In the marine Scottish waters, demersal fish species have ingested more plastics than pelagic species. [START_REF] Murphy | The Uptake of Macroplastic & Microplastic by Demersal & Pelagic Fish in the Northeast Atlantic around Scotland[END_REF] They also found a significant difference between coastal and offshore species, in which the former are more contaminated than the latter. Nevertheless, this latter category was only represented by demersal species, which may lead to the wrong conclusions about the involvement of the fish species coastal habitat on MP ingestion. Not all studies focusing on MP ingestion by pelagic and demersal fish found a difference between these two categories. 130 Moreover, Rummel et al. [START_REF] Rummel | Plastic Ingestion by Pelagic and Demersal Fish from the North Sea and Baltic Sea[END_REF] showed that pelagic feeders had more MP in their stomach than demersal fish. Again, as methods vary from one study to another and as MP ingestion depends on several factors, the results may vary when looking at only one factor.

Feeding strategy

A more commonly discussed parameter is the feeding strategy of fish and their trophic position. It is thought that (top-)predator fish from both fresh and marine environments might be more at risk than organisms at lower trophic levels because of high energy requirements and trophic transfer. [START_REF] Mcneish | Microplastic in Riverine Fish Is Connected to Species Traits[END_REF][START_REF] Campbell | Microplastics in the Gastrointestinal Tracts of Fish and the Water from an Urban Prairie Creek[END_REF]102,112,131 Other predators such as marine mammals have ingested MP that are too small to be preyed upon by such larger animals. [START_REF] Nelms | Investigating Microplastic Trophic Transfer in Marine Top Predators[END_REF]132,133 This suggests that secondary ingestion has occurred and thus, MP have accumulated through the food chain. The developmental stage of fish might also influence MP ingestion. [START_REF] Horton | The Influence of Exposure and Physiology on Microplastic Ingestion by the Freshwater Fish Rutilus Rutilus (Roach) in the River Thames, UK[END_REF] Indeed, many fish change their diet throughout their development. [134][135][136][137] Ferreira et al. 102 studied MP contamination in several ontogenetic phases of Cynoscion acoupa. The adults, which are mainly piscivorous, were more contaminated than juveniles and subadults, which exhibit an omnivorous diet. This result leads to the same conclusion previously reported: predator or piscivorous fish are more at risk than fish displaying another type of feeding strategy. Later, Ferreira et al. 112 made the same conclusion for two other species (the snook Centropomus undecimalis and Centropomus mexicanus) obtained from the same location, the Goiana Estuary. Three ontogenetic phases were sampled and compared. Juvenile snook registered the lowest MP ingestion level while the piscivorous adults, registered the highest. Moreover, peaks of MP ingestion by adults coincide with peak of fish ingestion. Other studies did not find such differences, 108 indicating the possibility for other parameters to be involved. For example, in the marine environment, debris categories were related to the feeding behavior of the sampled fish. 138 Opportunistic feeders ingested all debris categories (i.e., metals, wood, and plastics), while pelagic feeders had only swallowed plastic bags, benthic fish, and hard plastic pieces.

Based on the fish feeding habits, the visual characteristics of plastic pieces may be involved in MP ingestion by particulate-feeders. In contrast to ram-feeding, which consists of passively filtering water and is nonselective, particulate-feeding is a selective mode, where prey are visually detected before their capture. 139 Then, ram-feeders are less likely to be influenced by the visual characteristics of plastics. [START_REF] Collard | Morphology of the Filtration Apparatus of Three Planktivorous Fishes and Relation with Ingested Anthropogenic Particles[END_REF] Yuan et al. 110 noticed that white MP were more frequently found in fish stomachs than in both water and sediment. Yuan et al. 110 supposed that Carassius auratus could mistake MP for planktonic food. Ory et al. 140 studied Seriolella violacea juveniles in controlled conditions and exposed them to food-shaped and food-sized polyamide particles with different colors. As expected, the juveniles preferentially ingested black particles, which is the same color as food pellets. The other colored particles were supposed to be cocaptured with the food, as they were ingested together with the food pellets only. Nevertheless, attention should be paid to the digesting agent used in the extraction process as some agents, such as NaClO, change or discolor the particles. [START_REF] Collard | Detection of Anthropogenic Particles in Fish Stomachs: An Isolation Method Adapted to Identification by Raman Spectroscopy[END_REF] In a more global framework, the likelihood that beached macroplastics were previously bitten by fish and other organisms depends on the plastic colors. 141 Blue and yellow macroplastics were preferentially more than were other colors. Similarly, plastic threads were the main plastic type ingested by Cynoscion acoupa in the Goiana Estuary, while plastic threads represented only 1.4% of all the plastic debris in the water column. 102,142 Several colors of threads were ingested, suggesting that the plastic shape was more important than the color for this species. The same observation was made by Carson, 141 who found that bottle shaped macroplastics displayed more bite marks than other shapes. Visual and chemical cues thus interfere together with the feeding behavior of fish, making the understanding of MP ingestion in fish even more challenging. Further studies that will consider several parameters are needed to understand the different pathways involved. Increasing the knowledge of those factors will help to suggest and define species for the monitoring of plastic pollution in both fresh and marine waters.

