Coastal Nursery Research Articles

Do Sardinella aurita spawning seasons match local retention patterns in the Senegalese–Mauritanian upwelling region?

AbstractSardinella aurita is the most abundant small pelagic fish in the Senegalese–Mauritanian region. The success of its reproduction crucially depends on the local circulation as this determines whether larvae reach coastal nursery areas favorable to their survival or are dispersed into the open ocean. As a first step towards evaluating sardinella vulnerability to climate‐driven hydrodynamical changes, this study aims at underpinning how transport pathways drive optimal spatial and seasonal patterns for sardinella reproduction. We have used two estimates of the Senegalese–Mauritanian coastal seasonal circulation simulated by two hydrodynamical model configurations that differ in their forcing and topography. Nursery areas are determined by evaluating coastal retention with a Lagrangian individual‐based model that accounts for processes such as diel vertical migration and mortality as a result of lethal temperature exposure. Our results suggest that the shelf zones located at the Arguin Bank (19.5°N–21°N) and south of Senegal (12°N–14.75°N) are highly retentive. We find maximum retention rates in July–August and November–December over the Arguin Bank; from February–July and November–December over the southern Senegalese shelf; and lower retention rates over the central region (14.75°N–19.5°N) that are locally maximum in June–July when the upwelling weakens. These retention areas and their seasonality are in agreement with previously reported spawning patterns, suggesting that the Sardinella aurita spawning strategy may result from a trade‐off between retention patterns associated with the seasonal circulation and food availability. Exposure to lethal temperatures, although not well studied, could be a further limiting factor for spawning. The Lagrangian analysis reveals important connectivity between sub‐regions within and south of the system and hence underlines the importance for joint management of the Sardinella aurita stock.

Contrasting coastal and shelf nursery habitats of Pacific cod in the southeastern Bering Sea

Abstract Shallow, subtidal waters of coastal embayments are the primary nursery habitats of juvenile Pacific cod through much of their range. However, the importance of these habitats to the Bering Sea population is poorly understood as the Bering Sea offers relatively little of this habitat. In this study, we examined the use of demersal and pelagic habitats in the southeast Bering Sea by age-0 Pacific cod. In 4 years of demersal beam trawling on the shelf at depths of 20–146 m, fish were most abundant along the Alaska Peninsula (AKP) at depths to 50 m. In addition, 1 year of spatially intensive beam trawl sampling was conducted at depths of 5–30 m in a nearshore focal area along the central AKP. In this survey, age-0 cod were more abundant along the open coastline than they were in two coastal embayments, counter to patterns observed in the Gulf of Alaska. Demersal sampling in 2012 was conducted synoptically with surveys of surface and subsurface waters over the continental shelf. Age-0 cod were captured in pelagic waters over the middle and outer shelf, with maximum catches occurring over depths of 60–80 m. The similar size distributions of fish in coastal-demersal and shelf-surface habitats and the proximity of concentrations in the two habitat types suggests that habitat use in the Bering Sea occurs along a gradient from coastal to pelagic. While capture efficiencies may differ among trawl types, trawl-based estimates of age-0 cod density in demersal waters along the AKP was 10 times that observed in the highest density pelagic-shelf habitats, demonstrating the importance of coastal nursery habitats in this population. Despite representing a much smaller habitat area, the coastal waters along the AKP appear an important nursery area and support a significant fraction of the age-0 Pacific cod in the Bering Sea.

True Value of Estuarine and Coastal Nurseries for Fish: Incorporating Complexity and Dynamics

Coastal ecosystems, such as estuaries, salt marshes, mangroves and seagrass meadows, comprise some of the world's most productive and ecologically significant ecosystems. Cur- rently, the predominant factor considered in valuing coastal wetlands as fish habitats is the contribution they make to off- shore, adult fish stocks via ontogenetic migrations. However, the true value of coastal nurseries for fish is much more extensive, involving several additional, fundamentally important ecosystem processes. Overlooking these broader aspects when identifying and valuing habitats risks suboptimal conservation outcomes, especially given the intense competing human pressures on coastlines and the likelihood that protection will have to be focussed on specific locations rather than across broad sweeps of individual habitat types. We describe 10 key components of nursery habitat value grouped into three types: (1) connectivity and population dynamics (includes connectivity, ontogenetic migration and seascape migration), (2) ecological and ecophys- iological factors (includes ecotone effects, ecophysiological fac- tors, food/predation trade-offs and food webs) and (3) resource dynamics (includes resource availability, ontogenetic diet shifts and allochthonous inputs). By accounting for ecosystem com- plexities and spatial and temporal variation, these additional components offer a more comprehensive account of habitat value. We explicitly identify research needs and methods to support a broader assessment of nursery habitat value. We also explain how, by better synthesising results from existing re- search, some of the seemingly complex aspects of this broader view can be addressed efficiently.

