Distribution Of Small Pelagic Fishes Research Articles

Climate change impacts on small pelagic fish distribution in Northwest Africa: trends, shifts, and risk for food security

Climate change is recognised to lead to spatial shifts in the distribution of small pelagic fish, likely by altering their environmental optima. Fish supply along the Northwest African coast is significant at both socio-economic and cultural levels. Evaluating the impacts of climatic change on small pelagic fish is a challenge and of serious concern in the context of shared stock management. Evaluating the impact of climate change on the distribution of small pelagic fish, a trend analysis was conducted using data from 2363 trawl samplings and 170,000 km of acoustics sea surveys. Strong warming is reported across the Southern Canary Current Large Marine Ecosystem (CCLME), extending from Morocco to Senegal. Over 34 years, several trends emerged, with the southern CCLME experiencing increases in both wind speed and upwelling intensity, particularly where the coastal upwelling was already the strongest. Despite upwelling-induced cooling mechanisms, sea surface temperature (SST) increased in most areas, indicating the complex interplay of climatic-related stressors in shaping the marine ecosystem. Concomitant northward shifts in the distribution of small pelagic species were attributed to long-term warming trends in SST and a decrease in marine productivity in the south. The abundance of Sardinella aurita, the most abundant species along the coast, has increased in the subtropics and fallen in the intertropical region. Spatial shifts in biomass were observed for other exploited small pelagic species, similar to those recorded for surface isotherms. An intensification in upwelling intensity within the northern and central regions of the system is documented without a change in marine primary productivity. In contrast, upwelling intensity is stable in the southern region, while there is a decline in primary productivity. These environmental differences affected several small pelagic species across national boundaries. This adds a new threat to these recently overexploited fish stocks, making sustainable management more difficult. Such changes must motivate common regional policy considerations for food security and sovereignty in all West African countries sharing the same stocks.

Can we actually monitor the spatial distribution of small pelagic fish based on Sentinel-3 data? An example from the North Aegean Sea (Eastern Mediterranean Sea)

Fish population spatial distribution data provide essential information for fleet monitoring and fishery spatial planning. Modern high resolution ocean color remote sensing sensors with daily temporal coverage can enable consistent monitoring of highly productive areas, giving insight in seasonal and yearly variations. Here is presented the methodology to monitor small pelagic fish spatial distribution by means of 500m resolution satellite data in a geographically and oceanographically complex area. Specifically, anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) acoustic biomass data are modeled against environmental proxies obtained from the Sentinel-3 satellite mission. Three modeling techniques (Logistic Regression, Generalized Additive Models, Random Forest) were applied and validated against the in-situ measurements. The accuracy of anchovy presence detection peaked at 76% and for sardine at 78%. Additionally, the spatial distribution of the models’ output highlighted known fishing grounds. For anchovy, biomass modeling highlighted the importance of bathymetry, SST, and the distance from thermal fronts, whereas for sardine, bathymetry, CHL and chlorophyll fronts. The models are applied to a sample dataset to showcase a potential outcome of the proposed methodology and its spatial characteristics. Finally, the results are discussed and compared to other habitat studies and findings in the area.

Biological interactions potentially alter the large-scale distribution pattern of the small pelagic fish, Pacific saury Cololabis saira

We aimed to understand the effect of biological interactions on the distribution of small pelagic fishes. Surveys were conducted during 2003-2019 using sea surface trawl nets in the western and central North Pacific Ocean, covering an area between 143° E and 165° W, to estimate the distributions of 4 small pelagic fishes: Pacific saury Cololabis saira, Japanese sardine Sardinops melanostictus, chub mackerel Scomber japonicus, and Japanese anchovy Engraulis japonicus. These species are potential competitors, as they all occupy a similar niche. Japanese anchovy was abundant during 2003-2012, while Japanese sardine and chub mackerel were abundant during 2013-2019. Only Pacific saury was distributed throughout the survey area, while the others occurred within the area west of 180° longitude (WA). Pacific saury and its potential competitors showed adjacent distributions, with a slight overlap along sea surface temperature gradients in the WA. This distribution pattern remained even when Japanese sardine expanded its distribution to colder waters and increased in abundance after 2013, resulting in a shift in the distribution of Pacific saury to colder waters and a period of high densities of Japanese sardine. Such a distribution shift of Pacific saury was not observed in the area east of 180° longitude, where no Japanese sardine was observed. These results suggest the possibility that biological interactions such as inter-species competition can be an important factor in determining the distribution of small pelagic fishes.