Freshwater fish for the biomonitoring of MP contamination?

The occurrence of a contaminant in an ecosystem does not mean that the contaminant will cause harm to the populations inhabiting the ecosystem. Biota is needed to link the pollutant levels in abiotic compartments and in organisms' tissue to potential adverse effects. 143 Using fish species for the assessment of the quality of aquatic ecosystems is highly relevant. Fish are found everywhere in aquatic environments and have an intermediate trophic position, linking the lower trophic positions to the higher trophic positions (Beyer 1996 in van der Oost et al. 143 ). Nevertheless, attention must be paid when choosing a species for monitoring. The GESAMP [START_REF] Gesamp | Guidelines or the Monitoring and Assessment of Plastic Litter and Microplastics in the Ocean[END_REF] report suggested a list of criteria that helps to define good species for monitoring. This species, or group of species, must be representative of different life histories, phylogenies, sizes, ages and developmental stages. General strategies are proposed, which include opportunistic sampling, market sampling of commercial species and targeting biota. In this report, it is stated that fish, as targeted species or those purchased from the markets, could constitute a good monitoring group for the ingestion of MP. [START_REF] Gesamp | Guidelines or the Monitoring and Assessment of Plastic Litter and Microplastics in the Ocean[END_REF] Given the numerous factors influencing the ingestion of MP by fish, choosing only one indicator species or a group of species is challenging and perhaps impossible. The ecology and feeding habits of the chosen species must be known. It is important to know where the fish, as well as all its developmental stages, is located within the food web, for instance. Here, we present a summary of the parameters to be considered when choosing an indicator fish species, for both fresh and marine environments:

-Trophic position. As discussed in the previous section, predators, especially top-predators, are more likely to ingest MP through food transfer. However, monitoring MP pollution with toppredators will not accurately indicate the contamination level of the environment (water and sediment) but will give global information on the trophic web and the ecosystem.

-Feeding strategy. Ram-feeding fish passively filter water without any visual search for food.

They may be good indicators of the MP levels in the surrounding water. In this case, in addition to gut contents, we advise researchers to also examine the gill rakers, which are bony or cartilaginous structures attached to the branchial arches that retain filtered particles inside the mouth. Those analyses will complete the GC results.

-Migrations. Many fish migrate, either vertically (diel migrations) or horizontally (seasonal migrations), and sometimes fish migrate both ways. Depending on the area chosen to be studied, all types of migration can be valuable. Diel migrations could inform researchers about the MP levels in the pelagic zone, as the surface, seasonal and spatial migrations could provide data for the same species but in different areas. Utilizing the same species is important, as it avoids some bias caused by specific related factors, such as the fish length, the mouth structure and the fish vision. Whatever the chosen species and area, the sampling protocol must be designed considering those migration types. Fish must be sampled at the same time of the day or year if only one location or one water column compartment is targeted.

-Commercial value. Collecting commercially valued fish is advantageous as this is a costeffective method for the assessment of human exposure through the chosen species. [START_REF] Gesamp | Guidelines or the Monitoring and Assessment of Plastic Litter and Microplastics in the Ocean[END_REF] Indeed, commercial fish can be purchased at markets, which eliminates evident sampling costs.

However, it may not be possible to obtain precise information regarding the sampling location.

-Living habitat. Benthic and demersal species will provide information about a sea compartment that is different than the sea compartment of pelagic species. Demersal fish will give a less accurate overview of the benthic compartment than benthic fish, as demersal fish are not fully associated with the sediment.