Numerical modelling of physical processes governing larval transport in the southern North Sea

Abstract. A three-dimensional hydrodynamic model (GETM) was coupled with a particle tracking routine (GITM) to study the inter-annual variability in transport paths of particles in the North Sea and English Channel. For validation, a comparison with observed drifter trajectories is also presented here. This research investigated to what extent variability in the hydrodynamic conditions alone (reflecting passive particle transport) contributed to inter-annual variability in the transport of eggs and larvae. In this idealised study, no a priori selection of specific spawning grounds or periods was made and no active behaviour (vertical migration) or mortality was included. In this study, egg and larval development towards coastal nursery areas was based solely on sea water temperature, while settlement areas were defined by a threshold water depth. Results showed strong inter-annual variability in drift direction and distance, caused by a combination of wind speed and direction. Strong inter-annual variability was observed both in absolute amount of settlement in several coastal areas, and in the relative importance of the different areas. The effects of wind and temperature variability are minor for settlement along the western shores of the North Sea and in the English Channel, but have a very significant impact on settlement along the eastern shores of the North Sea. Years with strong south-westerly winds across the Dover Straight resulted in higher settlement figures along its eastern shores of the North Sea (standard deviation 37% of the mean annual settlement value). Settlement in the western Dutch Wadden Sea did not only show inter-annual variability, but patterns were also variable within each year and revealed seasonal changes in the origin of particles: during winter, stronger currents along with colder temperatures generally result in particles originating from further away.

Coastal Nurseries and Their Importance for Conservation of Sea Kraits

Destruction and pollution of coral reefs threaten these marine biodiversity hot stops which shelter more than two thirds of sea snake species. Notably, in many coral reef ecosystems of the Western Pacific Ocean, large populations of sea kraits (amphibious sea snakes) have drastically declined during the past three decades. Protecting remaining healthy populations is thus essential. In New Caledonia, coral reefs shelter numerous sea krait colonies spread throughout an immense lagoon (24,000 km2). Sea kraits feed on coral fish but lay their eggs on land. However, ecological information on reproduction and juveniles is extremely fragmentary, precluding protection of key habitats for reproduction. Our 10 years mark recapture study on Yellow sea kraits (L. saintgironsi >8,700 individuals marked) revealed that most neonates aggregate in highly localized coastal sites, where they feed and grow during several months before dispersal. Hundreds of females emigrate seasonally from remote populations (>50 km away) to lay their eggs in these coastal nurseries, and then return home. Protecting these nurseries is a priority to maintain recruitment rate, and to retain sea krait populations in the future.

Juvenile growth and mortality effects on white shrimp Litopenaeus setiferus population dynamics in the northern Gulf of Mexico

Processes regulating juvenile growth and mortality of white shrimp Litopenaeus setiferus in coastal nurseries may be particularly important in regulating offshore adult population size and sustainability. To advance the integration of these processes into fishery stock assessments, and to provide a better understanding of the functional role of coastal nurseries for fishery species, we explored the potential effects of variable juvenile growth and survival on white shrimp population growth rate. We developed a population model that incorporates available information on vital rates (growth, mortality, fecundity) for each shrimp life stage. We used the model to explore the potential impacts of variability in juvenile growth and mortality rates on the overall population growth rate. Modest changes in juvenile growth and mortality rates were projected to have a greater impact on stock size than the full range in fishing mortality over the past few decades. These results suggest that variability in juvenile survival may be a strong driver of adult stock size and that the processes that regulate juvenile growth and mortality need to be properly understood for the effective management of coastal nurseries and shrimp stocks.

