domain: sde shs

text:
Frédéric Bertucci
email: fred.bertucci@gmail.com

William E Feeney

Zara-Louise Cowan

Camille Gache

Rakamaly Madi Moussa

Cécile Berthe

Lana Minier

Tamatoa Bambridge

David Lecchini

Effects of COVID-19 lockdown on the observed density of coral reef fish along coastal habitats of Moorea, French Polynesia

Keywords: Anthropause, Coral reef fish, COVID-19, Coastline habitat, Human activities, Recreational activities

d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Background

Coastal habitats and environments associated with coral reefs support a wide range of essential socio-economic (e.g. food production, tourism, biotechnology) and biophysical (e.g. fish nursery area, coastal protection) processes (Lecchini et al. 2013;[START_REF] Aouiche | Anthropogenic effects on shoreface and shoreline changes: input from a multi-method analysis, Agadir Bay, Morocco[END_REF][START_REF] Moussa | Long-term coastline monitoring on a coral reef island (Moorea, French Polynesia)[END_REF]) that provide direct benefits to at least 500 million people living in tropical coastal regions worldwide [START_REF] Wilkinson | Coastal resource degradation in the tropics: does the tragedy of the commons apply for coral reefs, mangrove forests and seagrass beds[END_REF][START_REF] Costanza | Changes in the global value of ecosystem services[END_REF][START_REF] Hoegh-Guldberg | Reviving the ocean economy: the case for action -2015[END_REF]. While it is clear that the constant presence of humans affects coastal environments [START_REF] Hughes | Global warming and recurrent mass bleaching of corals[END_REF][START_REF] Moritz | Status and trends of coral reefs of the Pacific[END_REF][START_REF] Moritz | Status and trends of coral reefs of the Pacific[END_REF], the COVID-19 pandemic and associated government-mandated lockdowns that occurred in 2020 created a window during which the degree of human impact could be isolated and quantified [START_REF] Corlett | Impacts of the coronavirus pandemic on biodiversity conservation[END_REF][START_REF] Rice | Changes in recreational behaviors of outdoor enthusiasts during the COVID-19 pandemic: analysis across urban and rural communities[END_REF][START_REF] Sandford | Coronavirus: half of humanity now on lockdown as 90 countries call for confinement[END_REF]. Many studies have noted that the reduction of human activities quickly leads to positive effects on the environment and on wild organisms (e.g. [START_REF] Arora | Coronavirus lockdown helped the environment to bounce back[END_REF][START_REF] Rosenbloom | A COVID-19 recovery for climate[END_REF][START_REF] Zambrano-Monserrate | Indirect effects of COVID-19 on the environment[END_REF][START_REF] Bertucci | Changes to an urban marina soundscape associated with COVID-19 lockdown in Guadeloupe[END_REF][START_REF] Kumar | COVID-19 driven changes in the air quality; a study of major cities in the Indian state of Uttar Pradesh[END_REF]. Thus, lockdowns initially appeared to relieve marine ecosystems of anthropogenic pressure [START_REF] Bates | Global COVID-19 lockdown highlights humans as both threats and custodians of the environment[END_REF]; however, the cessation of tourism led to a greater dependence 1 3

16

Page 2 of 11 of some populations on natural resources [START_REF] Buckley | Conservation implications of COVID19: effects via tourism and extractive industries[END_REF], and COVID-19-related restrictions have negatively impacted small-scale fisheries (Bennet et al. 2020;[START_REF] Campbell | Immediate impact of COVID-19 across tropical smallscale fishing communities[END_REF].

The first COVID-19 infection was identified in French Polynesia on March 10, 2020, and the World Health Organization declared COVID-19 a pandemic on March 11, 2020. The country immediately started to prohibit international boats from stopping or anchoring in French Polynesian waters. On March 18, inter-island flights were strictly limited to travels that could not be postponed (with presentation of an exemption form) and international flights were cancelled on March 19. Between March 11 and March 19, approximately 3000 tourists left French Polynesia. The country instituted a total lockdown from March 20, during which public movements were restricted to short trips of up to 1 h per day and within a maximum radius of 2 km of households. Only a few grocery stores remained open for basic purchases (www. polyn esie-franc aise. pref. gouv. fr/ Dossi ers/ Infor mation-COVID-19/ Textes-offic iels/ 2020/ Textes-offic iels). As a result, the majority of maritime and tourism activities ceased for 6 weeks (until May 4, 2020). Many residents lost their jobs (permanently or temporarily) and turned to subsistence fishing along the coastline to feed themselves and their families, with spear or line fishing rather than the usual boat fishing, due to movement limitations (TB and DL personal observations). This differential pressure on local resources to meet food and subsistence needs along the coast, as well as the decrease in human presence at sea, is therefore likely to have affected fish communities [START_REF] Hoffman | The curse of the Caribbean[END_REF]. In a recent study, [START_REF] Lecchini | Effects of COVID-19 pandemic restrictions on coral reef fishes at eco-tourism sites in Bora-Bora, French Polynesia[END_REF] showed that fish density at ecotourism sites in Bora-Bora (French Polynesia) increased significantly from March to May 2020 in the absence of humans and returned to prelockdown levels in August 2020 (12 weeks after the lockdown), after most human activities and tourism resumed. Another study performed on the inner and outer barrier reefs inside and outside marine protected areas of Moorea [START_REF] Feeney | COVID-19 lockdown highlights impact of recreational activities on the behaviour of coral reef fishes[END_REF] compared fish densities between 2011 and 2020, with densities observed in May and July 2020. The study recorded an increase in post-lockdown densities compared to this long-term average on sites where human activity decreased (inner barrier reefs) but not in areas where human activity was already lower (outer barrier reefs). These results likely suggest that fish have moved into less frequented areas.

The present study aims to provide new information on the effects of the COVID-19 lockdown on observed coral reef fish density along coastal habitats in Moorea, French Polynesia. Specifically, we intend to identify whether the reduction in local fishing and boating activities in the Moorea lagoon caused changes in fish densities after the 2020 lockdown. We hypothesize that fish densities along the coastline should have increased during the lockdown period, but that this benefit should have been short-lived and thus densities would return to pre-lockdown levels when human activities resumed.

