Chia-Ting Chen
email: chiating.chen9692@gmail.com

François Carlotti
email: francois.carlotti@mio.osupytheas.fr

Mireille Harmelin-Vivien

Benoit Lebreton
email: benoit.lebreton@univ-lr.fr

Gaël Guillou
email: gael.guillou01@univ-lr.fr

Laura Vassallo

Marjorie Le Bihan

Daniela Bănaru
email: daniela.banaru@mio.osupytheas.fr

Diet and trophic interactions of Mediterranean planktivorous fishes

Keywords: Engraulis encrasicolus, Sardina pilchardus, Sardinella aurita, Sprattus sprattus, Cepola macrophthalma, Small pelagic fish, diet composition, prey-predator size-ratio, trophic niche overlap, food competition European sardine Sardina pilchardus, round

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Introduction

Small planktivorous fish represent a fundamental trophic group in many ecosystems [START_REF] Peck | Small pelagic fish in the new millennium: A bottom-up view of global research effort[END_REF]) as they feed on diverse plankton groups and are forage species for many predators including mackerel, hake and tuna [START_REF] Palomera | Small pelagic fish in the NW Mediterranean Sea: An ecological review[END_REF]; [START_REF] Bănaru | Trophic structure in the Gulf of Lions marine ecosystem (north-western Mediterranean Sea) and fishing impacts[END_REF][START_REF] Bănaru | Implementation of an end-to-end model of the Gulf of Lions ecosystem (NW Mediterranean Sea). I. Parameterization, calibration and evaluation[END_REF]. Small planktivorous fishes also have a high commercial value as they represent more than 50% of the annual landings in the Gulf of Lion and more widely in the Mediterranean Sea [START_REF] Bănaru | Trophic structure in the Gulf of Lions marine ecosystem (north-western Mediterranean Sea) and fishing impacts[END_REF]; FAO 2020).

Bottom-up changes in the pelagic food web induced by climate change via hydrological processes are suspected to be one of the major drivers of planktivorous fish populations, including in the North-Western Mediterranean Sea (NWMS) [START_REF] Moullec | Climate change induces bottom-up changes in the food webs of the Mediterranean Sea[END_REF] and references therein; [START_REF] Bănaru | Implementation of an end-to-end model of the Gulf of Lions ecosystem (NW Mediterranean Sea). I. Parameterization, calibration and evaluation[END_REF][START_REF] Feuilloley | Concomitant changes in the environment and small pelagic fish community of the Gulf of Lions[END_REF]). Moreover, the NWMS has also been impacted by changes in the management of sewage water, leading to a reduction of nutrient inputs [START_REF] Raimbault | Le milieu marin autour de Marseille[END_REF]) that may potentially impact plankton communities, as demonstrated in other Mediterranean areas [START_REF] Pagès | Changes in rivers inputs during the last decades significantly impacted the biogeochemistry of the eastern Mediterranean basin: A modelling study[END_REF]). Zooplankton play a pivotal role in marine ecosystems, as they support fisheries production and mediate fluxes of nutrients and chemical elements [START_REF] Banse | Zooplankton: Pivotal role in the control of ocean production: I. Biomass and production[END_REF]. Mesozooplankton of 200 µm to 1000 μm in length, mainly consisting of copepods, constitute the main prey of adult planktivorous fish, and the bottom-up control hypothesis involves changes in the quantity or quality of these prey (Le Bourg et al. 2015). In addition, recent studies have demonstrated that the quality of plankton depends on both taxonomic group and size class In coastal areas, the changes described above may have contributed to the reduction of planktivorous fish biomass and their relative proportions in fish communities [START_REF] Ourgaud | Response of a seagrass fish assemblage to improved wastewater treatment[END_REF][START_REF] Cresson | Functional traits unravel temporal changes in fish biomass production on artificial reefs[END_REF]). In the NWMS, it has been hypothesized that changes in both taxonomic composition and individual size of zooplankton may underlie recently observed changes in the diet of some planktivorous fish species such as the European pilchard and the European anchovy (Le Bourg Sea, this bottom-up control hypothesis highlights the need to study the composition of zooplanktivorous fish diet and trophic interactions, and to consider zooplankton prey groups and size in order to better estimate their role in the pelagic food webs. Knowledge of dietary specialization, including potential sharing of food resources among co-occurring members of the fish community, would help determine functional roles and to identify species for which potential competitive interactions during periods of prey scarcity may occur [START_REF] Burbank | Diet and isotopic niche of eastern sand darter (Ammocrypta pellucida) near the northern edge of its range: a test of niche specificity[END_REF]. This is essential for ecosystem management and conservation [START_REF] Bachiller | Trophic niche overlap between round sardinella (Sardinella aurita) and sympatric pelagic fish species in the Western Mediterranean[END_REF]).

In the NWMS, previous studies on planktivorous fish diet have focused on the taxonomic composition of stomach contents [START_REF] Stergiou | Feeding habits and trophic levels of Mediterranean fish[END_REF]Costalago and Palomera 2014). Some of these studies were based on the isotopic composition of muscle tissue and of the prey of planktivorous fish (Cresson et Combining stable isotope analyses (SIA) and stomach content analyses (SCA) offers different and complementary insights into trophic relationships. While SCA provide information on recently consumed prey (hours to one day), SIA (measurement of δ 13 C and δ 15 N values) of muscle tissues provides information on food sources integrated over time (weeks to months) [START_REF] Vander Zanden | Stable Isotope turnover and half-life in animal tissues: a literature synthesis[END_REF]). An underlying assumption is that isotopic compositions accurately reflect species dietary breadths over a period of time defined by tissue-specific turnover rates.

The present study focused on the diet and trophic interactions of seven planktivorous fish species: European anchovy Engraulis encrasicolus (Linnaeus, 1758) Our aim here was to determine the food resources consumed by planktivorous fish.

Our hypothesis is that competition for food resources may occur between the NWMS planktivorous species, leading to the observed changes in small pelagic fish populations. The information provided by our study will improve food web modeling, which is increasingly applied to predict fish population and ecosystem dynamics. We investigated the diet of pelago-demersal planktivorous fish by combining SCA and SIA. Specifically, we aimed: (1) to analyze fish diet in terms of prey taxonomic groups and prey sizes over the entire sampling period, (2) to estimate the trophic overlap among these species using SCA and SIA, and (3) to compare the dietary diversity and overlap of trophic niches using both methods, to test the hypothesis of potential trophic competition.

Material and methods

Study site

The Bay of Marseille (South of France) is located in the eastern part of the Gulf of Lion in the NWMS. In this bay, phyto-and zooplankton communities benefit from diverse marine and terrestrial nutrient input [START_REF] Bănaru | Seasonal variation of stable isotope ratios of size-fractionated zooplankton in the Bay of Marseille (NW Mediterranean Sea)[END_REF]Chen et al. 2019a).

The Bay of Marseille is influenced from the south by the intrusions of the offshore Northern Current [START_REF] Petrenko | Circulation in a stratified and wind-forced Gulf of Lions, NW Mediterranean Sea: in situ and modeling data[END_REF], and occasionally from the west by the intrusions of the Rhone River plume, and by the coastal Huveaune River, depending on wind and rain events [START_REF] Cresson | Spatio-temporal variation of suspended and sedimentary organic matter quality in the Bay of Marseilles (NW Mediterranean) assessed by biochemical and isotopic analyses[END_REF][START_REF] Fraysse | Intrusion of Rhone River diluted water into the Bay of Marseille: Generation processes and impacts on ecosystem functioning[END_REF]). The bay is also subject to anthropogenic and terrestrial inputs from the Marseille sewage treatment plant at Cortiou [START_REF] Millet | Unexpected spatial impact of treatment plant discharges induced by episodic hydrodynamic events: Modelling Lagrangian transport of fine particles by Northern Current intrusions in the bays of Marseille (France)[END_REF]) (Fig. 1).