Regional-Scale Migrations and Habitat Use of Juvenile Lemon Sharks (Negaprion brevirostris) in the US South Atlantic

Resolving the geographic extent and timing of coastal shark migrations, as well as their environmental cues, is essential for refining shark management strategies in anticipation of increasing anthropogenic stressors to coastal ecosystems. We employed a regional-scale passive acoustic telemetry array encompassing 300 km of the east Florida coast to assess what factors influence site fidelity of juvenile lemon sharks (Negaprion brevirostris) to an exposed coastal nursery at Cape Canaveral, and to document the timing and rate of their seasonal migrations. Movements of 54 juvenile lemon sharks were monitored for three years with individuals tracked for up to 751 days. While most sharks demonstrated site fidelity to the Cape Canaveral region December through February under typical winter water temperatures, historically extreme declines in ocean temperature were accompanied by rapid and often temporary, southward displacements of up to 190 km along the Florida east coast. From late February through April each year, most sharks initiated a northward migration at speeds of up to 64 km day−1 with several individuals then detected in compatible estuarine telemetry arrays in Georgia and South Carolina up to 472 km from release locations. Nineteen sharks returned for a second or even third consecutive winter, thus demonstrating strong seasonal philopatry to the Cape Canaveral region. The long distance movements and habitat associations of immature lemon sharks along the US southeast coast contrast sharply with the natal site fidelity observed in this species at other sites in the western Atlantic Ocean. These findings validate the existing multi-state management strategies now in place. Results also affirm the value of collaborative passive arrays for resolving seasonal movements and habitat preferences of migratory coastal shark species not easily studied with other tagging techniques.

The Great Melting Pot. Common Sole Population Connectivity Assessed by Otolith and Water Fingerprints

Quantifying the scale and importance of individual dispersion between populations and life stages is a key challenge in marine ecology. The common sole (Solea solea), an important commercial flatfish in the North Sea, Atlantic Ocean and the Mediterranean Sea, has a marine pelagic larval stage, a benthic juvenile stage in coastal nurseries (lagoons, estuaries or shallow marine areas) and a benthic adult stage in deeper marine waters on the continental shelf. To date, the ecological connectivity among these life stages has been little assessed in the Mediterranean. Here, such an assessment is provided for the first time for the Gulf of Lions, NW Mediterranean, based on a dataset on otolith microchemistry and stable isotopic composition as indicators of the water masses inhabited by individual fish. Specifically, otolith Ba/Ca and Sr/Ca profiles, and δ13C and δ18O values of adults collected in four areas of the Gulf of Lions were compared with those of young-of-the-year collected in different coastal nurseries. Results showed that a high proportion of adults (>46%) were influenced by river inputs during their larval stage. Furthermore Sr/Ca ratios and the otolith length at one year of age revealed that most adults (∼70%) spent their juvenile stage in nurseries with high salinity, whereas the remainder used brackish environments. In total, data were consistent with the use of six nursery types, three with high salinity (marine areas and two types of highly saline lagoons) and three brackish (coastal areas near river mouths, and two types of brackish environments), all of which contributed to the replenishment of adult populations. These finding implicated panmixia in sole population in the Gulf of Lions and claimed for a habitat integrated management of fisheries.

Density-dependence can be revealed by modelling the variance in the stock–recruitment process: an application to flatfish

Abstract Recruitment success in marine species is mostly driven by the high and variable mortality of first life stages, and the relationships between stock and recruitment are then largely dominated by residual variability. We show that analysing the residual variability may provide insights on the density-dependence process occurring during the recruitment. Following the seminal formulation of Minto et al. (Survival variability and population density in fish populations. Nature, 2008), we show that when recruitment is considered as a sequence of a pelagic stage with stochastic density-independent mortality followed by a second stage with stochastic density-dependent mortality, then the variability of the recruitment rate per spawning biomass (RPSB) should be a decreasing function of the spawning biomass. Using stock–recruit data of 148 stocks from the RAM legacy database, we provide a test of this hypothesis by showing that the variability of RPSB is lower for fish species with the higher concentration during juvenile stages. Second, a hierarchical Bayesian model (HBM) is built to derive a meta-analysis of stock–recruit data for 39 flatfish stocks, characterized by a high concentration of juveniles in coastal nursery habitats. Results of the HBM show that the variance of the RPSB decreases with the spawning biomass for almost all stocks, thus providing strong evidence of density-dependence during the recruitment process. Finally, we attempt to relate patterns in recruitment variance to relevant life-history traits of flatfish species.