Methods

Fish surveys

Fish density was assessed in two natural coastal marine habitats (bare sand and mangrove) and near a human-engineered shore structure (embankment) made of rocks and concrete on the island of Moorea (Madi [START_REF] Moussa | Long-term coastline monitoring on a coral reef island (Moorea, French Polynesia)[END_REF]) (Fig. 1). In each coastal habitat, two sites were selected, and visual surveys were conducted using SCUBA along three transects per site (30 m long and 2 m wide = 60 m 2 ) extending 30 m perpendicular to the shoreline [START_REF] Lecchini | How does shoreline development impact the recruitment patterns of coral reef fish juveniles (Moorea Island, French Polynesia)?[END_REF]. Along each transect, all observable fishes were counted and identified to the species level along with their ontogenetic stage (adult vs. juveniles). The distinction between juvenile and adult stages was based on size, colour, pattern and behaviour [START_REF] Lecchini | Spatial repartition and ontogenetic shifts in habitat use by coral reef fishes (Moorea, French Polynesia)[END_REF]. Count data were standardized to densities by dividing the total number of fish observed by the area covered by the 6 transects (360 m 2 ). Surveys took place between 10:00 and 16:00 both in 2019 and 2020. In 2019, visual surveys were conducted in February, March, April, July, September and November, and densities were averaged across months in order to obtain baseline fish density data for 2019 for each coastal habitat type (see Online Resource 1 for inter-annual variability). In 2020, no surveys could be conducted prior to lockdown and the first fish density surveys were conducted immediately after lockdown (first week of May 2020, in order to best capture the environment following six weeks of lockdown) and then 1 and 2 months after lockdown (June and July 2020, respectively). By the July sampling period, international flights and associated tourism activities had resumed. The same survey protocol was used for the 2019 and 2020 surveys. Although tidal fluctuations in Moorea are minimal (less than 0.4 m), all surveys were conducted at high tide for consistency. Water depth along all transects varied between 0.5 and 1 m in all coastal habitats.

Statistical analysis

All statistical analyses were conducted using R-Studio version 1.3.1093 (R version 4.0.3) (R Core Team 2020; R Studio Team 2020). To assess whether juvenile, adult and harvested fish densities changed after the period of reduced human presence due to the lockdown, generalized linear mixed effects models were run using the lme4 package [START_REF] Bambridge | Communs et océans: Le rahui en Polynésie. Au vent des îles[END_REF]. Full models included fish density as the dependent variable; sampling periods, i.e. 2019 average, end of lockdown, 1 month and 2 months after the end of lockdown, as the categorical fixed effect; and coastline habitats, i.e. bare sand, mangrove and embankment as a random effect with transect nested within habitat. Density did not follow a Normal distribution (Shapiro-Wilk tests, W = 0.50-0.60, all P < 0.001). The probability distribution that best fit the data was determined graphically using the packages car and MASS and the function "qqp" (see Online Resource 2). Three separate models were then fit using a log-normal distribution, each with the same covariates described above, for juveniles (including all species), adults (including all species) and harvested fish at the adult stage (i.e. species targeted by recreational, subsistence and commercial fishers). A significant effect of the fixed effect, i.e. sampling period, was assessed at the significance level α = 0.05.

In order to take into account inter-annual variations, the fish densities recorded in April 2019 were compared to the densities of early May 2020, i.e. at the end of the lockdown (Table 1), and densities recorded in July 2019 were compared to the densities of July 2020, i.e. 2 months after the end of the lockdown (Table 2). Comparisons were made for each type of habitat (embankment, mangrove and bare sand) by measuring the effect sizes calculated as the change in the log ratio of the 2020 densities of fishes recorded at the end of the lockdown and 2 months after it in the different habitats relative to the densities recorded in April and July 2019 respectively. The same approach was taken at the family level by considering the eight families showing the highest densities of individuals (juvenile and adult stages), i.e. Acanthuridae, Chaetodontidae, Labridae, Lutjanidae, Mugilidae, Mullidae, Pomacentridae and Scaridae. Differences were considered significant when the 95% confidence interval (95% CI) of the effect size was different from zero.

Results

A significant effect of the sampling period was found on juvenile density (Table 3) with significantly higher densities at the end of the lockdown, 1 month and 2 months after the end of lockdown compared to the average density of 2019 (Table 3). A similar significant effect of the sampling period was also found on adult density (Table 4) with significantly higher densities at the end of the lockdown, 1 month and 2 months after the end of lockdown than in 2019 (Table 4). Again, a significant effect of the sampling period was found on harvested fish density (Table 5) with significantly higher densities observed at the end of the lockdown, 1 month and 2 months after the end of lockdown compared to the 2019 average density (Table 5).

The overall density of juveniles at the end of the lockdown (May 2020) did not change compared to the same period in 2019 (0.09 fish per m 2 in April 2019 vs. 0.09 fish per m 2 in May 2020). Despite that, densities were 25% higher near embankments (0.08 fish per m 2 in April 2019 vs. 0.10 fish per m 2 in May 2020), or decreased by 50% in mangroves (0.06 fish per m 2 in April 2019 vs. 0.03 fish per m 2 in May 2020) and 25% on bare sand (0.12 fish per m 2 in April 2019 vs. 0.09 fish per m 2 in May 2020), 95% CI of the effect sizes did not differ from 0 for the three habitats (Fig. 2). In adults, the overall density increased by 50% (0.02 fish per m 2 in April 2019 vs. 0.03 fish per m 2 in May 2020) and was significantly higher near embankments and on bare sand (Fig. 2). Despite a 33% increase (0.03 fish per m 2 in April 2019 vs. 0.04 fish per m 2 in May 2020), the 95% CI of the effect size did not differ from 0 in mangroves. Overall harvested species density increased by 25% compared to 2019 (0.036 fish per m 2 in April 2019 vs. 0.045 fish per m 2 in May 2020). In mangroves, it increased by 197% (0.06 fish per m 2 in April 2019 vs. 0.19 fish per m 2 in May 2020), but the 95% CI of the effect size did not differ from 0. Densities significantly increased by 520% near embankments (0.005 fish per m 2 in April 2019 vs. 0.03 fish per m 2 in May 2020) and 1325% on bare sand (0.004 fish per m 2 in April 2019 vs. 0.057 fish per m 2 in May 2020) (Fig. 2). The densities recorded 2 months after the end of the lockdown (July 2020) were significantly higher in juveniles for all types of habitats compared to densities recorded in July 2019. Overall density increased by 257% for juveniles (0.016 fish per m 2 in July 2019 vs. 0.059 fish per m 2 in July 2020), ranging from a 119% increase on bare sand (0.036 fish per m 2 in July 2019 vs. 0.079 fish per m 2 in July 2020) to a 345% increase in mangroves (0.011 fish per m 2 in July 2019 vs. 0.049 fish per m 2 in July 2020) and a 557% increase on embankments (0.007 fish per m 2 in July 2019 vs. 0.046 fish per m 2 in July 2020). In adults, the overall density increased by 24% in comparison with the densities recorded in July 2019 (0.015 fish per m 2 in July 2019 vs. 0.019 fish per m 2 in July 2020), with relative variation ranging from 18% on bare sand to 28% in mangroves and 168% near embankments. Differences were significant only for bare sand and embankments (Fig. 2). Similar results were found in harvested species (Fig. 2), with a 61% increase in the overall density (0.016 fish per m 2 in July 2019 vs. 0.026 fish per m 2 in July 2020). Despite a 62% increase in mangroves (0.03 fish per m 2 in July 2019 vs. 0.049 fish per m 2 in July 2020), however, the 95% CI of the effect size did not differ from 0. Differences were significant near embankments with a 437% increase (0.004 fish per m 2 in July 2019 vs. 0.022 fish per m 2 in July 2020) and also on bare sand with a 537% increase (0.004 fish per m 2 in July 2019 vs. 0.026 fish per m 2 in July 2020) (Fig. 2).