Sampling and sample processing

Fish were sampled through a partnership with local fishermen working with lampara net, gillnets and trawling (Fig. 1). Where possible, 30 individuals of each species were sampled in the early morning, each month from March 2017 to September 2017.

Alongside this monthly sampling, data were also analyzed collectively -and not seasonally -because the number of individuals sampled each month widely differed between species. The potential impact of seasonality on the results is considered separately (see section 4.4.). The number of samples for body length measurements, Once caught, fish were immediately put on ice and kept at -20°C upon arrival at the laboratory to inhibit digestion of the stomach contents.

Zooplankton was collected monthly at the Solemio station (43.24°N; 5.29°E) simultaneously with the SOMLIT (Service d'Observation en Milieu LITtoral) long-term national program for littoral observation (Fig. 1). Zooplankton was sampled vertically from 50 m depth (close to the bottom) to the surface with an 80-μm mesh size net (50 cm opening diameter). Zooplankton samples were preserved in a buffered 4% formaldehyde -seawater solution.

In the laboratory, total body length (TL) of each fish was measured to the nearest millimeter. Each individual was sexed by examination of the gonads. Digestive tracts were collected and preserved individually in a buffered 4% formaldehyde -seawater solution for later SCA. Dorsal white muscle was sampled from each fish and frozen at -20°C for later SIA.

Analysis of fish diet and plankton composition

The number of analyzed individuals for total body length measurements, the minimum, maximum and mean total body length (cm), the proportion of empty stomachs and the degree of repletion per species are displayed in Table 1. The mean degree of stomach repletion (SRD) was estimated visually using a semi-quantitative scale of prey volume in stomach: 0 = empty, 1 = > 0 -25 % full, 2 = 25 -50% full, 3 = 50 -75% full, 4 = 75 -100% full. Empty stomachs were not considered in diet index calculations.

Zooplankton prey were identified and sorted for further taxonomic and size analyses.

The taxonomic groups of zooplankton found in both fish stomachs and plankton net samples were identified and the number of individuals in each group were counted using a stereomicroscope (Leica M205C) as described by [START_REF] Chen | Temporal variation in prey selection by adult European sardine (Sardina pilchardus) in the NW Mediterranean Sea[END_REF]. The main copepod groups were identified using the website 'Marine Planktonic Copepods' (https://copepodes.obs-banyuls.fr/en/loc.php?loc=14) [START_REF] Razouls | Diversity and geographic distribution of marine planktonic copepods[END_REF]).

The maximum length (sum of prosome and urosome lengths for copepods) was measured on the first 30 individuals of representative prey groups found in each stomach and zooplankton net sample. Size measurement allowed the calculation of a prey-predator size ratio (PPSR) between each fish and its prey, achieved by determination of the most consumed prey size range by species and comparison with the size of the available zooplankton prey in the 80-μm mesh size net. For graphic representation, PPSR values were multiplied by 1000. Dry weight (DW) of each organism was estimated from length-weight relations gathered from the literature (Table S1). The degree of stomach fullness (SFD) was used to assess feeding intensity (as described by [START_REF] Bachiller | Feeding ecology of Northeast Atlantic Mackerel, Norwegian Spring-Spawning Herring and Blue Whiting in the Norwegian Sea[END_REF]). The SFD, expressed in μg.cm -1 , is defined as the sum of prey dry weight in the stomach divided by the total length of the fish.

Trophic indices

Diet composition based on stomach content analyses (SCA) was analyzed using several complementary indices: frequency of occurrence (O%: percentage of non-empty stomachs containing a given prey item), numerical percentage (N%: mean percentage per stomach of the number of a given prey item related to the total number of all prey in each stomach), dry weight percentage (W%: mean percentage per stomach of the weight of a given prey item related to the total weight of all prey in each stomach). These indices were used to calculate the index of relative importance (IRI) following the formula [START_REF] Pinkas | Food habits of albacore, bluefin tuna, and bonito in California waters[END_REF]): IRI = O% × (N% + W%) and then expressed as a percentage (IRI%) [START_REF] Cortés | A critical review of methods of studying fish feeding based on analysis of stomach contents: application to elasmobranch fishes[END_REF]).

For consistency, IRI was also used to compute trophic niche width and dietary overlap.

To compare the community composition of zooplankton sampled with nets and the zooplanktonic prey in fish diet, IRI% was also calculated by month for zooplankton sampled with the 80-μm mesh size plankton net. Here, IRI% integrates information on the abundance, occurrence and biomass of zooplankton groups sampled with nets.

Diversity indices

The Shannon diversity index was determined to quantify the diversity of prey in fish stomachs and then for zooplankton net samples, using the formula:

𝐻′ = -∑ 𝑝 𝑖 𝑙𝑛 𝑝 𝑖 𝑠 𝑖=1
where 𝑝 𝑖 is the mean proportion (IRI%) of the prey group i, and S is the total number of prey groups.

The Pielou evenness index was computed to determine the evenness of prey group distribution in the diets of the studied species, using the formula: 𝐽 = 𝐻′ ÷ ln (𝑆)

where 𝐻′ is the Shannon diversity index and S is the total number of prey groups found in stomach contents per species. This index was also estimated for the 80-μm mesh size plankton net.

Trophic niche breadth and prey diversity in relation to increasing fish size was analyzed following the method proposed by Bachiller and Irigoien (2015).

Stable isotope analysis (SIA)

For the SIA, where possible, at least three fish were analyzed per month and per species among those previously analyzed for SCA (Table 1). Fish muscle tissues were freeze-dried, ground into a fine powder and a subsample was packed into a 5 × 8 mm tin cup. Carbon and nitrogen stable isotope analyses were performed using a continuous-flow isotope-ratio mass spectrometer (Delta V Plus, Thermo Scientific) with a Conflo IV interface coupled to an elemental analyzer (Flash EA 1112, Thermo Scientific) equipped with the Smart EA option. Analyses were conducted at the Littoral, Environment and Societies Joint Research Unit stable isotope facility (CNRS -University of La Rochelle, France). The δ 13 C and δ 15 N values are expressed in δ notation as deviations from standards (Vienna Pee Dee Belemnite for δ 13 C and N 2 in air for δ 15 N), in ‰, according to the formula:

δX = [( Rsample Rstandard ) -1] × 1000
where X is 13 C or 15 N, R sample is the isotopic ratio of the sample and R standard is the isotopic ratio of the standard. Calibration was carried out using reference materials (USGS-24, -61, -62, IAEA-CH6, -600 for carbon; USGS-61, -62, IAEA-N2, -NO-3, -600 for nitrogen). The analytical precision of the measurements was <0.15‰ for carbon and nitrogen based on analyses of USGS-61 and USGS-62 used as laboratory internal standards.