-Distribution. For comparison and repeatability purposes, the chosen species should have the widest distribution possible or should at least belong to a higher taxon (genus or family) that is distributed worldwide distributed in order to avoid biases linked to the different species used in various studies.

In summary, the perfect fish species used to monitor MP pollution in fresh and marine water is either a top-predator or a ram-feeding species with a large distribution area and a commercial value. If the species is restricted to a small area, similar species must be found in other areas of the world. The migration of the selected fish must be well known and considered when sampling. Researchers must then carefully choose the correct species according to the information they need.

Perspectives and recommendations

To provide perspectives for further research, we want to highlight three steps of protocols that are of great importance: the number of samples analyzed, the size limit of the extracted particles and their identification.

Sample sizes vary greatly between studies, as sizes vary from 1 to 330 individuals for a single species.

A sample size of 50 individuals has been determined to be sufficient for plastic pollution research. [START_REF] Galgani | Guidance on Monitoring of Marine Litter in European Seas[END_REF] A couple of samples is not useful for research; in addition to being statistically irrelevant, time and/or money can be saved by not sampling such small sample sizes and perhaps used for better purposes.

Achieving 50 individuals per studied criterion can be challenging, and therefore, we recommend sampling as close to 50 individuals as possible by sampling the same species from a specific location, for example.

Studies have rarely identified the target size range of MP extracted from fish gut contents (Table 2).

The only information available is the mesh size of the filters used during the extraction step when present. Usually, that mesh size is approximately a micrometer, which allows researchers to extract very small particles, but such particles are hardly observed visually and analyzed spectroscopically.

In contrast, focusing on large MP might lead to an underestimation of the number of ingested particles. Both large and small fish are likely to ingest small MP (< 100 µm) either by transfer from prey to predator or through their feeding behavior. It is thus critical to specify which threshold is applied in each study to promote comparability. This threshold must be defined according to the identification method used; FTIR and Raman spectroscopies allow researchers to analyze MP that are smaller than the particles visible by visual observation through a stereomicroscope.

MP have often been identified by visual inspection, although the identification techniques used have seemingly improved in recent studies. Only two studies on MP contamination in freshwater fish included all dyed particles, even if the particles were not made of plastic (Table 1). We believe that it is important to report all types of anthropogenic contamination in fish tissues, especially textile fibers, which are usually dyed and released in large amounts into the environment. 144,145 With regards to contamination, we suggest that (1) sample collection is performed with as few plastic materials as possible, (2) all solutions, including tap water, must be filtered with filters having the same porosity as the one used to filter the samples and stored in glass jars, (3) plastic materials should be avoided during sample treatment, (4) if the laboratory work cannot be performed in a clean room, the laboratory should then be ventilated as much as possible, (5) all equipment must be covered with clean non-plastic materials when not in use and ( 6) the laboratory operator should wear nonsynthetic clothes, a cotton lab coat and gloves. Avoiding plastic during the sampling and treatment steps is not easy, especially regarding the textiles worn, solution storage and use of nonsynthetic gloves, but for the latter at least, an FTIR or Raman spectrum can be recorded and matched with the potential contaminating particles found in the samples. If contamination does occur, those particles can then be excluded from the results.

As previously recommended by published reviews, [START_REF] O'connor | Microplastics in Freshwater Biota: A Critical Review of Isolation, Characterization, and Assessment Methods[END_REF][START_REF] Lusher | Sampling, Isolating and Identifying Microplastics Ingested by Fish and Invertebrates[END_REF][START_REF] Hermsen | Quality Criteria for the Analysis of Microplastic in Biota Samples: A Critical Review[END_REF] standardized protocols are required. Thus we briefly recommend an overall optimal methodology that can be used when evaluating MP pollution by working with fish gut samples: (1) the number of sampled individuals should be as close to 50 as possible, (2) a MP target size down to 1 μm should be considered to fit the MP definition,

(3) enzymes should be used as digesting agents, (4) identification should be performed by spectroscopy, (5) qualitative observations should be made by light microscopy, (6) as many data as possible (averages and ranges, global contamination or per category, etc.) should be provided, (7) the results should be expressed using several units and per category studied (feeding strategy, developmental stage, etc.), and (8) procedural blank samples should be prepared and analyzed.

Spectroscopic analyses can be performed on a subsample of the total amount of extracted particles.