The observed increases did not apply equally across the sampled Families. At the end of the lockdown (May 2020), the densities of Acanthuridae (harvested species), Lutjanidae (harvested species), Mugilidae (harvested species), Mullidae (harvested species), Pomacentridae (nonharvested species) and Scaridae (harvested species) were not significantly different from those observed in the same period in 2019 (all 95% CI of the effect size did not differ from zero). However, in Chaetodontidae (non-harvested species), densities were significantly higher near embankments (95% CI: -0.80 to -0.15) and significantly lower in mangroves (95% CI: 0.14-3.20) in May 2020 compared to April 2019. Labridae (non-harvested species) were also significantly more abundant in May 2020 near embankments (95% CI: -1.83 to -0.74) and on bare sand (95% CI: -2.57 to -0.45) compared to April 2019 (Fig. 3). Two months after the end of the lockdown (July 2020), all families but Lutjanidae and Mugilidae (all 95% CI of the effect sizedid not differ from zero) showed significantly higher densities in at least one habitat compared to July 2019. Near embankments, Acanthuridae (95% CI: -2.57 to -1.99), Labridae (95% CI: -3.50 to -2.52), Pomacentridae (95% CI: -2.26 to -1.82) and Scaridae (95% CI: -3.04 to -1.31) were more abundant in 2020 than in 2019. In mangroves, only Mullidae (95% CI: -7.04 to -1.04) and Pomacentridae (95% CI: -3.17 to -0.63) showed higher densities in July 2020 compared to July 2019. On bare sand, Chaetodontidae (95% CI: -2.99 to -0.67), Labridae (95% CI: -2.95 to -1.09) and Scaridae (95% CI: -6.64 to -0.23) appeared significantly more abundant in July 2020 compared to July 2019 (Fig. 3).

Discussion

Coastal surveys conducted on the island of Moorea in May 2020 revealed significantly higher densities of adults and harvested species of fish near embankments and bare sand sites immediately after a 6-week lockdown, compared to densities observed at a similar time in 2019. Due to the sudden nature of the lockdown and the unpredictable course of the coronavirus pandemic, it was not possible to plan any survey in early 2020. Thus, the higher density of fish observed in May might be the result of comparatively higher recruitment in early 2020 [START_REF] Lecchini | Spatial repartition and ontogenetic shifts in habitat use by coral reef fishes (Moorea, French Polynesia)[END_REF]. However, the lack of significant differences in juveniles at the end of the lockdown compared to 2019 suggests that this explanation is unlikely. In addition, the period from May to August usually corresponds to the period of lowest sea surface temperatures in French Polynesia, when fishes are generally less abundant and larval recruitment is lower [START_REF] Galzin | Structure of fish communities of French Polynesian coral reefs[END_REF][START_REF] Lo-Yat | Extreme climatic events reduce ocean productivity and larval supply in a tropical reef ecosystem[END_REF]. Therefore, the higher apparent density of fish observed in May, June and July 2020 compared to the overall 2019 average could be related to behavioural changes due to the lockdown period (similar to [START_REF] Feeney | COVID-19 lockdown highlights impact of recreational activities on the behaviour of coral reef fishes[END_REF]).