The δ 13 C values were corrected for the effect of lipids when the C/N ratio was higher

Resource partitioning

The total dietary trophic niche width (DNW) based on stomach content analysis was estimated to compare fish species trophic diversity [START_REF] Schoener | Nonsynchronous Spatial Overlap of Lizards in Patchy Habitats[END_REF]

: DNW = e 𝐻 ′ with 𝐻 ′ = -∑(𝐼𝑅𝐼 𝑖 × 𝑙𝑜𝑔 2 𝐼𝑅𝐼 𝑖 ).
Overlap in species diet (T%; Schoener 1970) was calculated according to the formula:

T% = (1 -0.5∑|𝐼𝑅𝐼 𝑥𝑖 % -𝐼𝑅𝐼 𝑦𝑖 %|) × 100
where 𝐼𝑅𝐼 𝑥𝑖 % and 𝐼𝑅𝐼 𝑦𝑖 % are IRI% for the prey item i in the fish species x and y.

In theory, T% varies from zero (when the two groups of fish feed on totally different prey items) to one (when they consume the same prey items in the same proportions). Index values higher than or equal to 0.6 are considered significantly elevated [START_REF] Keast | Trophic and spatial interrelationships in the fish species of an Ontario temperate lake[END_REF]).

Statistical analyses

Data normality and homogeneity were tested using Shapiro-Wilk and Levene tests.

Non-parametric analysis of variance Kruskal-Wallis tests were used to examine differences in total body length, average prey size, PPSR, and isotopic compositions (δ 13 C and δ 15 N) among species. Dunn's multiple non-parametric post-hoc tests were used to pinpoint which specific means significantly differed from the others. Spearman's correlation was used to test the relationship between the total body length and stable isotopic compositions (δ 13 C and δ 15 N) of fish species. Differences in diet composition between species were tested using one-way analysis of similarity (ANOSIM). SIMPER analysis provided the average dissimilarity of their diets. The prey groupings were based on Bray-Curtis mean dissimilarities. Differences of prey size distribution among species were tested with the Chi-square test. The Bray-Curtis dissimilarity was also calculated for interspecific pairwise comparisons of prey size distribution. Changes in the range of the Shannon-Wiener diversity index (H') and PPSR of each individual predator with increasing size were analyzed according to the methods described in [START_REF] Scharf | Predator size-prey size relationships of marine fish predators: interspecific variation and effects of ontogeny and body size on trophic-niche breadth[END_REF] and Bachiller andIrigoien (2013, 2015). To represent interspecific differences in dietary and isotopic compositions, non-metric multi-dimensional scaling (nMDS) was carried out on the similarity matrix of the diet (IRI%) and the stable isotopic compositions (δ 13 C and δ 15 N) of fishes, respectively.

The differences between the isotopic niche and the dietary niche overlaps were tested with a χ 2 test and Spearman's rank order correlation. Data processing was carried out using the software R (R Core Team 2021).

Results

Prey availability

Available prey were often smaller than 1.5 mm with the most abundant prey ranging in size from 0.5 to 0.8 mm (36.4%) (Fig. 2). Zooplankton was dominated by copepods (IRI% = 84.8%), especially calanoid copepods (IRI% = 72.3%) (Fig. 3). Other zooplankton groups such as Monstrilloid and Sapphirinid copepods, fish larvae, tintinnids, echinoderm and cephalopod larvae, and other microzooplankton groups were also observed in net samples and contributed to overall diversity (Shannon diversity index = 1.00 ± 0.25 (mean ± standard deviation)).

Biometry and stomach fullness of caught fish

Body size was measured from a total of 715 fish, SCA was measured from 446 individuals and SIA from 173 individuals (Table 1). Total body length (Table 2) differed significantly among species (H = 129.4, P < 0.001). Cepola macrophthalma was the longest (29.3 ± 6.8 cm), followed by S. aurita, B. boops, S. pilchardus, C. chromis and E. encrasicolus, the lowest mean total length being recorded in S. sprattus (11.2 ± 1.0 cm). The proportion of empty stomachs was lowest for S. pilchardus (2.1%) and highest for C. chromis (20.6%), and stomach content was about half full for most species (Table 2). The SFDs varied significantly between species (H = 36.64, P < 0.001), with highest values for C. macrophthalma and C. chromis and lowest values for S. pilchardus (Table 2).

Prey group composition

For each species, trophic niche breadth based on prey diversity (i.e. H') did not vary with fish size (Fig. S1) or between sexes (not displayed, P > 0.05 for all species).

The average diet composition differed significantly among species (ANOSIM R = 0.15, P < 0.001) (Table 3, Fig. 3).

Engraulidae and Clupeidae species mainly preyed on copepods (IRI% > 86).

Although small harpacticoid (Microsetella/Macrosetella spp. and Euterpinidae) and cyclopoid (Oncaeidae and Corycaeidae) copepods were not the most abundant in the environment, they were present in high proportions as prey (Fig. 3, Table S2).

Engraulis encrasicolus and S. aurita consumed higher proportions of calanoid copepods than S. pilchardus and S. sprattus. Cepola macrophthalma consumed a high proportion of decapod larvae in addition to calanoid copepods (Acartiidae and Calanidae). Chromis chromis and B. boops both consumed other prey that were not abundant in the environment during the same sampling periods. Chromis chromis consumed a high proportion of gastropods (pteropods), cypris larvae and ostracods, while B. boops consumed benthic polychaetes and cypris larvae. Both species ingested high proportions of amphipods and decapod larvae.

The SIMPER analysis (Table 3) showed that the IRI% of copepods contributed the most (> 60%) to the dissimilarities of diet between the studied species, but the IRI% of other prey such as gastropod and decapod larvae was also high (> 5%).

Bray-Curtis dissimilarity indices on prey composition were lowest between S. pilchardus and S. aurita (Table 4), while the highest values were found between S. sprattus and both C. macrophthalma and C. chromis.

Prey diversity (χ 2 = 26) and evenness (χ 2 = 24) indices differed significantly among fish species (P < 0.001 for both). Sprattus sprattus exhibited the highest prey diversity (H' = 0.95 ± 0.08) and C. chromis presented the highest prey evenness (J = 0.76 ± 0.11). Cepola macrophthalma showed the lowest values for both indexes (H' = 0.29 ± 0.11, J = 0.32 ± 0.14) (Table 5). The zooplankton composition of the 80-μm mesh size net generally showed a higher group diversity but had about the same evenness index as fish stomach content (Table 5).

Prey size structure

Prey size distribution differed significantly among species (χ 2 = 405.23, P < 0.001), with one or two modes of prey size between 0.1 and 2 mm (Fig. 4). Sardina pilchardus and S. sprattus both presented a high consumption of prey ranging between 0.5 and 1.0 mm in size, while S. aurita, C. macrophthalma, C. chromis and B. boops also consumed a high proportion of prey > 2 mm (up to 40% for B. boops).

Average prey size and PPSR differed significantly among species (H = 54 and H = 61, respectively, with P < 0.001 for both). Boops boops presented the highest values for average prey size (8.54 ± 6.31 mm), and S. pilchardus the lowest (0.99 ± 0.21 mm) (Fig. 5A). Boops boops also had the highest average value for PPSR, and C. macrophthalma the lowest (Fig. 5B). A significant increase of trophic niche breadth based on PPSR with increasing fish size was observed in B. boops (Fig. S2). For other species, no change in mean prey size or of trophic niche breadth based on PPSR and prey diversity was observed with increasing body size.

Prey size distribution was significantly different between B. boops, S. pilchardus and S. sprattus (Table 6) and the most similar between the Clupeidae (< 0.30).