However, when focusing on fish contamination, it is uncommon to find tens of particles in a single individual. Therefore, we recommend analyzing all particles extracted from the fish GCs. Visual observations are not recommended for sorting or identifying microplastics but are required to take pictures and to measure the MP found. We encourage researchers to keep that step only for those purposes. The global evaluation of the protocol quality can be made thanks to the quality assessment system proposed by Hermsen et al. [START_REF] Hermsen | Quality Criteria for the Analysis of Microplastic in Biota Samples: A Critical Review[END_REF] This quality assessment has been adapted for MP ingestion from the CRED scoring system 146 and provides a good overview of the protocol strength.

This review examines MP in the digestive tracts of freshwater fish, but MP appear to translocate to other organs in several aquatic taxonomic groups, including freshwater fish. [START_REF] Collard | Anthropogenic Particles in the Stomach Contents and Liver of the Freshwater Fish Squalius Cephalus[END_REF][START_REF] Avio | Experimental Development of a New Protocol for Extraction and Characterization of Microplastics in Fish Tissues: First Observations in Commercial Species from Adriatic Sea[END_REF]111,[147][148][149] The impacts of MP translocation are not well known, and MP pathways still need to be understood. The liver has primarily been studied, but other tissues are worth investigating. Based on the human health perspective, muscle is obviously the first tissue that comes to mind, as muscle is almost the only part of the fish we consume. Few studies have examined MP in fish muscle, [START_REF] Collard | Anthropogenic Particles in the Stomach Contents and Liver of the Freshwater Fish Squalius Cephalus[END_REF]106,111,147 and when MP are found in the muscle, their pathways are unknown. In this regard, in vitro studies using biofouled plastics are needed to provide more information about the translocation process(es).

Selecting one or more species suitable for monitoring is currently challenging. First, studies must be performed on MP ingestion by fish and MP contamination in their nearby environment. Studies should be performed at the same time and, of course, by utilizing proper methods with chemical treatment and identification. This will allow scientists to determine which species reflects the best environmental contamination and thus to decide which species can be defined as an indicator.

Furthermore, assessing the species monitoring process is only the first step towards obtaining a more global understanding of MP pollution in biota. To date, most studies on MP ingestion by fish are quite descriptive and contain suggested explanations that are usually not demonstrated, and potential impacts are listed. The quantity and quality of research require improvements to be made to assess the ecological risk posed by MP. 150 A description of the contamination levels is obviously of great importance to the scientific community, but we think that only studying MP contamination is not sufficient. Microplastics alone have been found to negatively impact fish, [START_REF] Anbumani | Ecotoxicological Effects of Microplastics on Biota: A Review[END_REF]151,152 but MPs are part of a cocktail of pollutants containing additives, e.g., phthalates and bisphenols, dyes, organic pollutants, among others. These pollutants are also known to affect fish in several ways. [153][154][155] Consequently, MP should be monitored concomitantly with other pollutants to more accurately reflect the ecological state of freshwaters with regards to pollution.

In conclusion, this review aims to highlight the need for the standardization of methodologies and to give an overview of the factors involved in the ingestion of MP by freshwater fish. The first step of standardization can be to follow several recommendations, such as suggesting that a sufficient number of individuals be sampled and recommending the systematic use of spectroscopy to identify polymer particles. With regards to biomonitoring, studies should be performed using the same overall methodology and the same or very similar fish species. These species must be chosen for the purpose of monitoring and based on their biological characteristics: top-predator and ram-filtering species provide useful information with regards to plastic contamination.

Supporting information

The Supporting Information is available free of charge on the ACS Publications website at DOI:

XXX.

An overview of the studies reporting plastic contamination in freshwaters and a summary of data regarding MP contamination in abiotic compartments and fish at the same location.

Acknowledgments

Figure 1 .

 1 Figure 1. Global overview of the studies performed on MP ingestion by wild freshwater fish.

Figure 2 .

 2 Figure 2. Flowchart of the main steps used to analyze microplastics in fish gut contents. The

Figure 3 .

 3 Figure 3. Number of species for each category of sampling size. This chart is based on studies

Figure 4 .

 4 Figure 4. Proportion of studies according to the main digestive agent used.

Figure 5 .

 5 Figure 5. Proportion of studies according to their lowest limit of MP extracted. Based on Table

Table 1 .