Moorea's coastal sites are generally subject to high human pressure as they are frequented by local residents and international tourists. Therefore, the decrease in activity resulting from the lockdown was likely to affect the density of fish observed by affecting the behaviour of individuals and causing more fish to venture to the study sites or by or Chlorurus sordidus (Scaridae), which is fisheries target species, were only observed in May and July of 2020. These species may be more negatively affected than others by normal human activities and may therefore have become more visible when this pressure was relieved. This hypothesis of a change in behaviour further highlights a known potential bias in the effectiveness of traditional visual monitoring of fish populations by SCUBA divers in areas frequented by humans, where fish have already been shown to remain at a greater distance and hide from divers [START_REF] Kulbicki | How the acquired behaviour of commercial reef fishes may influence the results obtained from visual censuses[END_REF][START_REF] Lindfield | Silent fish surveys: bubble-free diving highlights inaccuracies associated with SCUBA-based surveys in heavily fished areas[END_REF][START_REF] Gray | Comparison of reef fish survey data gathered by open and closed circuit SCUBA divers reveals differences in areas with higher fishing pressure[END_REF]. This suggests that complementary and less invasive methods, such as environmental DNA or video monitoring, might also be useful in future surveys in order to provide more precise evaluation of biodiversity. Additionally, current surveys often consider only diurnal communities while populations are known to display marked biological rhythms [START_REF] Galzin | Structure of fish communities of French Polynesian coral reefs[END_REF][START_REF] Galzin | The fish communities of a coral reef transect[END_REF]. To investigate whether a diminution in human activities, which occur mainly during daytime, would also benefit nocturnal organisms, the use of passive acoustic monitoring would appear as a particularly suited alternative to investigate potential impacts on communities without further disturbing marine organisms [START_REF] Bertucci | Local sonic activity reveals potential partitioning in a coral reef fish community[END_REF][START_REF] Bertucci | Changes to an urban marina soundscape associated with COVID-19 lockdown in Guadeloupe[END_REF]. Before the lockdown, nautical activities were mainly present near the bare sand sites, with three family guesthouses (ca. 15 rooms in total and provision of kayaks for tourists), a campsite (ca. 50 people) and a diving club (two sea trips per day and open all year round). Likewise, the mangroves were mainly used as fishing areas from kayaks and pirogues by local fishermen, and embankments were used as launching sites (boats to bring surfers close to the ocean waves) and for fishing on foot or with rods by local fishermen (TB and DL personal observations). In contrast to [START_REF] Hoffman | The curse of the Caribbean[END_REF], the present ecological surveys showed that the density of adults and harvested species was significantly greater at the end of the lockdown and 2 months after for juveniles, adults and harvested species. Unfortunately, there are no data or indicators to quantify the change in human use of the areas in question nor is there a control site such as a no-entry marine reserve that would have been less or not at all impacted by the lockdown. However, despite the inability to statistically compare the relationship between fish density and nautical/fishing activities on the three coastline habitats, we can postulate that any change in the fish community would mainly depend on the degree of human activity existing at the location ordinarily and the intensity of the change after the lockdown, as this has been suggested by other recent studies (e.g. [START_REF] Edward | COVID-19 lockdown improved the health of coastal environment and enhanced the population of reef-fish[END_REF][START_REF] Lecchini | Effects of COVID-19 pandemic restrictions on coral reef fishes at eco-tourism sites in Bora-Bora, French Polynesia[END_REF][START_REF] Feeney | COVID-19 lockdown highlights impact of recreational activities on the behaviour of coral reef fishes[END_REF]. Furthermore, at Moorea, we are not aware of any particular variations or events (potentially related to weather, cyclone, coral bleaching or environmental conditions) that may have occurred during the same period that could explain the observed changes. [START_REF] Zambrano-Monserrate | Indirect effects of COVID-19 on the environment[END_REF] showed that some internationally known beaches, which welcome millions of tourists every year (e.g. Acapulco in Mexico, Barcelona in Spain and Salinas in Ecuador) looked cleaner and had crystal clear waters during the early 2020 lockdown. This resembles our results, where the impact of reduced human activity appears strong in areas where human presence is usually high. At Moorea, the main change was due to the absence of human activities along the coastline during the lockdown. For instance, the lockdown seems to have had a strong impact on adult and harvested fish densities on bare sand sites which are frequently used for nautical activities and by many locals and tourists. In contrast, even though mangrove habitats are good fishing grounds, few local people actually use the sites studied and no significant effect of the reduced human activity could be detected (April 2019 vs. May 2020) for juveniles, adults or harvested species at the end of the lockdown. Additionally, the existence of a greater number of refuges potentially used by fish when humans are present would support the hypothesis of a change in the behaviour of individuals venturing outside their shelters [START_REF] Gotanda | Body size and reserve protection affect flight initiation distance in parrotfishes[END_REF][START_REF] Madin | Field evidence for pervasive indirect effects of fishing on prey foraging behavior[END_REF] After the lockdown, schools remained closed, there were many public holidays, and economic activities (especially in hotels) remained low. However, by mid-June, schools Fig. 2 Effect sizes ± 95% confidence interval measured on bare sand, mangroves and embankments, in juveniles, adults and harvested fish, for densities at the end of the lockdown (May 2020) and two months after the lockdown (July 2020). Effect sizes are expressed as the change in the log ratio of the 2020 densities in the different habitats relative to their levels in April and July 2019 respectively. Higher negative values of effect size indicate higher densities in 2020 and are considered significant when the 95% confidence interval of the effect does not overlap zero reopened, there were no more holidays, and more economic sectors resumed their activities (www. polyn esie-franc aise. pref. gouv. fr/ conte nt/ downl oad/ 45236/ 273700/ file/). Correspondingly, fish density along the Moorea coastline appeared to be lower at the end of June. These results may indicate that although there were apparently more fish in the coastal habitats at the end of the lockdown, the resumption of local activity with the return of international tourists may have led to a decrease in the observed fish density. However, when comparing densities observed 2 months after the end of the lockdown, fish were still more abundant than in 2019 at the same time. This was also the case for juveniles in all habitats including mangroves, where an increase in density was observed in all families in at least one habitat type, including the parrotfishes (Scaridae) which comprise many species usually targeted by fishermen. Increased damselfish (Pomacentridae) density was also observed in mangroves and near embankments, which generally support substantial levels of boating and fishing activities and hence high noise levels. Species in this family have been shown to be particularly sensitive to noise pollution with negative effects on larval survival, altered response to predators, reduced faststart kinetics, altered swimming behaviour and increased aggressive behaviour [START_REF] Jain-Schlaepfer | Impact of motorboats on fish embryos depends on engine type[END_REF][START_REF] Mccormick | Effects of boat noise on fish fast-start escape response depend on engine type[END_REF][START_REF] Mills | Hormonal and behavioural effects of motorboat noise on wild coral reef fish[END_REF]). This again illustrates the possible benefit of reduced human activity along the coastline and may inspire long-term management plans in which access to certain areas by tourists or fishermen is reduced and/or allowed only at certain times of the year in order to allow species, particularly those of commercial value, to reproduce and also in order to promote biodiversity. Such achievement could be realized within the framework of the United Nations Sustainable Development Goals (SDGs).