Stable isotope composition

Mean δ 13 C and δ 15 N values and the estimated trophic level differed significantly among species (δ 13 C: H = 35.67, P < 0.001; δ 15 N: H = 61.52, P < 0.001, TL: H = 61.50, P < 0.001). Sprattus sprattus had the highest δ 13 C values and B. boops and S. pilchardus had the lowest (Table 7). Boops boops and C. macrophthalma had the highest δ 15 N values and estimated trophic levels, while S. sprattus and E. encrasicolus presented the lowest. Spearman's correlation tests highlighted significant negative correlations between δ 13 C values and total body length for S. pilchardus (Table 7).

Significant positive correlations were found between δ 15 N values and both estimated trophic levels and fish size for C. chromis and B. boops, while significant negative correlations were found for E. encrasicolus.

Dietary niche widths and diet overlaps

Chromis chromis had the highest DNW and C. macrophthalma the lowest (Table 8).

Higher and significant dietary niche overlaps were found between S. pilchardus, S.

aurita and E. encrasicolus, while overlap was also observed between E. encrasicolus and C. macrophthalma.

The nMDS analysis based on SCA showed no ellipse overlaps between S. sprattus and the other species except for S. pilchardus. Sprattus sprattus differed the most from E. encrasicolus, S. aurita and C. macrophthalma (Fig. 6A). Sardina pilchardus, C. chromis and B. boops had an intermediate position between the two groups. The nMDS analysis based on isotopic compositions displayed more extensive ellipse overlaps among all fish species, with S. sprattus on one side, close to E. encrasicolus and S. aurita, while the other species appeared on the other side of the biplot (Fig. 6B).

With both SCA and SIA, S. sprattus and C. macrophthalma were the most distant species.

Discussion

This study characterized the diet of seven Bay of Marseille fish species with the complementary use of stomach content (SCA) and stable isotope (SIA) analyses, and also considered the size of prey consumed. Diet studies generally focus on composition of prey species, data on prey size tending to be rare, despite prey size being a defining factor in pelagic fish growth [START_REF] Thoral | Changes in foraging mode caused by a decline in prey size have major bioenergetic consequences for a small pelagic fish[END_REF]). For planktivorous species, inter-specific differences in diet and prey size depend on the gill raker density as well as the number of gill arches, whereas gape height determines the maximum size of prey ingested (Velip and Rivonker 2018). Moreover, prey energy content is generally closely linked to both species/groups composition and size (Harmelin-Vivien et al.

2019; Chen et al. 2019a

). We adopted this approach in the context of recently observed changes to small pelagic fish populations in the NWMS, assumed to be related to changes in the composition of zooplankton, fish diet and trophic interactions [START_REF] Brosset | Linking small pelagic dietary shifts with ecosystem changes in the Gulf of Lions[END_REF][START_REF] Saraux | Small pelagic fish dynamics: A review of mechanisms in the Gulf of Lions[END_REF]).

Prey group and prey size composition

Our study shows that diets differed from ambient prey availability, suggesting found a high proportion of salps, appendicularians, chaetognaths and polychaetes in S. aurita, but not in S. pilchardus. These dietary differences may suggest more opportunistic feeding in S. aurita, as observed in the Aegean Sea [START_REF] Lomiri | Size-related and temporal variation in the diet of the round sardinella, Sardinella aurita (Valenciennes, 1847), in the central Mediterranean Sea[END_REF]).

The prey size distribution of S. aurita observed in the Bay of Marseille was similar to that observed in the Ebro River Delta by Albo-Puigserver et al. (2019). Although S. aurita had a higher average prey size than the other Clupeidae, it presented the lowest PPSR due to its larger body size. The high consumption of Microsetella/Macrosetella spp. copepods by the four small sized pelagic fish (Engraulidae and Clupeidae) may be related to the high input of aggregates in the Bay of Marseille with which these copepods were associated (Chen et al. 2021 and references therein).

Among bentho-demersal species, C. macrophthalma mainly preyed on decapod larvae, calanoid (Acartiidae and Calanidae) copepods, and small proportions of gelatinous zooplankton with their associated Phronima spp. amphipods. Cepola macrophthalma is considered a benthic opportunist preying mainly on diverse zooplankton groups, but also ingesting some benthic organisms [START_REF] Stergiou | Abundance-depth relationship, condition factor and adaptive value of zooplanktophagy for red bandfish, Cepola macrophthalma[END_REF] sprattus, a corresponding increase with increasing fish size was observed only for B. boops in the present study, possibly reflecting the poorer feeding conditions in the Bay of Marseille compared to those of the Atlantic Sea.

Stable isotope composition of planktivorous fish species

Stable isotope analysis revealed that strictly pelagic planktivorous fish (i.e. E. encrasicolus, S. pilchardus, S. aurita, S. sprattus) demonstrated generally higher mean δ 13 C and lower mean δ 15 values than E. encrasicolus, and S. aurita presenting higher values than S. pilchardus, allowing some isotopic segregation of these four species in spite of high isotopic overlaps. The values reported were similar to those observed for these species in the Gulf of Lion in 2010-2014, but differed from those recorded in 2004-2005 [START_REF] Brosset | Linking small pelagic dietary shifts with ecosystem changes in the Gulf of Lions[END_REF]). This may be linked to changes in zooplankton community structure and/or in the organic matter composition at the base of the pelagic food webs. Sardina pilchardus also presented higher relative δ 15 N values than E. encrasicolus and S. aurita in our studies, corresponding to the observations in the nearby Western Mediterranean Sea. In our study, this observation can be related to the higher proportion of eggs in the diet of S. pilchardus, as isotopic compositions of fish eggs are generally similar to those of adult fish [START_REF] Chen | Diet of planktivorous fish species of the Marseille region (Northwest of the Western Mediterranean)[END_REF]). The higher δ 15 N values and trophic levels of B. boops and C. macrophthalma may be related to their consumption of crustacean larvae, chaetognaths and benthic organisms. The higher dispersion of δ 13 C and δ 15 N values observed in these two bentho-demersal species may result from a higher diversity of sources of organic matter at the base of the pelagic and the benthic food webs in the Bay of Marseille [START_REF] Cresson | Spatio-temporal variation of suspended and sedimentary organic matter quality in the Bay of Marseilles (NW Mediterranean) assessed by biochemical and isotopic analyses[END_REF]).

While no relationship was detected between fish size and diet composition, or between fish size and trophic niche breadth, changes in fish isotopic compositions occurred with size for some species. We observed a decrease of δ 13 C values with growth in S. pilchardus and of δ 15 N values in E. encrasicolus, while δ 15 N values of C. chromis and B. boops increased. These alterations may be linked to changes in diet or foraging zones, or to zooplankton community changes related to changes of primary producers, and thus of carbon food sources, as observed in the Gulf of Lion [START_REF] Costalago | Ontogenetic and seasonal changes in the feeding habits and trophic levels of two small pelagic fish species[END_REF]). The increase of δ 15 N values and of trophic niche breadth (i.e. PPSR) with growth of B. boops may be related to the higher ability of this species to capture larger prey when growing. In contrast to our observations, Bachiller et al. (2021) showed increasing δ 13 C values with size for E. encrasicolus and S. aurita and decreasing δ 15 N values with size for S. pilchardus. This may be related to differences in foraging patterns with growth, as larger prey and better body condition were observed in the fish populations from the southern areas of the Mediterranean Sea [START_REF] Bachiller | A trophic latitudinal gradient revealed in anchovy and sardine from the Western Mediterranean Sea using a multi-proxy approach[END_REF]).