 1 Summary of studies reporting MP ingestion by wild freshwater fish and their methodologies.

	No. sp.	n/sp.	Country	Water body	Targeted particles AP PL	Extracted particle size (µm)	Visual sorting	Digesting agent Hyd. Acid Enz.	Ox.	Spectroscopi c analyses Rn FTIR	Procedural blanks	Reference
		60, 60, 62	Brazil	E	-	X	?	X	-	-	-	-	-	-	-	Possatto et al. 97
		239 & 330	Brazil	E	-	X	?	X	-	-	-	-	-	-	-	Dantas et al. 49
		240, 141, 44	Brazil	E	-	X	?	X	-	-	-	-	-	-	-	Ramos et al. 50
		186	France	R	-	X	> 1.2	X	-	-	-	-	-	-	-	Sanchez et al. 98
		20	Tanzania	L	-	X	> 500	X	X	-	-	-	-	X	-	Biginagwa et al. 99
		10	Switzerland	L	-	X	?	X	-	-	-	-	-	-	-	Faure et al. 100
	44	1-67	U.S.A.	R & E	-	X	?	X	-	-	-	-	-	X	-	Phillips & Bonner 101
		530	Brazil	E	-	X	?	X	-	-	-	-	-	-	-	Ferreira et al. 102
		318 & 118	U.S.A.	R	-	X	> 53	X	-	-	-	-	-	-	-	Peters & Bratton 103
		10-75	Canada	R	-	X	> 5	X*	-	-	-	X	-	-	X	Campbell et al. 96
		20-40	China	L	-	X	> 5	X	-	-	-	X	-	X	X	Jabeen et al. 44
		10 & 66	England	R & E	-	X	?	X	-	-	-	-	-	X	-	McGoran et al. 104
	11	1-21	Argentina	E	-	X	?	X*	-	-	-	X	-	-	-	Pazos et al. 56
		48	Brazil	R	-	X	> 63	X	-	-	-	-	-	-	-	Silva-Cavalcanti et al. 105
	27	2-215	Brazil	E	-	X	?	X	-	-	-	-	-	-	-	Vendel et al. 51
	13	1-6

Table 2 .

 2 Summary of plastic characteristics from gut contents of freshwater fish.

	Plastic concentration or occurrence

339

data and the data published Collard et al.

ACS Paragon Plus EnvironmentEnvironmental Science & Technology

This work has been performed at both the University of Paris-Est Créteil (UPEC) and Norwegian Polar Institute. While working at the UPEC, F. Collard received funding from the People Programme (Marie Curie Actions) of the European Union Seventh Framework Programme (FP7/2007-2013) under the REA grant agreement n. PCOFUND-GA-2013-609102 and through the PRESTIGE Programme coordinated by Campus France (postdoctoral grant).


F Grillot

A Gavrielides

O Spitz

T C Newell

M Carras

Talbot Coupling of an array of quantum cascade lasers

Keywords: Quantum cascade lasers, Talbot cavity, high power lasers

Quantum cascade lasers (QCL) are semiconductor lasers based on ultrafast intersubband transitions with picosecond timescale that have become the most suitable laser sources from the mid-infrared to the THz range, due to their compactness, efficiency and high room temperature performances. In particular, high-power QCLs are powerful sources for optical countermeasures, including night vision blinding and missile out steering. This work investigates the nonlinear dynamical features of coupling of linear arrays of emitters in the so-called Talbot configuration for phase-locking operation using broad area emitters. These initial results are of paramount importance for creating future bright infrared sources with Watt-level power.