Tourism is an important source of income and employment in many developing and island countries and can hardly be banned. Developing sustainable tourism, i.e. that considers economic, social and environmental impacts, that meets the needs of visitors and professionals, would align with SDG 8 "Decent work and economic growth" which intends to "promote sustained, inclusive and sustainable economic growth, employment and decent work for all". Monitoring potential impacts of such activities on sustainable development is also central in the SDG 12 "Responsible consumption and production". The present results finally support that these efforts will benefit biodiversity and promote the conservation and sustainable use of the oceans, seas and marine resources as encouraged by the SDG14 "Life below water". The COVID-19 pandemic has raised awareness of our dependence on a functional food system [START_REF] Gordon | The COVID-19 pandemic stress the need to build resilient production ecosystems[END_REF]. To understand the effects of future similar pandemics, it is essential to acquire data on the impact of alternating breaks and recoveries of human activities on the resilience of fish, humans and their interactions. Several studies have suggested that a rapid reduction in anthropogenic stressors can lead to behavioural changes in animals. For instance, many wild species moved closer to rural and urban areas, including parks and beaches, where they have not been seen for many years, when traffic and other human activities were reduced (e.g. [START_REF] Bates | Global COVID-19 lockdown highlights humans as both threats and custodians of the environment[END_REF][START_REF] Kumar | COVID-19 driven changes in the air quality; a study of major cities in the Indian state of Uttar Pradesh[END_REF]). Our study suggests that limiting human presence in coastal areas that are highly affected by tourism and nautical activities may be an effective management policy to minimize total human impacts on coastline habitats in the future. The COVID-19 crisis has thus called for a new balance between sustainable management of common resources, such as coral reef fish for Pacific Island fisheries and tourism [START_REF] Bambridge | Communs et océans: Le rahui en Polynésie. Au vent des îles[END_REF], and human well-being.

Fig. 1

 1 Fig. 1 Map of Moorea Island showing the location of the different study sites. Map was drawn by the authors using PhotoFiltre 7 software (version 7.1.2-www.photofiltre.com) from an aerial photograph taken by the CRIOBE in 2008 from a private plane

  et al. 2011). As an example, species of wrasse (Labridae), which often hide in sand or lie against hard substrate were more abundant near embankments and on bare sand sites in May 2020 (at the end of the lockdown) than in April 2019. Embankments likely showed large effect sizes because they typically host fishing and recreational activities even though these are practiced by a limited number of people. These sites may also have experienced a reduction in boat engine noise associated with human activities and, as a consequence, fish may have been less stressed[START_REF] Buckley | Conservation implications of COVID19: effects via tourism and extractive industries[END_REF][START_REF] Ferrier-Pagès | Noise pollution on coral reefs? A yet underestimated threat to coral reef communities[END_REF]). However, the potential effects of the lockdown (April 2019 vs. May 2020) remained non-significant for the most abundant fish families.

Fig. 3

 3 Fig. 3 Effect sizes ± 95% confidence interval measured on bare sand (blue), mangroves (green) and embankments (red), in all species of families Scaridae, Pomacentridae, Mullidae, Mugilidae, Lutjanidae, Labridae, Chaetodontidae and Acanthuridae at all stages (juveniles and adults), for densities at the end of the lockdown (May 2020) and two months after the lockdown (July 2020). Effect sizes are expressed as the change in the log ratio of the 2020 densities in the different

Table 1 .

 1 List of adult (AD) and juvenile (JUV) fish species observed in the different habitats in April 2019 and May 2020, i.e. at the end of the lockdown.

					Bare sand			Mangrove			Embankment
				April 2019 May 2020	April 2019 May 2020	April 2019 May 2020
		Family	Species	AD JUV	AD JUV	AD JUV	AD JUV	AD JUV	AD JUV
		Acanthuridae	Acanthurus nigricauda				3						3 0	2
			Acanthurus olivaceus										1
			Acanthurus triostegus	3	4	21	31	1	6		1	1 8	3 2	2 6
	Species in grey		Ctenochaetus striatus Naso lituratus				1					5	28 1	35	22
	are harvested species (fisheries targets). Numbers are the total	Apogonidae	Naso unicornis Apogon exosƟgma Cheiliodipterus quiquelineatus										5
	number of observations made along the 6 transects (3 transects 2 sites) of each habitat	BalisƟdae	Ostorhinchus angustatus PrisƟapogon kallopterus Ballistapus undulatus Rhinecanthus aculeatus	8 6		4 1	3 10			1 1	1	1 8	1 8	7
		Bothidae	Bothus mancus
		Caranguidae	Caranx melampygus		83		12		4			1	1
			Caranx sexfasciatus										9
		ChaetodonƟdae	Chaetodon auriga	9	6	4	8	7	4			210	1 7
			Chaetodon citrinellus	4								8	6	2
			Chaetodon ephippium			6		1
			Chaetodon lunula	12	8	6	5	4			2	1	14
			Chaetodon lunulatus									2
			Chaetodon trifascialis	1
			Chaetodon unimaculatus										3
			Chaetodon vagabundus			9	2 0				1		24	9
			Heniochus chrysostomus
		DasyaƟdae	Himantura fai			2
		Fistulariidae	Fistularia commersoni									2	3
		Gobiidae	Asterropteryx semipunctata			5	2			11	9
			Fusigobius neophytus			4				6
			Gnatholepis anjerensis	2				11
			Valenciennea strigata									1
		Holocentridae	Neoniphon microstoma										2
			Neoniphon sammara			6	2
		Labridae	Cheilinus chlorourus		37		1		2				2	9
			Cheilinus trilobatus		2								1
			Coris aygula										7
			Coris gaimard										1
			Gomphosus varius
			Halichoeres hortulanus			1
			Halichoeres margaritaceus			26	25			1			39
			Halichoeres trimaculatus	7	1 3	9 9 115	12	27	19	2	8	28	52	6
			Stethojulis bandanensis		1	1 3	4 5	1				6	5	26	35
			Thalassoma hardwicke	12	9	1 3	2 8	1	1			815	64	31
			Thalassoma purpureum										1
		Lethrinidae	Gnathodentex aureolineatus										3
			Lethrinus nebulosus										1
			Monotaxis grandoculis										15	1
		Lutjanidae	Lutjanus fulvus	2	2	31		2	2	21	1	1	1
			Lutjanus kasmira		1		1
			Lutjanus monosƟgma			5
		Mugilidae	Crenimugil crenilabis		117	3 1 195	87 124	1 5		3178
			Ellochelon vaigiensis		156			1 14	4				1 60
			Liza vaigiensis				100			1 41	39
		Mullidae	Mulloidichthys flavolineatus		827	3 5 139		360		1 38		1827 100 540
			Mulloidichthys vanicolensis		12		19						1	1082
			Parupeneus barberinus										3
			Parupeneus ciliatus
			Parupeneus mulƟfasciatus		1		2		1				1 1	1 4
		Muraenidae	Echidna nebulosa			1							1
			Gymnothorax javanicus		1	1							1
		Ostraciidae	Ostracion cubicus					1					1
		Pomacanthidae	Centropyge flavissima									1
		Pomacentridae	Abudefduf septemfasciatus					3	7		10	9	1
			Abudefduf sexfasciatus	2	6	11	10		1			1 0	1 5	1 0
			Abudefduf sordidus	1			1		1		1 0	1 6	7	2 6
			Chromis viridis	97 185 100 338
			Chrysiptera brownriggii										2
			Chrysiptera glauca	11		8	5
			Dascyllus aruanus	77	99 146 455	5					1
			Dascyllus trimaculatus			1
			Pomacentrus pavo	2								2
			Stegastes albifasciatus			25
			Stegastes fasciolatus				12
			Stegastes nigricans	82	32 132 116	16	2	1		185	5	65	83
		Scaridae	Calotomus carolinus										7
			Chlorurus spilurus	1 159		67				1		3 15	233
			Hipposcarus longiceps	1		3	34				2		4 1
			Scarus psiƩacus		208		1 20		41				274	110
			Scarus rubroviolaceus						13
			Scarus schlegeli			40	55						4 0
			Chlorurus sordidus				170						85
		Serranidae	Epinephelus merra					2			1		2
		Siganidae	Siganus argenteus		6			223				22
			Siganus spinus		6								1 4
		Syngnathidae	Corythoichthys flavofasciatus
		SynodonƟdae	Synodus binotatus									1
			Saurida gracilis
		Zanclidae	Zanclus cornutus	1								2	4	1