This study highlighted differences and complementarity between SCA and SIA, as the two methods demonstrated contrasting results. We found a higher degree of dietary overlap with the SIA than with the SCA between S. pilchardus, S. aurita and S. sprattus.

However, even if the different species ingested different zooplankton groups, these prey may have similar isotopic composition due to similar POM sources at the base of the food web and similar trophic levels [START_REF] Bănaru | Seasonal variation of stable isotope ratios of size-fractionated zooplankton in the Bay of Marseille (NW Mediterranean Sea)[END_REF]). Le Bourg et al. (2015) demonstrated that in the last decade S. pilchardus and E. encrasicolus had similar diets to S. sprattus in the Gulf of Lion. However, E. encrasicolus appeared to be less in competition with the Clupeidae species in the Bay of Marseille. The results of SCA also suggested less competition among bentho-demersal fish, indicating specialization on different prey groups and sizes, despite the high similarities of their isotopic compositions. In addition, we found some species (i.e. S. aurita, C. macrophthalma, C. sprattus fed mainly on copepods and seemed to be less opportunist feeders. However, with sufficient food availability, dietary overlap does not necessarily imply competition for food resources [START_REF] Coe | Responses of Elk and Mule Deer to Cattle in Summer[END_REF]. Our data thus tend to show a higher potential for competition between the Engraulidae and Clupeidae species than between the other species studied. Data on the quality and quantity of zooplankton available as prey remain essential to confirm or disprove the hypothesis on the limitation of food resources.

Methodological issues and improvements

The main difficulty for multi-species fish diet studies is in simultaneously obtaining both plankton prey and fish samples. Here, monthly fish and zooplankton sampling was carried out several days and a few kilometers apart. However, as some of our target species consumed high proportions of large prey (> 2 mm), certain prey categories that the fish were feeding on locally may have been missed. Combining multiple nets to capture a wider range of prey size and categories would allow a more precise comparison of diet with available zooplankton ( It was not possible to collect monthly data for all our study species, as not all belong to targeted fisheries, necessitating time-aggregation of data. For example, a large sample of S. pilchardus was compared to small samples (< 30) of S. sprattus, C. macrophthalma, C. chromis, and B. boops. Planktivorous species diets may also vary monthly/seasonally (Chen et al. 2021) along with the isotopic compositions of the primary producers (i.e. baseline), as previously shown in the Bay of Marseille by [START_REF] Bănaru | Seasonal variation of stable isotope ratios of size-fractionated zooplankton in the Bay of Marseille (NW Mediterranean Sea)[END_REF]. For improved fisheries management, better temporal sampling would better determine whether the observed differences of fish isotopic compositions are truly related to dietary differences or to changes in the isotopic compositions of the primary food sources. Furthermore, global climate change, and associated changes in water convection, and urban and river inputs, could drastically alter the nutrient inputs at the base of the food web, thereby altering the availability of different plankton prey groups.

Nevertheless, our extensive survey provided the best possible results. Most previous studies in the Mediterranean Sea have focused on the feeding of E. encrasicolus, S. pilchardus and S. sprattus with samples obtained from oceanographic surveys in targeted areas and periods, and have faced similar difficulties in relating plankton availability to fish feeding [START_REF] Costalago | Ontogenetic and seasonal changes in the feeding habits and trophic levels of two small pelagic fish species[END_REF](Costalago et al. , 2014;;Le Bourg et al. 2015;[START_REF] Bachiller | A trophic latitudinal gradient revealed in anchovy and sardine from the Western Mediterranean Sea using a multi-proxy approach[END_REF]). In addition, as the commercial value of species tends to define research priorities, more scientific knowledge is available for E. encrasicolus and S.

pilchardus, but remains more limited for S. aurita, S. sprattus [START_REF] Palomera | Small pelagic fish in the NW Mediterranean Sea: An ecological review[END_REF] and the other species in this study.

Finally, better harmonization between fish and zooplankton sampling might include the simultaneous use of acoustic systems to detect fish species at given feeding depths (prior to capture) and of multiple net stratified-horizontal tows (such as the multiple plankton sampler Hydro-Bios) to target zooplankton at the same depths. In addition, as different prey types have different digestion rates, the rapid conservation of fish samples immediately following capture could also ensure better preservation of stomach contents. The use of DNA barcoding can also improve the prey identification, especially gelatinous prey [START_REF] Lamb | How quantitative is metabarcoding: A meta-analytical approach[END_REF][START_REF] Bachiller | Trophic niche overlap between round sardinella (Sardinella aurita) and sympatric pelagic fish species in the Western Mediterranean[END_REF]).

Conclusion

Similarities in the dietary and isotopic composition revealed by this study suggest higher potential trophic competition among strictly pelagic species under conditions of scarce food resources, while little competition was evident among bentho-demersal species.

Our results highlight that inter-specific similarities in isotopic composition does not necessarily imply similar diets. While SCA differed significantly between species, similar inter-specific isotopic compositions may result from similar POM sources at the base of the food web. Long-term monitoring of zooplankton communities and planktivorous fish diets is necessary to improve our understanding of the variability of species interactions in the context of climate change and to underpin ecosystem-based studies for fisheries management in the NWMS.

(

  Harmelin-Vivien et al. 2019; Chen et al. 2019a).

  et al. 2015; Chen et al. 2019a). A recent decline in planktivorous fish biomass, abundance, body size and relative body condition has been observed and may have led to changes in fish diet (STECF 2015; Ourgaud et al. 2015; GFCM 2019; Saraux et al. 2019; Cresson et al. 2019; Chen et al. 2021). In the changing environmental context of the Mediterranean

  al. 2014; Costalago et al. 2014; Brosset et al. 2016), and some of them also provided information on prey size structure (Le Bourg et al. 2015; Albo-Puigserver et al. 2019; Chen et al. 2021).

(

  Engraulidae), European sardine Sardina pilchardus (Walbaum, 1792) (Clupeidae), round sardinella Sardinella aurita (Valenciennes, 1847) (Clupeidae), European sprat Sprattus sprattus (Linnaeus, 1758) (Clupeidae), red bandfish Cepola macrophthalma (Linnaeus, 1758) (Cepolidae), damselfish Chromis chromis (Linnaeus, 1758) (Pomacentridae) and bogue Boops boops (Linnaeus, 1758) (Sparidae) in the Bay of Marseille, North-Western Mediterranean Sea. The study was performed from March to September 2017. These species represent a range in body sizes, shapes, and habitat occupations (entire water column in the case of pelagic Engraulidae and Clupeidae species, benthic and pelagic environments for the demersal B. boops and C. chromis, and burrows in soft-bottoms for C. macrophthalma (Stergiou and Karpouzi 2002)). Several previous studies have focused on diet, notably in commercially important E. encrasicolus and S. pilchardus, which demonstrated diet changes and reduced prey diversity (Le Bourg et al. 2015; Brosset et al. 2016). Using a multi-proxy approach, Bachiller et al. (2020, 2021) highlighted a trophic latitudinal gradient in E. encrasicolus and S. pilchardus in the Western Mediterranean Sea and a trophic niche overlap between S. pilchardus and S. aurita. Information on other pelago-demersal planktivorous species from the Gulf of Lion is lacking.

than 3 .