Comparison of species-specific qPCR and metabarcoding methods to detect small pelagic fish distribution from open ocean environmental DNA.

Environmental DNA (eDNA) is increasingly used to noninvasively monitor aquatic animals in freshwater and coastal areas. However, the use of eDNA in the open ocean (hereafter referred to OceanDNA) is still limited because of the sparse distribution of eDNA in the open ocean. Small pelagic fish have a large biomass and are widely distributed in the open ocean. We tested the performance of two OceanDNA analysis methods-species-specific qPCR (quantitative polymerase chain reaction) and MiFish metabarcoding using universal primers-to determine the distribution of small pelagic fish in the open ocean. We focused on six small pelagic fish species (Sardinops melanostictus, Engraulis japonicus, Scomber japonicus, Scomber australasicus, Trachurus japonicus, and Cololabis saira) and selected the Kuroshio Extension area as a testbed, because distribution of the selected species is known to be influenced by the strong frontal structure. The results from OceanDNA methods were compared to those of net sampling to test for consistency. Then, we compared the detection performance in each target fish between the using of qPCR and MiFish methods. A positive correlation was evident between the qPCR and MiFish detection results. In the ranking of the species detection rates and spatial distribution estimations, comparable similarity was observed between results derived from the qPCR and MiFish methods. In contrast, the detection rate using the qPCR method was always higher than that of the MiFish method. Amplification bias on non-target DNA and low sample DNA quantity seemed to partially result in a lower detection rate for the MiFish method; the reason is still unclear. Considering the ability of MiFish to detect large numbers of species and the quantitative nature of qPCR, the combined usage of the two methods to monitor quantitative distribution of small pelagic fish species with information of fish community structures was recommended.

The use remote sensing technology to determine the distribution of small pelagic fish in IFMA 713

This study aimed to better understand the effect of oceanographic parameters on the spatial distribution of small pelagic fish (SPF) in Indonesia Fisheries Management Area (IFMA) 713. This research is a preliminary study in identifying preferences habitat for SPF by using some oceanographic parameters (i.e. depth, sea surface temperature (SST), and sea surface chlorophyll-a (SSC)). The depth data was gained from The ETOPO2 satellite database. Furthermore, the SST and SSC data were derived from Terra/MODIS. Fish finder was used to detect and determine the position of fish schooling in the water column and provide water depth information. The In-situ data collected from June to August 2020 in Makassar Strait and Bone Gulf by followed the fishing operation by lift net gear in the Spermonde Island waters and Luwu Regency waters, respectively. The fishery data and oceanographic satellite imaginary overlay and mapped using Geographic Information System techniques. The result founded that the SPF distributions, which is dominated by anchovies, sardine, scad, and mackerel, occupied areas with range from 30.0 to 31.0°C SST with corresponding with SSC of 0.30 to 0.50 mg/m−3, and inhabit the water depth in inshore areas with the fish swimming layers mostly in maximum depth at 20 m or less based on fish finder recording display. This information is needed for the sustainable management of small pelagic fish in IFMA 713.

Forecasting shifts in habitat suitability across the distribution range of a temperate small pelagic fish under different scenarios of climate change

Climate change often leads to shifts in the distribution of small pelagic fish, likely by changing the match-mismatch dynamics between these sensitive species within their environmental optima. Using present-day habitat suitability, we projected how different scenarios of climate change (IPCC Representative Concentration Pathways 2.6, 4.5 and 8.5) may alter the large scale distribution of European sardine Sardina pilchardus (a model species) by 2050 and 2100. We evaluated the variability of species-specific environmental optima allowing a comparison between present-day and future scenarios. Regardless of the scenario, sea surface temperature and salinity and the interaction between current velocity and distance to the nearest coast were the main descriptors responsible for the main effects on sardine's distribution. Present-day and future potential “hotspots” for sardine were neritic zones (<250 km) with water currents <0.4 m s−1, where SST was between 10 and 22 °C and SSS > 20 (PSU), on average. Most variability in projected shifts among climatic scenarios was in habitats with moderate to low suitability. By the end of this century, habitat suitability was projected to increase in the Canary Islands, Iberian Peninsula, central North Sea, northern Mediterranean, and eastern Black Sea and to decrease in the Atlantic African coast, southwest Mediterranean, English Channel, northern North Sea and Western U.K. A gradual poleward-eastward shift in sardine distribution was also projected among scenarios. This shift was most pronounced in 2100 under RCP 8.5. In that scenario, sardines had a 9.6% range expansion which included waters along the entire coast of Norway up and into the White Sea. As habitat suitability is mediated by the synergic effects of climate variability and change on species fitness, it is critical to apply models with robust underlying species-habitat data that integrate knowledge on the full range of processes shaping species productivity and distribution.