Table 2 .

 2 List

	of adult (AD) observed in the different and juvenile (JUV) fish species	Acanthuridae Family	Acanthurus nigricauda Species	Bare sand AD JUV AD JUV July 2019 J uly 2020	Mangrove 2 AD JUV AD JUV J uly 2019 J uly 2020	Embankment 22 AD JUV AD JUV J uly 2019 J uly 2020
	habitats in July 2019 and July 2020, i.e. 2 months after the end of the lockdown. Species	Apogonidae	Acanthurus triostegus Ctenochaetus striatus Naso unicornis Apogon novenfasciatus	2	3	6 14		10	9	1 1		5 5	4 1 1	4 2 2 8 127 6 3 1 3
	in grey are harvested species (fisheries targets). Numbers are	BalisƟdae Caranguidae	Ballistapus undulatus Rhinecanthus aculeatus Caranx melampygus		1	5		2	4			68	1	10
	the total number of observations made along the 6 transects (3 transects 2 sites) of each habitat	ChaetodonƟdae	Caranx sexfasciatus Chaetodon auriga Chaetodon citrinellus Chaetodon ephippium	1	1	11 1	6	7	4		4	511 5 2	2 8	1
			Chaetodon lunula	2	3	4	2		2		4			4
			Chaetodon lunulatus									2
			Chaetodon trifascialis			2								2
			Chaetodon ulietensis		1
			Chaetodon unimaculatus										3
			Chaetodon vagabundus			14	10			5				9	10
			Heniochus chrysostomus									3
		Gobiidae	Asterropteryx semipunctata	2				1					1
			Fusigobius neophytus						1
		Kuhliidae	Kuhlia mugil			2
			Kuhlia sandvicensis											12
		Labridae	Cheilinus trilobatus											1	1
			Epibulus insidiator									1		1
			Halichoeres hortulanus											6
			Halichoeres margaritaceus			1	8		1				14	5 7
			Halichoeres trimaculatus	6	4	27	36	4	1	9	3	2	2	28	29
			Stethojulis bandanensis	1			5							6	1 0
			Thalassoma hardwicke	2		2 1				2		4	1	37	44
		Lethrinidae	Monotaxis grandoculis										1
		Lutjanidae	Lutjanus fulvus			17		5	1		35			5
			Lutjanus monosƟgma					2
		Mugilidae	Crenimugil crenilabis	1	2 4	4 5 165	1		7 0 148
			Ellochelon vaigiensis			35 160			3 6	8 5
			Liza vaigiensis		38			64	13
		Mullidae	Mulloidichthys flavolineatus			30	13		1		2 7	1
			Mulloidichthys vanicolensis								30
			Parupeneus barberinus											1
			Parupeneus mulƟfasciatus											3	1 3
		Muraenidae	Echidna nebulosa									1
		Ostraciidae	Ostracion cubicus										1
		Pomacanthidae	Centropyge flavissima											1
		Pomacentridae	Abudefduf septemfasciatus		1	1 1		2		5	2 0	5	2	1 8	1 8
			Abudefduf sexfasciatus			8				2	14	2	2	8	1 5
			Abudefduf sordidus	2				31			2	1
			Chromis viridis	7	6 4	7 0 230
			Chrysiptera brownriggii									1	1
			Chrysiptera glauca		1	3	6			3				4	1 9
			Chrysiptera leucopoma							3				2	16
			Dascyllus aruanus	104	9 8	8 3 247
			Pomacentrus pavo			8	2 6
			Stegastes albifasciatus			7	2 0							1 3
			Stegastes fasciolatus									1
			Stegastes nigricans	112	6 4	5 9 125		1			1 4	4	40 117
		Scaridae	Chlorurus spilurus		5				1 5				3 5
			Hipposcarus longiceps					18	12			2	2
			Scarus psiƩacus		1	7 105							7 3 143
			Scarus schlegeli											2
			Chlorurus sordidus				85							37	90
		Serranidae	Epinephelus merra									1		1
		Siganidae	Siganus argenteus	2								2	2
			Siganus spinus					7					4	1
		Syngnathidae	Corythoichthys flavofasciatus									2
		SynodonƟdae	Saurida gracilis											1
		TetraodonƟdae	Arothron hispidus			1
			Arothron meleagris											1
			Canthigaster jenƟnoptera											1
		Zanclidae	Zanclus cornutus					1				1

Acknowledgements

The authors would like to thank all the CRIOBE staff who assisted with this study and Rohan M. Brooker for his constructive comments on earlier versions of the manuscript. We also thank two anonymous reviewers for their comments on a first version of our manuscript.

Funding This work was supported by several grants: Fondation de France (2019-08602), Ministère de l'Economie verte et du domaine -Délégation à la Recherche de Polynésie française (contrat N3622 MED-EPHE), LabEx CORAIL (projects 2018 Emul and 2019 Plasti-Perl), ANR-19-CE34-0006-Manini, ANR-19-CE14-0010-SENSO, the Rāhui Forum and Ressource Center and the Vibrant Oceans Initiative supported by Bloomberg Philanthropies.