Modeling the population dynamics of lemon sharks

BackgroundLong-lived marine megavertebrates (e.g. sharks, turtles, mammals, and seabirds) are inherently vulnerable to anthropogenic mortality. Although some mathematical models have been applied successfully to manage these animals, more detailed treatments are often needed to assess potential drivers of population dynamics. In particular, factors such as age-structure, density-dependent feedbacks on reproduction, and demographic stochasticity are important for understanding population trends, but are often difficult to assess. Lemon sharks (Negaprion brevirostris) have a pelagic adult phase that makes them logistically difficult to study. However, juveniles use coastal nursery areas where their densities can be high.ResultsWe use a stage-structured, Markov-chain stochastic model to describe lemon shark population dynamics from a 17-year longitudinal dataset at a coastal nursery area at Bimini, Bahamas. We found that the interaction between delayed breeding, density-dependence, and demographic stochasticity accounts for 33 to 49% of the variance in population size.ConclusionsDemographic stochasticity contributed all random effects in this model, suggesting that the existence of unmodeled environmental factors may be driving the majority of interannual population fluctuations. In addition, we are able to use our model to estimate the natural mortality rate of older age classes of lemon sharks that are difficult to study. Further, we use our model to examine what effect the length of a time series plays on deciphering ecological patterns. We find that—even with a relatively long time series—our sampling still misses important rare events. Our approach can be used more broadly to infer population dynamics of other large vertebrates in which age structure and demographic stochasticity are important.ReviewersThis article was reviewed by Yang Kuang, Christine Jacob, and Ollivier Hyrien.

Effect of wind stress on the catch of Japanese anchovy Engraulis japonicus off northwestern Kyushu, Japan

Effect of wind stress on the annual catch of Japanese anchovy Engraulis japonicus off northwestern Kyushu for the period between 1963 and 2009 was investigated. Regime shift analysis detected several step changes in catch and environmental variables. Since the mid-1980s, the anchovy catch in the coastal fishery zones has declined, while the catch in the offshore zone has increased. The decline of catch in the coastal zones showed a significant correlation with the long-term variations in prevailing north-northeastward wind stress over the Goto-Nada Sea during spring spawning season. The results indicated that weakened north-northeastward winds caused the recent low recruitment of anchovy through low levels of wind-induced eggs and larval transport from the offshore spawning ground to the coastal nursery areas, resulting in the potential shift of nursery area to the northwestern offshore region. Thus, as well as the growth-favorable ambient temperature, transport process would play a key role on long-term fluctuations in anchovy abundance in these coastal seas.

A hierarchical Bayesian model for embedding larval drift and habitat models in integrated life cycles for exploited fish

This paper proposes a hierarchical Bayesian framework for modeling the life cycle of marine exploited fish with a spatial perspective. The application was developed for a nursery-dependent fish species, the common sole (Solea solea), on the Eastern Channel population (Western Europe). The approach combined processes of different natures and various sources of observations within an integrated framework for life-cycle modeling: (1) outputs of an individual-based model for larval drift and survival that provided yearly estimates of the dispersion and mortality of eggs and larvae, from spawning grounds to settlement in several coastal nurseries; (2) a habitat suitability model, based on juvenile trawl surveys coupled with a geographic information system, to estimate juvenile densities and surface areas of suitable juvenile habitat in each nursery sector; (3) a statistical catch-at-age model for the estimation of the numbers-at-age and the fishing mortality on subadults and adults. The approach provided estimates of hidden variables and parameters of key biological significance. A simulation approach provided insight to the robustness of the approach when only weak data are available. Estimates of spawning biomass, fishing mortality, and recruitment were close to the estimations derived from stock-assessment working groups. In addition, the model quantified mortality along the life cycle, and estimated site-specific density-dependent mortalities between settled larvae and age-0 juveniles in each nursery ground. This provided a better understanding of the productivity and the specific contribution of each nursery ground toward recruitment and population renewal. Perspectives include further development of the modeling framework on the common sole and applications to other fish species to disentangle the effects of multiple interacting stress factors (e.g., estuarine and coastal nursery habitat degradation, fishing pressure) on population renewal and to develop risk analysis in the context of marine spatial planning for sustainable management of fish resources.