Analysis of small pelagic fishing grounds using a generalized additive model in the Makassar Strait

Spatial and temporal studies on the distribution of small pelagic fish by applying oceanographic parameters are essential for fisheries management. An understanding of the spatial characteristics of small pelagic fish distributions is important to optimize the sustainability of marine resource utilization. This study examined the characteristics of small pelagic fish fishing grounds through the analysis of oceanographic parameters using remote sensing satellite data and generalized additive model. Remote sensing satellite data (SST and chlorophyll-α) covering the area of Makassar Strait were obtained from NASA databases, with a spatial resolution of 4 km and monthly temporal resolution. Data on the geographical location and catch volume of small pelagic fish were obtained from a fisheries survey in the Makassar Strait. Remote sensing satellite data and fisheries survey data were plotted graphically to determine the environmental conditions at the sites where the small pelagic fish were caught. Catch volume and catch positions were mapped to show fish distribution. The results show that the highest point was around Barru waters with a catch of 30.70kg - 36.75kg. Small pelagic fish were caught in the SST range between 29.82° C - 31.32°C and mostly caught in chlorophyll-a between 0.34 mg/m3 - 0.62 mg/m3.

Climate-related changes in the biomass and distribution of small pelagic fishes in the eastern Bering Sea during late summer, 2002–2018

Climate change is altering the distribution and biomass of marine species in Arctic and sub-Arctic waters. In this study, we investigate the influence of sea temperature on the annual distribution and biomass of pelagic fishes in the eastern Bering Sea during late summer, 2002–2018. The distribution (easting, northing, and area occupied) and biomass of capelin, Pacific herring, juvenile chum salmon, juvenile pink salmon, juvenile sockeye salmon, and age-0 walleye pollock collected by surface trawl were estimated using a standardized geostatistical delta-generalized linear mixed modeling approach. Species showed varied responses to warming on a temporal scale. Warming corresponded with more northerly distributions for capelin and juvenile sockeye salmon, a more westerly distribution of juvenile sockeye salmon, and range expansions for juvenile chum and sockeye salmon. Warming corresponded to a decrease in the annual biomass of capelin and an increase in the biomass of herring, age-0 pollock, and juvenile sockeye salmon. The spatio-temporal covariation in sea temperature and the distribution was nonlinear for juvenile pink salmon and age-0 pollock, positive for juvenile chum salmon and juvenile pink salmon, and negative for capelin indicating different responses of the distribution of pelagic fishes to warming in the eastern Bering Sea during late summer. In warmer areas, we found that the catch rates were higher for juvenile pink salmon, lower for capelin, and not significantly different for juvenile chum salmon, herring, age-0 pollock, and juvenile sockeye salmon. Juvenile sockeye salmon, a southerly distributed species in the survey area, appeared most responsive to warming. In this study, sockeye salmon and pollock are the most commercially important species while chum salmon are important for subsistence fishing. These temperature related changes during early life history stages for survival may have impacts on the numbers of these fishes recruiting to the fisheries.

Damage or benefit? How future scenarios of climate change may affect the distribution of small pelagic fishes in the coastal seas of the Americas

Species occurrence and distribution have already been directly affected by climate change, a scenario that is likely to be accentuated as the temperature rise is expected to exceed 2 °C by 2100. Owing to climate change, organisms are forced to migrate or adapt to new climatic conditions, and if they fail to do so, they are at risk of declining and becoming extinct. However, some species are adapted to overcome and even benefit from these new conditions, increasing their occurrence area or even their abundance. Using Bayesian Species Distribution Models (B-SDMs), we evaluated the current distribution of two halfbeak fishes, Hyporhamphus unifasciatus and Hemiramphus brasiliensis, and the effect of climate changes predicted for 2050 and 2100 on the distribution of these populations in coastal waters of the Americas. We used species occurrence data from bibliographical sources and online databases. One biotic (net primary production - NPP) and four abiotic variables (sea surface temperature - SST), sea surface salinity - SSS), depth, and sea bottom rugosity) were used as potential predictors of species distribution. Results indicated that both species are more likely found in shallower, warmer, and saltier waters. Model prediction suggests that they will probably benefit from climate change, with potential increase in their occurrence area in coastal regions of the Americas, especially due to temperature rise and increased salinity.