Data Availability

The datasets generated during and/or analysed during the current study are available from the corresponding author upon reasonable request.

Supplementary Information

The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s10113-022-02011-0.

Author contribution Frédéric Bertucci: formal analysis, visualization, writing-reviewing and editing; William E. Feeney: writing-reviewing and editing; Zara-Louise Cowan: writing-reviewing and editing; Camille Gache: investigation; Rakamaly Madi Moussa: investigation; Cecile Berthe: investigation; Lana Minier: investigation; Tamatoa Bambridge: supervision; David Lecchini: supervision, investigation, formal analysis, writing-reviewing and editing.

Declarations

Conflict of interest The authors declare no competing interests.


Pambou Nziengui

Claude Feldman
email: pclaudefeldman@gmail.com

Ioana Ionel

Manfoumbi Boussougou

Moutou Pitti Rostand

Craciunescu Corneliu

Investigation on Monopetalanthus behavior wood from Gabonese forest, during a natural moistening and drying phases

Keywords: Natural drying, Natural moistening, Congo Basin Forest, Tropical Forest, Biobased material

This work deals with the study of physico-mechanical behaviour of Monopetalanthus (Andoung) specie from Gabonese forest under natural moistening and drying phases. Wood which is generally used for the conception of footbridges in Gabon and therefore exposed to the weather conditions. Some pieces of this specie have been sized following its radial and tangential plane. In the first, the section of each specimen has been analysed by using a scanning electron microscope to have maps of the internal arrangement of cells of this specie according to RT and TL axis planes. Then physical tests (drying and moistening) have been done on specimens in a room where condition of temperature T (°C) and pressure P (mmHg) are considered constant. The principal parameters followed during the tests are: (i) the dimensions of the radiale plane (R, T)(b, h) and (ii) the weight of each specimen studied. The results show a link between the variations of water in the specimen and its geometrical parameters. Specifically, a pronounced effect of drying and moistening on its dimensional variations before the Fiber Saturation Point (FSP). This work is the beginning of a series of studies conducted on several species endemic to the Gabonese forest and whose use in structures is proven. The goal is to provide a database of their intrinsic characteristics for better structural design.

Introduction:

Presently, in the field of construction, the debates revolve around the need to offer materials capable of reducing the carbon impact, which is due in major cases, to the resurgence of the use of heavy materials such as steel and concrete (Nkene [START_REF] Mezui | Strain and cracks investigations on tropical green wood slices under natural drying: experimental and numerical approaches[END_REF]. Having this perspective as scope, wood, and its derivatives, which have more advantageous operating, production, and recycling costs, appear as ideal palliatives for the fight of greenhouse reduction gases emissions [START_REF] Nziengui | Beam creep of Douglas Fir and White Fir in Outdoor conditions: Experimental study[END_REF]. Indeed, international policies on the fight against climate change define new frameworks whose objectives are to promote sustainable construction using so-called green materials (hemp concrete, straw, raw earth, cellulose wadding, wood, etc.) [START_REF] Niyigena | Variability of the mechanical properties of hemp concrete[END_REF][START_REF] Yadav | Biobased building materials for sustainable future: An overview[END_REF] .

That is one of the reasons why Gabon, whose forest cover is today estimated at more than 87 % of its territory, is implementing drastic policies for the regulation and management of timber exploitation in their forest. These policies are essentially based on the studies carried out at the scale of material on different wood species, endemic to its forest, and which are used in the field of construction [START_REF] Odounga | Mixed mode fracture of some tropical species with the grid method[END_REF][START_REF] Manfoumbi Boussougou | Creep behaviour of timber structures in variable tropical climates: Application to Dacryodes buettneri and baillonella toxisperma species[END_REF]. The objective is to have the necessary information on the type of species and their main characteristics founded within the Gabonese forest to better use them in structure. In rural areas, so-called "plank houses" are mostly built by the poorest class of the population due to the relatively low costs of materials that are accessible to everyone. Based on this observation, it is easy to understand that it is impossible to ban woodcutting in the forest, but it is necessary to regulate its exploitation. In Gabon, logging represents more than 60% of GDP (excluding hydrocarbons), with around 17,000 direct and indirect jobs in the private sector, making it a resource that the state cannot do without today.

The present paper focuses on some specific, physical tests, which occurred upon the Monopetalanthus (Gabonese wood commonly called Andoung). with the aim to understand its behavior in structure. First, the test tubes are sized to carry out microscopic studies to understand their internal structure. Then, other drying and moistening tests are carried out. The objective of these tests is to understand the effects of variations in the internal moisture of the wood on its dimensional variations in structure and to be able to propose mathematical models to better understand this behavior. Indeed, the Andoung wood is currently being used in Gabon for the construction and implementation of footbridges intended for pedestrians. Considering the equatorial environment in which these footbridges are structured, the wood, which is the main material, is faced with the very aggressive climate that characterizes this region. Hence the need to determine physical parameters affecting their mechanical behavior in structure during variations in humidity and temperature. This study will therefore ultimately be based on an experimental model proposed upstream based on the different tests carried out. A long-term scope is to propose some analytical model of the link which exists between the variations of its physical parameters and its mechanical behavior in structures.

Material and methods:

The material studied is wood samples of the Monopetalanthus species collected in the Gabonese forest, in the Congo Basin region and commonly called Andoung. The experimental setup is met for the study of wooden pieces with sizes of 202030(bhL), all expressed in mm, during a drying phase of the probes. All the analyzed samples are machined along the fibers direction and the cross section is carried by the directions (R, T) and (T, L), as in Figure 1. The tests are carried out in a room where conditions of temperature (°C), relative humidity (%) and pressure (mmHg) are considered constant and respectively equal to 22  2°C, 68  2% and 760 mmHg. Then, some specimens are cut, following the cross sections carried by the (R, T) directions and the (T, L) directions as presented in the photo from Figure 1a. These cross sections are analyzed by a scanning electron microscope machine (results presented in Figure 1b). On this machine, several scans can be carried out at different levels of scales, to understand the disposition of the fibers and the cells vertical canals on the cross section of the specimen considered. After the tests carried out using the scanning electron microscope on the different samples, humidity and drying tests are then performed on all the test tubes to highlight the impact of humidity variations on the shrinkage-swelling phenomenon of Andoung wood. The samples are previously machined (Figure 2a) and then immersed in a beaker (Figure 2b) until they are saturated (Figure 2c and2d). The difference in watercolor observed in figures 2b, 2c, and 2d is due to the deposition of nutrient elements at the bottom of the beakers. Finally, the samples are removed from the water and all the dimensions, and the mass of the samples are measured during the shrinkage phase using respectively, a caliper and a balance.