INTRODUCTION

High-power quantum cascade lasers (QCLs) are powerful sources for optical countermeasures, including night vision blinding and missile out steering. [START_REF] Faist | Quantum Cascade Lasers[END_REF] In order to increase the power, several solutions have been proposed such as using external optical feedback, which has been shown to be an efficient technique to control the beam quality and dynamical stability, without resorting to integrated solutions. [START_REF] Ferré | Beam shaping in high-power broad-area quantum cascade lasers using optical feedback[END_REF] This paper aims at investigating the coupling of linear arrays of emitters in the Talbot configuration based external cavity. Talbot effect is a well-known lensless optical phenomenon that the intensity pattern of an array of coherent emitters reproduces itself after a specific distance of propagation. [START_REF] Talbot | Facts relating to the optical science[END_REF][START_REF] Rayleigh | On copying diffraction gratings and on some phenomenon connected therewith[END_REF][START_REF] Wen | The Talbot effect: recent advances in classical optics, nonlinear optics, and quantum optics[END_REF] This effect has been exploited to phase-lock lasers in the nearinfrared, which is called diffraction coupling scheme phase-locked array. [START_REF] Evans | Surface Emitting Semiconductor Lasers and Arrays[END_REF] In the past, Talbot arrays have been investigated using single transverse mode diode laser arrays but not QCLs except in two very recent papers in which phase-locking using broad area emitters is unveiled. [START_REF] Wang | Phase-locked array of quantum cascade lasers with an integrated talbot cavity[END_REF][START_REF] Meng | Coherent emission from integrated Talbot-cavity quantum cascade lasers[END_REF] In this paper, we investigate both theoretically and experimentally the complex dynamics arising in an array of QCLs. [START_REF] Peterson | Extraction characteristics of a one dimensional Talbot cavity with stochastic propagation phase[END_REF] We provide a comprehensive analysis of the Talbot cavity and show the existence of distinct regions depending on whether all the lasers are locked to the same frequency. Impact of the external cavity length and detuning between lasers is discussed. The results are promising for creating a bright infrared source with Watt-level power.

SINGLE MODE QCL COUPLED BY THE TALBOT EFFECT

The setup consists in an array of N QCLs placed side by side with a center-to-center distance d. The facet of the QCLs are cleaved on both sides so that emission occurs on both sides. On one side, the global emission properties of the array can be studied with that specific configuration. On the other side, a mirror is placed at a multiple of Zt 8 , with Z t being the Talbot distance and scaling as 2nd 2 λ . λ is the emission wavelength of the QCL and n is the refractive index of the medium between the QCLs array and the mirror. For instance, if the mirror is set at a distance equal to Zt 8 as shown on Fig. 1, the pattern derived from the Talbot effect that is found at a distance Zt 4 is back reflected to the array. This induces a coupling which can improve the emission properties of the linear QCL array in particular for creating a bright infrared source with Watt-level power. The model that contains the essential features of coupling N QCLs grown on a chip consists of N equations for the field and 2N equations for the carriers, 10

dE n dt = iδ n E n + (1 + iα)N n E n + η N m=1 R nm (τ )E m (t -τ ) , (1)
dZ n dt = γ 1 [P + γ 3 V n -γ 4 Z n -(1 + 2Z n )|E n | 2 ] , (2)
dV n dt = γ 2 [2Z n -V n + (1 + 2Z n )|E n | 2 ] , (3)
where the time t is measured in units of photon lifetime τ p . The parameters γ i for i between 1 and 4, and P , which are the normalized dimensionless parameters, are defined as follows in terms of the physical parameters of the QCLs: Also, δ n is the relative detuning from the frequency of a selected reference laser, for instance laser 1 is related to δ 1 = 0. The coupling coefficients R nm (τ ) are obtained from the specific geometry of feedback as determined from the Talbot cavity or of the Fourier Transform cavity configuration. For example, in the case of a Talbot cavity, the coefficients R nm (τ ) are given using the approximations in Ref. 9 such as

γ 1 = τ P τ 32 , (4)
γ 2 = τ 32 τ 21 -1 τ P τ 32 , (5)
γ 3 = τ 32 2τ 21 -1 - τ 32 2τ 31 1 τ32 τ21 -1 , (6)
γ 4 = 2 1 + τ 32 2τ 31 , (7)
P ≡ N p gτ P τ 32 2 I in -I th q , (8)
R nm (τ ) = e iωτ 1 + iτ 2τ0 exp i π 2 (n -m) 2 τ t τ ( 9
)
where τ is the roundtrip time of the external coupling path, τ 0 = ωa 2 2c 2 tp is related to the waist a, or the aperture of the lasers of the radiation emitted by each QCL in normalized time units, and τ t = ωd 2 πc 2 tp is the Talbot distance expressed in normalized time units.