 3 5 according to the formula developed by Post et al. (2007):δ13 C normalised = δ 13 C sample -3.32 + 0.99 × C/N Individual and mean trophic level (TL) of fish species (TL fish ) were calculated using δ 15 N values according to the following formula (Vander Zanden and Rasmussen 2001): 𝑇𝐿 𝑡𝑒𝑙𝑒𝑜𝑠𝑡 = 𝜆 + (δ 15 𝑁 𝑡𝑒𝑙𝑒𝑜𝑠𝑡 -δ 15 𝑁 𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒 ) ÷ 3.4 where δ 15 N fish is the δ 15 N value of the consumer, λ the trophic level of the baseline and δ 15 N baseline the δ 15 N value of the baseline. The mean annual δ 15 N value of the 200-300 μm size class of zooplankton sampled in the framework of SOMLIT (see section 2.2.) (Chen et al. 2019a) was considered as the baseline for primary consumers (i.e. trophic position λ = 2) and a trophic fractionation factor of 3.4 was used for the computations (Post 2002).

  differential prey selection (Karachle and Stergiou 2013; Costalago and Palomera 2014; Chen et al. 2021). Dietary differences may be related to body shape, size, jaw morphology and habitat (Karachle and Stergiou 2013; Frédérich et al. 2014). In our study, differences in prey composition and diversity, prey size structure and dietary overlaps allowed the differentiation of two main groups, which also differed in mean size, morphology and foraging habitats. The first group included the strictly pelagic Clupeidae (S. pilchardus, S. aurita and S. sprattus), which preyed mostly on small sized copepods. The second group contained the bentho-demersal Cepolidae (C. macrophthalma), Pomacentridae (C. chromis) and Sparidae (B. boops), which consumed larger copepods, other zooplankton groups and varying proportions of benthic prey. The Engraulid E. encrasicolus was intermediate due to the wide spectrum of planktonic prey consumed. Engraulis encrasicolus, S. pilchardus and S. sprattus consumed mainly small and medium size copepods, such as harpacticoid (Microsetella/Macrosetella spp., Euterpina acutifrons) and cyclopoid (Oncaeidae, Corycaeidae) copepods, in accordance with previous observations from the literature (Costalago et al. 2014; Le Bourg et al. 2015; Albo-Puigserver et al. 2019; Bachiller et al. 2020, 2021). Sprattus sprattus, which had the highest relative proportion of copepods in its stomach contents, fed on a higher proportion of cyclopoid copepods than the other Clupeidae species, as reported by Le Bourg et al. (2015). Bachiller et al. (2020) also observed a latitudinal gradient of diet variation in E. encrasicolus and S. pilchardus along the NWMS coast. Our results seem to confirm poorer feeding conditions previously described for northern Mediterranean populations, with smaller prey consumed compared to the southern populations which consumed high proportions of decapods, malacostracans, and euphausiids (Bachiller et al. 2020). In terms of prey size, the preferred prey size range of E. encrasicolus, S. pilchardus and S. sprattus was wider in the Bay of Marseille (0.4 -0.9 mm) than in the Gulf of Lion (0.2 -0.5 mm) (Le Bourg et al. 2015), with larger prey (1.1 -1.5 mm) consumed in higher proportions. The larger plankton prey size in the Bay of Marseille was probably linked to the high production induced by terrestrial inputs (Fraysse et al. 2014). Mean PPSR values of these three species were higher in the Bay of Marseille (11.50 ± 1.94, 44.83 ± 15.76, and 11.09 ± 3.68, respectively) than in the Gulf of Lion (7.32 ± 6.57, 5.20 ± 1.86, and 6.21 ± 1.61, respectively) during the summer months, possibly due to temporal variation in prey size (Chen et al. 2021). It seems important therefore that PPSR comparisons should be undertaken contemporaneously. The diet of S. aurita was similar to that of S. pilchardus in the Bay of Marseille, but presented higher prey diversity and evenness. Bachiller et al. (2021) also observed more diet similarities between S. aurita and S. pilchardus than with E. encrasicolus.This was likely due to their similar filtering apparatus structure, with smaller gill raker spaces than in E. encrasicolus(Costalago and Palomera 2014).[START_REF] Bachiller | Trophic niche overlap between round sardinella (Sardinella aurita) and sympatric pelagic fish species in the Western Mediterranean[END_REF]

  . Similarly, C. chromis consumed both planktonic and benthic organisms (Mapstone and Wood 1975; Bell and Harmelin-Vivien 1983; Dulčić 2007; Pinnegar et al. 2007; Pinnegar 2018). We observed that C. chromis consumed high proportions of pteropods, copepods, decapod larvae, and amphipods. Pteropods are prey that contain high protein and lipid contents (Harmelin-Vivien et al. 2019), but were rare in the diet of other species in our study. The high proportion of pteropods in C. chromis diet may be explained by powerful jaws specialized in ram suction feeding like other phylogenetically close pomacentrid species (Frédérich et al. 2013, 2014; Chen et al. 2019b). Boops boops consumed a wide diversity of benthic prey in addition to zooplanktonic crustaceans, as previously observed in the Bay of Marseille (Bell and Harmelin-Vivien 1983; Cresson et al. 2014). This demersal species can be considered an opportunistic omnivore (Stergiou and Karpouzi 2002; El-Haweet et al. 2005). In the present study B. boops consumed mainly annelids, gelatinous zooplankton and pteropods. Despite the generally low energy content of small gelatinous zooplankton (Harmelin-Vivien et al. 2019; Chen et al. 2019a), this diet is reflective of its opportunistic predation behavior. Cepola macrophthalma, together with C. chromis and B. boops, but also S. aurita, consumed a high proportion of prey > 2 mm in size, related to both larger body, and thus a larger mouth size for C. macrophthalma and S. aurita, and to the powerful jaws for C. chromis (Frédérich et al. 2013, 2014; Chen et al. 2019b). While Bachiller and Irigoien (2013) observed an increase in trophic niche breadth based on PPSR in E. encrasicolus and S. pilchardus, and a decrease with growth in S.

  N values than bentho-demersal species (C. macrophthalma, B. boops), excluding C. chromis. This is consistent with previous observations in the Gulf of Lion (Le Bourg et al. 2015) and the Ebro River Delta (Albo-Puigserver et al. 2019). Similar mean δ 15 N values between E. encrasicolus and S. sprattus, and between S. pilchardus and S. aurita respectively, indicate close trophic level consistency between these species (Hussey et al. 2014). However, δ 13 C values differed significantly between species pairs, with S. sprattus presenting higher δ 13 C

  chromis and B. boops) to be highly opportunist, in agreement with the observations of other authors (El-Maremie and El-Mor 2015; Pinnegar 2018; Albo-Puigserver et al. 2019; Bachiller et al. 2020, 2021). By contrast, E. encrasicolus, S. pilchardus and S.

  Costalago and Palomera 2014; Albo-Puigserver et al. 2019; Bachiller et al. 2020).

Fig. 1 .

 1 Fig. 1. Planktivorous fish (black dots) and zooplankton sampling sites (Solemio, red star) in the Bay of Marseille, NW Mediterranean Sea. Huveaune river (bold line), Cortiou sewage treatment plant (black triangle).

Fig. 2 .

 2 Fig. 2. Size spectrum (0.1 mm size classes) of zooplankton sampled in the Bay of Marseille between March and September 2017 with an 80-μm mesh size net. Zooplankton (N = 2394) from 14 different samples were measured.

Fig. 3 .