Distribution of sea surface temperature and chlorophyll-a concentration its correlation with small pelagic fish catch in Dodinga Bay

Current technology development provides us convenience through utilization of data time series to improve fish catch production, which may also be performed in Dodinga Bay through an oceanographic factor analysis on small pelagic fish catch aiming at examining the role of oceanographic factors and their correlation with the distribution of small pelagic fish. The use of fishing experimental method for data collection, image analysis, statistical analysis and their interpretation is expected to answer the research objective for optimized wild fishery production. The research finds that simultaneously sea surface temperature and chlorophyll-a significantly influence pelagic fish catch with Fhit value 90.403 and significance 0.000, while partially surface temperature and chlorophyll-a are the oceanographic factors which contribute to the distribution of small pelagic fish in Dodinga Bay with r square value 0.8525 for sea surface temperature and 0.6966 for chlorophyll-a.

Spatial distribution of small pelagic fishes: Implications for fishing quota allocations

The upwelling ecosystem of central southern Chile sustains the pelagic common sardine and anchovy fisheries, which are managed by setting a global quota allocated to small-scale (artisanal) and industrial fishers. A prearranged spatial allocation of quotas for small-scale fishers is based on historical landings rather than the current spatial distribution of the target species. Artisanal fishers can only fish in their original Spatial Administrative Units (SAUs) according to fishing rights. Marked environmental variability, such as that caused by ENSO events, trigger changes in the local availability of resources. Thereafter, artisanal and industrial fishers compensate for low yields by transferring their fishing quota to quota holders (artisanal-artisanal, industrial-artisanal, artisanal-industrial) from other SAUs. The legal procedure for transferring quotas among users is efficient to harvest the total allowable catch. This procedure favors the quota holders but is detrimental to the crew of vessels that transfer quotas to other fishers. A mechanism is recommended to distribute the profits obtained by quota transfers between ship owners and crews to avoid this negative effect.

Hubungan Parameter Biofisik Perairan dengan Hasil Tangkapan Ikan Pelagis Kecil di Perairan Pulau Ternate

This research was experimental fishing methods and regression analysis to examine the effect and relationship between biophysical parameters namely sea surface temperature and chlorophyll-a and the results of small pelagic fish conversations in Ternate Island, the results of the study showed that the biosphysical parameters including sea surface temperature (SPL) and chlorophyll-a have a significant influence on the distribution of small pelagic fish, with the correlation coefficient values of each sea surface temperature parameter and chlorophyll-a with the results of the pelagic fish tagging are 0.6121 and 0.7042

Targeted prey fields determine foraging effort thresholds of a marine diver: Important cues for the sustainable management of fisheries

Abstract Understanding functional relationships between seabirds and prey can play an important role in the sustainable management of fisheries that compete for the same resources yet data linking the foraging performance of seabirds directly with concurrent information on prey supplies remain limited. We assess the influence of prey availability on the foraging performance of breeding African penguins Spheniscus demersus using data from instrumented birds during 14 pelagic fish surveys carried out over 3 years around their two largest breeding colonies, St Croix and Bird islands in Algoa Bay. Characteristics of prey fields targeted by African penguins were identified by assessing the response of foraging birds to the acoustically determined distribution of small pelagic fish. Path‐ and dive‐derived foraging performance metrics were modelled as a response to overall and targeted prey abundance estimates to assess potential nonlinear functional relationships between penguin foraging effort and prey availability. African penguins displayed marked spatial heterogeneity in their selection of foraging sites, but birds from both colonies invariably foraged on fish schools &lt;40 m deep. Generalized additive model fits improved for all foraging performance responses when using targeted prey abundance (i.e. only fish in areas frequented by penguins and at depths &lt;40 m, as opposed to estimates using all prey). Models for trip duration (r2 = 0.45) and foraging path length (r2 = 0.44) showed a negative nonlinear form in their response to targeted prey abundance, with significant improvement in penguin foraging effort when prey abundance was greater than 45% of the maximum prey biomass recorded. Synthesis and applications. Functional relationships between seabird foraging performance and prey supplies have the potential to inform the sustainable management of fisheries that compete for the same prey. Our findings highlight the importance of quantifying prey that is available to seabirds when assessing critical thresholds in prey abundance below which seabird foraging performance is adversely affected. These findings can be integrated into dynamic ocean management strategies to mitigate resource competition around seabird colonies. A framework for flexible, colony‐centred closures is proposed as a more dynamic alternative to time/area closures with the potential to respond to prevailing habitat conditions in near real‐time.