The coefficients δ n are the detunings of the fundamental uncoupled frequency from laser No. 1. We then estimate that the detunings will be random to within δν the HWHM of the gain of the QCLs if the lengths of the cavities are random. The initial computations will assume that the detunings are equal to zero since we expect that the QCLs will be grown on the same substrate. For an order parameter i.e. an estimate of the coherence between the lasers is:

r(t) = 1 N < | N n=1 E n (t)| 2 > < N n=1 |E n (t)| 2 > , (10)
which is inversely proportional to the beam quality as it can be determined from the near field and far field data. E n (t) is the field from each laser in the array. The order parameter r(t) can be consider as an interference term of several fields. For equal phases, r converges towards the unity while for anti-phase or random phases it leads to 0. This is the same effect in the far field except the overall envelope is the Fourier transform hence on the axis one either get r → 1 for equal phases or r → 0 for random or anti-phase. In general we would like to monitor the time development of the r(t) order parameter in time to identify any possible global bifurcations.

NUMERICAL RESULTS FOR Z T

Effects of the detuning

In this section we will try to understand the effects of the detuning between the lasers. We assume that either due to the manufacture or due to thermal effects there may be imperfections in the lengths of the individual cavities that may lead to the possibility that the laser will not operate on the same frequency, thus introducing detuning to the array. This effect was included in the model equations Eq.( 1)-(3). To examine this more carefully we would like to recast the Eq.( 1)-(3) so that the frequency of the individual laser ω n = Ω n τ p is included explicitly:

dE n dt = iω n E n + (1 + iα)N n E n + η N m=1 R nm (τ )E m (t -τ ) , (11)
We will only need at this time the equations for the fields as the equations for the populations will be unchanged. We can then define the average frequency of the N lasers collection by

ω 0 =< ω n >= 1 N N n=1 ω n , (12)
We then redefine the field as A n = e -iω0t E n and obtain

dA n dt = iδ n A n + (1 + iα)N n A n + ηe -iω0τ N m=1 R nm (τ )A m (t -τ ) , (13)
We can eventually regard δ n as random numbers with zero mean on an interval of (-D, D) with probability P (δ) = 1/2D. Similarly, we will restrict ω 0 τ on the interval of (-π, π) under symmetry considerations. In Fig. 2, we compute the order parameter as a function of time for two values of mean square deviation of the detuning. It is obvious that as the correlation increases, the order parameter decreases and at a value close to 0.1004 the locking is broken and the lasers are operating at their own frequencies. To obtain an idea of the actual physical detuning required to break locking we find that if λ = 5µm, and the cavity lifetime is of the order of the picosecond, then the RMS of ∆ν = 7GHz corresponds to the correlation of √ 0.1004.

Numerical results for zt 4

We will examine a set of seven QCLs coupled by the Talbot effect in which the external cavity is set at 2L = zt 4 . The coupling phases are symmetric and approximately equal to each other. In the notation introduced earlier the pumping is assumed to be P =2.0, the enhancement factor α = 2 with a fill factor of f = 0.1.

In Fig. 3 we show the bifurcation of amplitude and frequency of laser 1 as well as of the order parameter as a function of the feedback η. Three distinct regions are apparent immediately. In the first region it is clear that the laser is not at a steady state for values of η ≤ 0.17. An example of the time dependence of the order parameter in this region for a value of η = 0.05 is shown in Fig. 4. It is clear that it is oscillating at a small frequency and the numerical calculation indicates that the frequencies of the lasers are not locked and not related to each other as well as the laser amplitudes. For the region of η ≥ 0.17 and η ≤ 0.62 all three variables indicated in Fig. 3 are constant and therefore the lasers are in an in-phase steady state and therefore the order parameter approaches 1. All the lasers are locked to the same frequency and it develops as a function of η in the usual manner as in single lasers with an external cavity. The order parameter for η = 0.55 is shown in Fig. 4 and is flat at r=0.988 once the transients have died down. Finally, in the third region η ≥ 0.62 we find regular pulsations. In particular we show in Fig. 4 for η = 0.65 the time development of the order parameter. It is clear that the lasers show a mode-locked behavior since each laser lase at its own frequency and with relative phases equal to zero or in a in-phase condition. In addition, the frequencies as well as the amplitudes obey.

S n = S N -n+1 , (14)
where S stands for either A or ωτ . We have only a preliminary glimpse in this region but it appears that this may have interesting properties for mode-locking applications, in particular as the number of lasers in a bar becomes rather large.

EXPERIMENTS

As a precursor to the Talbot cavity array phasing we initially investigate a broad-area QCL (BA-QCL) subject to external cavity feedback. The motivation for this first step comes in part from the behavior of BA-QCLs. These often exhibit lasing on one principal transverse mode [START_REF] Bai | High power broad area quantum cascade lasers[END_REF] with other transverse modes strongly suppressed.