 3 Fig. 3. Average index of relative importance (IRI%) of zooplankton in the stomach contents of each fish species compared to the composition of zooplankton sampled with the 80-μm mesh size net (potential prey) from March to September 2017 in the Bay of Marseille. For abbreviations of species: [EE] = E. encrasicolus, [SP] = S. pilchardus, [SA] = S. aurita, [SPSP] = S. sprattus, [CM] = C. macrophthalma, [CC] = C. chromis, [BB] = B. boops.

Fig. 4 .

 4 Fig. 4. Proportion of consumed prey by 0.1 mm size classes for the studied species (A -G) in the Bay of Marseille (March to September 2017). "Predator n" is the number of stomachs of fish analyzed. "Prey n" is the number of measured prey items in their stomach content.

Fig. 5 .

 5 Fig. 5. (A) Average prey size (± standard deviation) and (B) average predator-prey size ratio (PPSR, values multiplied by 1000) by species in the Bay of Marseille (March to September 2017). Superscript letters represent post-hoc groups and species with same letters are not significantly different (P > 0.05).

Fig. 6 .

 6 Fig. 6. Non-metric multi-dimensional scaling (nMDS) plot of the diet composition similarities (A) based on stomach content analysis (IRI%, Bray-Curtis) and (B) based on isotope compositions (δ 13 C and δ 15 N values, in ‰, Euclidean) of the studied planktivorous teleost species in the Bay of Marseille (March to September 2017).

Fig. S1 .

 S1 Fig. S1. Scatter plots illustrating Shannon-Wiener diversity index (H') of prey as functions of predator total body length (cm) for the studied species in the Bay of Marseille from March to September 2017. Each point represents the prey diversity in the stomach content of a single fish. Dotted quantile regression lines represent changes in the range of diversity as trophic niche breadth (TNB) with increasing predator size. Upper-(UBS) and lower-bound slopes (LBS) are quantile regressions estimating 90 th (upper) and 10 th (lower) quantiles of diversity vs. predator length comparisons (*** P < 0.0001, ** P < 0.001, * P < 0.01, ' P < 0.1, none = not significant). SE is the standard error of each quantile regression. Results from quantile regressions of diversity vs. predator length comparisons are also presented. CHANGE TNB represents decreases or increases in trophic niche breadth based on statistically significant differences between UBS and LBS (P > 0.05 and thus no change in TNB for all).

Fig. S2 .

 S2 Fig. S2. Scatter plots illustrating prey-predator size ratios (PPSR)*1000 as functions of predator total body length (cm) of the studied species in the Bay of Marseille from March to September 2017. Each point represents the PPSR*1000 in the stomach content of a single fish. Solid quantile regression lines represent significant changes in the range of the PPSR*1000 as trophic niche breadth (TNB) with increasing predator size while dotted quantile regression lines represent non-significant changes. Upper-(UBS) and lower-bound slopes (LBS) are quantile regressions estimating 90 th (upper) and 10 th (lower) quantiles of PPSR*1000 vs. predator length comparisons (*** P < 0.0001, ** P < 0.001, * P < 0.01, ' P < 0.1, none = not significant). SE is the standard error of each quantile regression. Results from quantile regressions of PPSR*1000 vs. predator length comparisons are also presented. CHANGE TNB represents decreases or increases in trophic niche breadth based on statistically significant differences between UBS and LBS (P > 0.05). There was no change in TNB for all species, except for B. boops (P = 0.004).

Table 1 .

 1 Number of samples of planktivorous fish for total length measurements (TL), stomach content analyses (SCA) and stable isotope analyses (SIA) by month.

		E. encrasicolus	S. pilchardus		S. aurita			S. sprattus
	Month	TL	SCA	SIA	TL	SCA	SIA	TL	SCA	SIA	TL	SCA	SIA
	Mar.	4	4	4	97	44	11	7	10	5	0	0	0
	Apr.	0	0	0	64	44	8	6	10	4	0	0	0
	May	4	4	4	45	44	6	6	7	5	7	7	4
	Jun.	0	0	0	28	28	8	26	8	9	0	0	0
	Jul.	31	15	9	21	21	7	36	12	10	0	0	0
	Aug.	0	0	0	35	20	8	30	17	8	0	0	0
	Sep.	0	0	4	44	23	9	30	13	9	8	8	4
	Total	39	23	21	334	224	57	141	77	50	15	15	8

Table 1 (

 1 continued). Number of samples of planktivorous fish for total length measurements (TL), stomach content analyses (SCA) and stable isotope analyses (SIA) by month.

		C. macrophthalma		C. chromis			B. boops
	Month	TL	SCA	SIA	TL	SCA	SIA	TL	SCA	SIA
	Mar.	0	0	0	0	0	0	33	11	11
	Apr.	0	0	0	0	0	0	6	2	0
	May	0	0	0	3	3	3	23	9	0
	Jun.	17	17	9	18	18	9	16	6	0
	Jul.	0	0	0	4	4	4	13	8	0
	Aug.	0	0	0	1	1	1	27	22	0
	Sep.	0	0	0	0	0	0	25	6	0
	Total	17	17	9	26	26	17	143	64	11

Table 2 .

 2 Details of biometric measurements (n), minimum (min), maximum (max) and mean (± SD, standard deviation) of total length (cm), proportion of empty stomachs, mean degree of stomach repletion (SRD) (0 = empty, 5 = full) (± SD) and stomach filling degree (SFD) (± SD) of the studied planktivorous fish species. Superscript letters represent post-hoc groups after Kruskal-Wallis tests comparing the total body length (χ 2 = 1059.90, P = 0.002). Species with the same letters have similar sizes (P > 0.05).

	Species	Family	n	Total length (cm)	Empty stomachs (%)	SRD	SFD
				min max mean SD
	E. encrasicolus	Engraulidae	39 10.5 12.7 11.6 b 0.6	6.3	2.1 ± 1.2 21.1 ± 10.0 bc
	S. pilchardus	Clupeidae	334 10.0 18.1 13.0 d 0.6	2.1	2.3 ± 1.3 13.0 ± 0.8 c
	S. aurita	Clupeidae	141 15.5 27.5 23.4 c 1.3	19.7	2.4 ± 1.3 30.2 ± 26.4 ab
	S. sprattus	Clupeidae	15 10.1 12.5 11.2 b 1.0	15.1	1.8 ± 1.1 50.4 ± 3.9 a
	C. macrophthalma	Cepolidae	17 21.2 43.9 29.3 c 6.8	18.6	3.0 ± 1.3 68.9 ± 17.5 a
	C. chromis	Pomacentridae 26	9.3 13.8 11.7 b 0.6	20.6	2.1 ± 1.1 68.6 ± 38.0 a
	B. boops	Sparidae	143 15.1 35.0 19.9 a 1.8	13.6	2.2 ± 0.9 22.6 ± 6.9 bc

Table 3 .

 3 Results of SIMPER analysis of prey groups that contributed to the dissimilarities (based on Bray-Curtis distances) of the diet of the studied planktivorous teleost species in the Bay of Marseille. Contrib. = Contribution, Cumulative % = cumulative contribution.