Oceanographic Conditions on Small Pelagic Fishery in the Gulf of Bone Waters

This study aimed to investigate the relationship between the dinamic oceanographic condition and fluctuation in the catch of small pelagic fish. Study on the dinamic oceanographic conditions were focused on the sea surface temperature (SST), chlorophyll-a concentration (SSC) and water depth. The study took place in the area of Gulf of Bone located, data collection was started from April to September 2017. The data were collected using experimental fishing metode (large liftnet) and applications of remote sensing in satellite oceanography, which then analysed using Geographic Information System (GIS) dan Generalized Additive Models (GAMs). The result showed that the distribution of small pelagic fish tends to be within the area of temperature ranging from 29.5 to 30.0oC, the chlorophyll-a from 0.7 to 0.9 mg.m-3 and concentrated within the coastal area with in waters depth maximum of 100 m.Keywords: Oceanography, small pelagic fish, distribution, fishing ground, Gulf of Bone

Spatial distribution of main clupeid species in relation to acoustic assessment surveys in the continental shelves of Senegal and The Gambia

This work compiles hydroacoustic recordings and catch data over Senegambia (Senegal and The Gambia) from assessment surveys on the major clupeid species to identify sources of bias in abundance estimates caused by their horizontal distribution. The latitudinal distribution of small pelagic fish is often well known, while their “across shelf” distribution on the continental shelf is less understood. The southern part of the Senegambian shelf has a wide shallow water (&lt;10 m) area that makes up 20% (1500 NM2) of the total shelf surface, while the northern part accounts for 3% (200 NM2). These areas are not assessed by conventional fisheries acoustics surveys and therefore increase the uncertainty of the assessment of these species. Our findings show that this likely introduces a bias in the assessment of Sardinella maderensis, while for S. aurita no major estimation-error is caused by their horizontal distribution. The data confirm that Ethmalosa fimbriata and Ilisha africana are challenging to assess by conventional surveys, due to their mostly inshore distribution. We emphasise the usefulness of assessing S. aurita through fisheries independent hydroacoustic surveys, and propose alternative methods to survey shallow water areas to reduce biases in biomass estimates and distribution mapping.

Assessing trophic adaptability is critical for understanding the response of predatory fishes to climate change: a case study of Pomatomus saltatrix in a global hotspot

There is a growing need to incorporate biotic interactions, particularly those between predators and their prey, when predicting climate-driven shifts in marine fishes. Predators dependent on a narrow range of prey species should respond rapidly to shifts in the distribution of their prey, whereas those with broad trophic adaptability may respond to shifts in their prey by altering their diet. Small pelagic fishes are an extremely important component of the diet of many marine predators. However, their populations are expected to shift in distribution and fluctuate in abundance as the climate changes. We conducted a comparative study of the seasonal diet of adult Pomatomus saltatrix over two periods (June–December 2006 and 2012) and examined the available data on small pelagic fishes biomass in a global hotspot (the coastal region of southern Angola, southern Africa) to gain an understanding of the tropic adaptability of the species. Despite a drop (630 000 t to 353 000 t) in the abundance of their dominant prey (Sardinella aurita) in the region, it remained the most important prey item during both study periods (Period 1 = 99.3% RI, Period 2 = 85.3% RI, where %RI is a ranking index of relative importance). However, the diet during Period 2 was supplemented with prey typically associated with the nearshore zone. The seasonal data showed that P. saltatrix were capable not only of switching their diet from S. aurita to other prey items, but also of switching their trophic habitat from the pelagic to the nearshore zone. These findings suggest that P. saltatrix will not necessarily co-migrate if there is a climate-driven shift in the distribution of small pelagic fishes (their dominant prey). Accordingly, understanding the trophic adaptability of predators is critical for understanding their response to the impacts of climate change.

From species distributions to ecosystem structure and function: A methodological perspective