The importance to the Talbot cavity is that each emitter in the array may not have to be constrained for the fundamental transverse mode. With BA-QCLs lasing on a single mode and with Talbot feedback selecting a supermode, then we may be able to widen each emitter so as to increase the gain and total output power while remaining in the fundamental supermode. As a test of feedback we place a 40 µm wide, 3 mm long, λ=4.7 µm QCL in an external cavity, see Fig. 5 which is a schematic of the external cavity (ECL) experimental arrangement. The ECL is bounded by the QCL and rotatable partially reflective mirror (PR). The components consist of a f =1.87 mm collimating lens (CL), pellicle beam splitter with low reflectivity at 4.7 µm (BS), a midinfrared polarizer to vary the feedback intensity (P), f=200 mm focusing lens (FL), integrating sphere and detector (PM), an amplified Mercury-Cadmium-Telluride (MCT) detector, mirrors (M) and Spectral Products DK240 monochromator. The primary component is the mirror that is rotated to create different cavity modes, thus steering the beam or inciting various dynamical effects. Near-field images at the facet are obtained using the CL and FL lens pair magnifying the image onto a Xenics Onca 3369 InSb camera. From the camera image a line scan is extracted. In Fig. 6,(a) and (b) show the near field intensity plotted versus the position on the laser facet. Figure 6(c) shows the light-current curves for the two cases. Fig. 6 (a) is the free-running case. A principal M=9 mode is observed although the intensity of each antimode varies and the furthest to the right is quite dim. In the far-field, two peaks are observed at angles 33 degrees as predicted although other lesser peaks are also observed. This image is not normally observed in diode counterparts that would show an irregular flat-top peak indicative of many transverse modes along with filamentation. Fig. 6 (b) is taken with feedback. In this case the created mode shows one dominant peak in the near-field. While it is not the fundamental mode of the BA-QCL, it does show that distinct modes can be created with feedback. The light-current curves in Fig. 6 (c) are measured from a sampling of the light and show that the feedback improves the threshold current while also increasing the modal gain. Note that the total power emitted at the facet is much higher. The significant result that may be applied to the Talbot question, suggests that the emitter width may be widened with the array maintaining coherent phase-locking. However we note that the created mode is highly mirror angle dependent. Curious modes are created and destroyed as the mirror is rotated. Furthermore, these modes can compete showing temporal oscillations.

CONCLUSIONS

This paper gives initial theoretical results on the nonlinear dynamical features of coupling of linear arrays of emitters in the Talbot configuration for phase-locking operation. We provide a comprehensive analysis of the Talbot cavity and show the existence of distinct regions depending on whether all the lasers are locked to the same frequency. These initial results are of paramount importance for creating future bright infrared sources with Watt-level power.

Figure 1 .

 1 Figure1. Schematic of the experimental setup when an array of N lasers is considered. In that case, the length of the Talbot cavity is set to Z t 8 and therefore the image at Z t 4 is sent back to the array.

Figure 2 .

 2 Figure 2. Order parameter r(t) as a function of time for two mean square deviations of the detuning, σ 2 = 0.1004 and σ 2 = 0.067.

Figure 3 .

 3 Figure 3. Amplitude of the laser 1 A1 (blue scatter plot), r(t) (green scatter plot), and frequency shift due to the external cavity φ1(t) -φ1(t -τ ) (red scatter plot) as a function of feedback strength η.

Figure 4 .

 4 Figure 4. Bifurcation using the order parameter r(t) as a function of η.

Figure 5 .

 5 Figure 5. Schematic of experimental arrangement

Figure 6 .

 6 Figure 6. (a) Line scan from near-field facet image for the free-running case and (b) feedback forcing single peak mode. (c) Light-current curves for the free-running and single peak feedback case.

Table 1 .

 1 Experimental parameters as estimated or measured and published in various publications

	References 11	12	13	14
	γ 1	2.75	1.85	15.59/15.02 0.67/0.67
	γ 2	16.49 10.48 90.56/55.77 4.1/0.17
	γ 3	0.35	0.34	0.41/0.37	0.42/1.5
	γ 4	2.81	2.82	2/2	2/2

Acknowledgments

TCN would like to thank the Air Force Office of Scientific Research for support.