Results and discussions:

-Internal spatial structure of Andoung:

Figure 3 shows a zoom of the typical results of the internal structural analysis performed on an Andoung sample. We observe cavities present along the (R, T) and (T, L) planes. These cavities are vessels through which tree sap is conducted but also the places where free water lodges [START_REF] Medzegue | Radial growth and characterization of juvenile and adult wood in plantation grown okoumé (Aucoumea klaineana Pierre) from Gabon[END_REF][START_REF] Ondo | Caractérisation et variabilité des propriétés physiques et structurelles du bois du kevazingo, Guibourtia tessmannii, et de l'okoumé, Aucoumea klaineana, provenant des forêts naturelles du Gabon[END_REF]. These structural characteristics will help to better understand the phenomenon of shrinkageswelling observed within the wood samples during the drying and wetting phases. The results of the study of these different cavities show that on average they have diameters of the order of 220.19 ± 21.57 μm. The literature [START_REF] Nziengui | Beam creep of Douglas Fir and White Fir in Outdoor conditions: Experimental study[END_REF] shows that during the humidification phase, the wood gains moisture, and swells, which is why after a certain time (Figure 2b), the wooden test specimens sink to the bottom of the beaker. This phenomenon is defined as the saturation phase of the wood, which occurs when the internal moisture content of the wood is higher than the Fiber Saturation Point of the wood species being tested.

-Shrink-swell tests:

During the tests, the evolutions of three kind of parameters have been followed and measured on five (5) specimens of Andoung: the length (L) of the test specimen size following the fibers direction; the average of the thickness (e) carried by the directions (R, T); and the weight (m) of each specimen. The evolution of average thickness e(t) is given by the following equation:

(R,T,L)(2,2,3) cm

(a) (b) (c) (d) 𝑒(𝑡) = 𝑏 + ℎ 2 (𝑡) (1)
Figures 4 shows the evolution of the relative average thickness er(t) measured during the experimental campaign performed on the Andoung samples: Figure 4a shows the influence of moisture content on its evolution; and Figure 4b its evolution in the time during a drying phase.

Figure 4 shows that, the moisture content effects on the Andoung wood's dimensional variation are characterized by two steps: (1) a first step when the drying is slower (𝜆(2) 2.310 -5 s -1 ) and where the moisture content of the wood is below the Point of Fiber Saturation (here PFS24%); (ii) a second step where the drying is faster (𝜆(1)410 -4 s -1 ). Thus, one has shown on Figure1 that the cross section of Andoung species shows porous cavities. According to the literature [START_REF] Guitard | Modèles prévisionnels de comportement élastique tridimensionnel pour les bois feuillus et les bois résineux[END_REF] it is in these cavities that the free water of wood lodges. In fact, during the drying phase, this water evaporates hence the speed of dimensional variation observed (14 times higher) whereas below the PFS, water is difficulty evaporated because it is linked to the structure of wood. Considering these informations and the results showed by Figure 4, the thickness could be expressed by the following expression:

𝑒(𝑡) = 𝛼 (<𝐹𝑆𝑃) 𝐻 + (𝑡) + 𝑏 (<𝐹𝑆𝑃) (2)
Where 𝛼 (<𝐹𝑆𝑃) is the swelling coefficient for a moisture content below the fiber saturation point, 𝐻 + (t) is the internal moisture content of the specimen at time t, and 𝑏 (<𝐹𝑆𝑃) is a constant.

Table 1 presents the values of all coefficients in equation ( 2) during a humidification phase. The determined swelling coefficients reinforce the results found on the moisture absorption and shrinkage rate within the tested specimens. It is noticeable that during the humidification phase, swelling (or moisture absorption) is more significant than below the FSP, but above the FSP, the absorption is very slow or almost non-existent ( = 0.028). By analogy, it can be concluded that during the shrinkage phase (drying), above the FSP, the shrinkage is more significant than below the FSP.

Conclusion and perspectives

One has shown that the physical mechanisms of moisture content variations, within wood, leads to a dimensional variation of this latter, phenomenon which can be problematic for a timber structure.

Fortunately, this phenomenon is considered by Eurocode 5 (Eurocode 5, 2009) in its chapter devoted to the

(b) (a)

service classes of wood in structure. But this normalization is not adapted for tropical species, because it's not considering all the intrinsic parameters of tropical species and their environmental conditions. Indeed, the results obtained in the context of this study show that the shrink-swell issues within tropical woods are aspects that deserve to be further investigated as the tropical environment is highly aggressive towards wooden structures. This demonstrates the need for standardization (national or regional) that considers all these aspects and considers the environment in which these structures are installed. Therefore, the next step of this research will be focused on the proposition of physico-mechanical analytical model of this tropical species during a drying or moistening phase. These models will allow us to limit our total dependence on experimental studies whose equipment can be very expensive for certain laboratories.

Figure 1 :

 1 Figure 1: Examples of samples used by the scanning electron microscope tests: (a) pictures taken by the camera; (b) map taken by the electron microscope;  represents the transversal plan of the specimen, carried by (R, T) directions and  is the tangential plan, carried by (T, L) directions.

Figure 2 :

 2 Figure 2: Main stages of sample moistening prior to drying: (a) typical tested sample; (b) first day of moistening; (c) 22nd day of moistening (beginning of sample saturation); (d) 27th day of moistening (all samples have leaked).

Figure 3 :

 3 Figure 3: Close-up of the typical radial section of the Andoung species.

Figure 4 :

 4 Figure 4:Typical evolution of Andoung wood's dimensional variations: (a) evolution during the moistening phase; (b) evolution during a drying phase.

Table 1 :

 1 Swelling coefficient of the Andoung species.

	H  FSP	H  FSP

Acknowledgements

The authors would like to strongly acknowledge the "Agence Universitaire de la Francophonie (AUF)" throughout the "Eugen Ionescu" scholarship funded for the first author by the Romanian Ministry of Foreign Affairs, at the Politehnica University of Timisoara, without which the research would not have been possible.