As species biology and ecology is profoundly influenced by climate, any climatic alteration may have severe consequences on marine pelagic ecosystems and their food webs. It remains challenging to estimate the influence of climate on both structural and functional properties of food webs. In this study, we proposed an innovative approach to assess the propagating effects of climate change on ecosystem food web. The approach is based on a sensitivity analysis of a food-web model, a linear inverse model using a Monte Carlo method coupled with a Markov Chain, in which changes in the values of parameters are driven by external Ecological Niche Model outputs. Our sensitivity analysis was restricted to parameters regarding a keystone functional group in marine ecosystems, i.e. small pelagic fish. At the ecosystem level, the consequences were evaluated using both structural and functional ecological network indices. The approach is innovative as it is the first time that these three methods were combined to assess ecological network indices sensitivity to future climatic pressure. This coupling method was applied on the French continental shelf of the Bay of Biscay for which a food-web model already exists and where future changes in the distribution of small pelagic fish have already been examined through model building and projections. In response to the sensitivity analysis corresponding to an increase in small pelagics production only, our results suggested a more active system with an intense plankton-small pelagics-seabirds chain and an efficient recycling to maximize detritus use in the system in relation with detritus export. All results combined together seemed to be in favor of a system adapting to sustain the tested increase in production of small pelagic planktivores. Finally, regarding the innovative combination of numerical tools presented, even if further investigations are still necessary to get a more realistic view of cumulative effects resulting from one given pressure (or more) on a food web (e.g. altering different biological compartments at the same time), the Ecological Network Analysis indices values showed a higher variability under the scenarios of change. Our study thus pointed out a promising methodology to assess propagating changes in structural and functional ecosystem properties.

Effects of ocean climate on life cycles and distribution of small pelagic fishes in the California Current System off Baja California

Due to their sensitivity to climate forcing that may alter their abundance and distribution, small pelagic fish are important ecological indicators of the state of the California Current System. They are schooling, planktivorous fish that provide forage for higher trophic levels. We describe the life cycle histories and patterns of distribution of sardine, anchovy, and mackerel species occupying the waters along the west coast of the Baja California Peninsula (Mexico). The links between the physical and biological forcing and the structure and condition of their habitats, their patterns of movement, productivity, and stock structure (where information is available) are described in relation to the fisheries. We have used the catches as a proxy for the history of the different stocks, while recognizing that fisheries-derived information is biased by the operational scale and only covers areas where fish are available. We have relied mainly on studies of sardines to evaluate the validity of the principal paradigms in fisheries oceanography. We describe how the environment can structure a population by relating an example given by the spatiotemporal variability of the northern and southern stocks of Pacific sardine (Sardinops sagax) and Pacific mackerel (Scomber japonicus) from 2000 to 2014. During this period, the northern stocks showed a marked tendency to contract southwards, as far south as Magdalena Bay, in response to the regional cooling that began in the previous decade. It appears that the combined effects of fishing and climate change may sufficiently alter habitat characteristics so that both the distribution and productivity of a population are shifted.

Checklist of the marine fishes collected during hydroacoustic surveys in the southeastern brazilian bight from 1995 to 2010

The spatial distribution of small pelagic fishes off the Southeastern Brazilian coastal area (22°-29°S) was investigated over six periods, from 1995 to 2010, in the context of the Program Prospection and Assessment of the Biomass of the Sardine, Sardinella brasiliensis in order to provide data about their pattern of distribution and biomass. A checklist of the species collected is presented herein with data on geographic position and depth of captures. Such information is intended to be a reference about the fauna that occupied the region in the period under investigation, and allow future comparisons about the resilience of the local fauna, i.e., the detection of future changes caused by environmental factors or human activities. Sampling was conducted during hydroacoustic surveys with mid water trawling plus a few purse seine operations between 10 m and 100 m depth. A total of 86 species were collected.

Spatial structure and distribution of small pelagic fish in the northwestern Mediterranean Sea.

Understanding the ecological and anthropogenic drivers of population dynamics requires detailed studies on habitat selection and spatial distribution. Although small pelagic fish aggregate in large shoals and usually exhibit important spatial structure, their dynamics in time and space remain unpredictable and challenging. In the Gulf of Lions (north-western Mediterranean), sardine and anchovy biomasses have declined over the past 5 years causing an important fishery crisis while sprat abundance rose. Applying geostatistical tools on scientific acoustic surveys conducted in the Gulf of Lions, we investigated anchovy, sardine and sprat spatial distributions and structures over 10 years. Our results show that sardines and sprats were more coastal than anchovies. The spatial structure of the three species was fairly stable over time according to variogram outputs, while year-to-year variations in kriged maps highlighted substantial changes in their location. Support for the McCall's basin hypothesis (covariation of both population density and presence area with biomass) was found only in sprats, the most variable of the three species. An innovative method to investigate species collocation at different scales revealed that globally the three species strongly overlap. Although species often co-occurred in terms of presence/absence, their biomass density differed at local scale, suggesting potential interspecific avoidance or different sensitivity to local environmental characteristics. Persistent favourable areas were finally detected, but their environmental characteristics remain to be determined.