	Prey groups	Average	Contrib. %	Cumulative %	E.	S.	S.	S.	C.	C.	B.
		dissimilarity			encrasicolus	pilchardus	aurita	sprattus	macrophthalma	chromis	boops
	Calanoid copepods	17.64	28.25	28.25	61.40	34.70	48.80	0.00	81.80	18.90	13.90
	Cyclopoid copepods	12.05	19.30	47.55	30.80	25.10	9.36	60.70	0.10	17.80	3.09
	Harpacticoid copepods	8.67	13.89	61.44	1.74	26.20	31.90	31.20	0.24	0.10	1.54
	Others	5.69	9.12	70.56	0.00	0.33	0.00	0.00	0.00	0.00	39.60
	Gastropods	4.84	7.75	78.31	1.74	0.00	0.01	0.00	0.00	32.70	0.19
	Decapod larvae	4.27	6.83	85.14	3.44	0.08	2.01	0.00	16.20	3.49	18.60
	Amphipods	3.03	4.86	90.00	0.00	0.02	0.09	0.00	1.60	11.30	13.10
	Cypris larvae	2.08	3.34	93.33	0.01	0.33	0.00	0.00	0.00	10.50	5.97
	Teleost eggs	2.08	3.32	96.66	0.03	12.00	1.46	2.66	0.00	2.45	3.24
	Ostracods	0.97	1.55	98.21	0.00	0.00	5.71	0.00	0.00	1.60	0.03
	Other crustaceans	0.65	1.04	99.25	0.00	0.01	0.30	3.59	0.00	1.16	0.55
	Cladocerans	0.41	0.66	99.90	0.78	1.12	0.23	1.85	0.00	0.00	0.00
	Bivalve larvae	0.03	0.05	99.96	0.10	0.00	0.01	0.00	0.00	0.00	0.17
	Copepod nauplii	0.03	0.04	100.00	0.00	0.14	0.06	0.00	0.00	0.00	0.00

Table 4 .

 4 Bray-Curtis' dissimilarity indices for the pairwise comparisons of the composition of stomach content between the studied planktivorous fish species. The most significant (> 0.6) values of the index are in bold.

	Species	E. encrasicolus	S. pilchardus	S. aurita	S. sprattus	C. macrophthalma	C. chromis
	S. pilchardus	0.61
	S. aurita	0.59	0.28
	S. sprattus	0.50	0.42	0.37
	C. macrophthalma	0.76	0.78	0.77	0.83
	C. chromis	0.68	0.79	0.76	0.83	0.54
	B. boops	0.72	0.67	0.62	0.74	0.63	0.61

Table 5 .

 5 Shannon's diversity (H') and Pielou's evenness index (J) calculated according to %IRI of the diet of the studied planktivorous fish species and for the zooplankton sampled with the 80 µm mesh size net in the Bay of Marseille (March to September 2017). Superscript letters represent post-hoc groups after Kruskal-Wallis tests. Values with the same post-hoc letters are not significantly different

	(P > 0.05).
	Species	H'	H'	J	J
		Stomach	Zooplankton	Stomach	Zooplankton
		content		content
	E. encrasicolus	0.69 ± 0.05 bc	0.85 ± 0.32	0.61 ± 0.15 ab	0.35 ± 0.12
	S. pilchardus	0.61 ± 0.08 bc	1.00 ± 0.25	0.63 ± 0.11 bc	0.41 ± 0.10
	S. aurita	0.69 ± 0.32 bc	1.00 ± 0.27	0.61 ± 0.22 ab	0.41 ± 0.11
	S. sprattus	0.95 ± 0.08 c	1.83 ± 0.24	0.67 ± 0.07 abc	0.60 ± 0.07
	C. macrophthalma	0.29 ± 0.11 a	1.25 ± 0.08	0.32 ± 0.14 a	0.49 ± 0.05
	C. chromis	0.71 ± 0.33 c	1.08 ± 0.26	0.76 ± 0.11 c	0.44 ± 0.10
	B. boops	0.47 ± 0.29 ab	1.00 ± 0.25	0.48 ± 0.21 ab	0.41 ± 0.10

Table 6 .

 6 Bray-Curtis' dissimilarity indices for the pairwise comparisons of the prey size distribution between the studied planktivorous fish species. The highest significant (> 0.6) values of the index are in bold.

	Species	E. encrasicolus	S. pilchardus	S. aurita	S. sprattus	C. macrophthalma	C. chromis
	S. pilchardus	0.49
	S. aurita	0.34	0.25
	S. sprattus	0.44	0.20	0.26
	C. macrophthalma	0.44	0.53	0.35	0.53
	C. chromis	0.48	0.39	0.27	0.37	0.31
	B. boops	0.63	0.61	0.48	0.62	0.54	0.39

Table S2 - 1 .

 S21 Mean stomach content composition of the studied planktivorous fish species in the Bay of Marseille from March to September 2017. O% = percentage of prey occurrence, N% = percentage of prey numerical abundance, W% = percentage of prey weight. IRI% = percentage of relative importance index. The highest percentages by species are in bold. O% N% W% IRI% O% N% W% IRI% O% N% W% IRI% O% N% W% IRI% Harpacticoid copepods 0.3 3.0 3.0 1.7 0.6 26.2 23.4 26.2 0.7 32.3 31.2 31.9 0.5 30.3 26.4 34.7 Cyclopoid copepods 0.8 28.2 29.4 30.8 0.6 22.8 24.1 25.1 0.3 10.4 10.5 9.4 1.0 15.8 19.1 42.9

	Prey category	E. encrasicolus	S. pilchardus	S. aurita	S. sprattus
	Calanoid copepods	0.9 57.7 56.5 61.4 0.6 32.5 33.4 34.7 0.7 42.7 43.6 48.8 0.2 49.1 49.

Acknowledgements

Thanks are due to the fishermen: Hubert Baty, Patrick Fernandez and Mr. Esposito for their interest in our study and their willingness to cooperate and provide fish samples; to the crew of the R.V. Antedon II and colleagues at the Mediterranean Institute of Oceanography (M.I.O.): Dorian Guillemain, Michel Lafont from the Service Atmosphère et Mer (S.A.M.) and to Guillermo Feliu and Lucas Lhomond for their help during sampling; to Patrick Raimbault, Christian Grenz and Loïc Guilloux in charge of the SOMLIT long-term survey in Marseille; to Célia Losson, Tom Robert, Emeline Bard, Nour Lahat, Manon Desplechin, Manon Gaultier and Guillaume Lassus for their help during dissections; to Valentina Tirelli and Yves Letourneur for their valuable advice on the manuscript of the PhD thesis of C.-T. Chen and on this paper, and to Michael Paul for improvement of the English. The authors gratefully acknowledge the Microscopy and Imaging facilities of the Mediterranean Institute of Oceanography (Aix-Marseille University) and the staff. Thanks are also due to the French Ministry of Higher Education, Research and Innovation for providing the PhD funding for C.-T. Chen. D. Bănaru received funding within the ANR CONTAMPUMP project (Nº ANR-19-CE34-0001). The project leading to this publication received funding from the European FEDER Fund under project 1166-39417 and the project MERITE (Marine Ecosystem Response to the Input of contaminants in the coasTal zonE) which was funded by MERMEX MISTRAL CNRS. Thanks are expressed to the two anonymous reviewers and to the Associate Editor Dr Ewan Hunter for their helpful comments for improving our manuscript.

Declarations

Conflict of interest

The authors declare that they have no conflicts of interest.

Ethical approval

All applicable national and/or institutional guidelines for sampling, care, and experimental use of organisms for the study were followed.

Table 7. Mean (± standard deviation) δ 13 C and δ 15 N values and trophic levels (TL) of the planktivorous fish species in the Bay of Marseille. Spearman's rho and associated P-value testing the rank correlation between the total body length and δ 13