Ecological relationship between environmental factors and Pacific cod (Gadus macrocephalus) catch in the southern East/Japan Sea

The habitat characteristics of Pacific cod (Gadus macrocephalus) and the ecological relationship between Pacific cod catch and environmental parameters in the southern East/Japan Sea were investigated. Pacific cod is sexually matured at age of 4–5. The highest catch of Pacific cod off the eastern Korean Peninsula occurred in seawater temperature range of 0–4°C and salinity range of 33.8–34.2 practical salinity unit at 100 m. The Cross-Correlation Function analysis showed that there was a significant negative correlation between Pacific cod catch and February seawater temperature at 75 m nearby spawning area with a time lag of 5 years in Korean waters (r = –0.360, p < 0.05). The annual catch of Pacific cod also had a significant correlation (p < 0.05) with zooplankton biomass in coastal nursery areas with a time lag, i.e., the highest ones, r = 0.432 and r = 0.452, for total zooplankton biomass in June of 5 years ago, and for euphausiids abundance in February of 4 years ago from the period when Pacific cod was caught, respectively. Furthermore, total catches of Pacific cod in the southern East/Japan Sea (i.e., sum of Korean and Japanese catches) were negatively correlated with winter Arctic Oscillation Index, and the highest negative correlation (r = –0.364, p < 0.05) was shown with a time lag of 4 years. Considering sexual maturation at ages 4–5, such statistical analyses imply that biotic and abiotic environmental factors during the early life stages of Pacific cod have a significant influence on recruitment after 4–5 years of spawning.

Modeling Fish Egg Production and Spatial Distribution from Acoustic Data: A Step Forward into the Analysis of Recruitment

To date, there are numerous transport simulation studies demonstrating the relevance of the hydrodynamics for the advection, dispersion and recruitment of early stages of marine organisms. However, the lack of data has conditioned the use of realistic locations for the model setup and configuration in transport studies. This work (I) demonstrates the key role played by the use of the realistic initial position of the eggs of small pelagic fishes in the analysis of late-larval recruitment in coastal nursery areas and (II) provides a general solution for deriving future egg positions and abundances from adult biomass obtained from acoustic surveys and available fecundity data. Using European anchovy in the NW Mediterranean as a case study, we first analyzed the impact of the initial location, timing, egg buoyancy and diel vertical migration of larvae on the potential late-larval recruitment to coastal areas. The results suggested that prior knowledge of the initial spawning grounds may substantially affect the estimates of potential recruitment. We then integrated biological and acoustics-derived data (the biomass and size structure, sex ratio, a weight-batch fecundity model, mean weight, number of fish and mean spawning) to build a predictive model for interannual egg production. This model was satisfactorily contrasted with field data for two years obtained with the Daily Egg Production Method (DEPM). We discuss our results in the context of the fluctuations of European anchovy egg abundance from 2003 through 2010 in the NW Mediterranean and in terms of the potential applicability of the acoustics-based spatial predictive egg production model.

The influence of oceanographic processes on jack mackerel (Trachurus murphyi) larval distribution and population structure in the southeastern Pacific Ocean

Abstract Vásquez, S., Correa-Ramírez, M., Parada, C., and Sepúlveda, A. 2013. The influence of oceanographic processes on jack mackerel (Trachurus murphyi) larval distribution and population structure in the southeastern Pacific Ocean. – ICES Journal of Marine Science, 70: 1097–1107. The distribution of jack mackerel larvae in the main oceanic spawning area of the southeastern Pacific Ocean was investigated through three consecutive spring bio-oceanographic surveys (2003–2005). In this study, otolith microstructure analysis revealed a spatial age gradient with the smallest/youngest larvae specimens found primarily in the offshore area and the largest/oldest found in the coastal area, implying offshore-inshore larval drift. This suggests a connection between the oceanic spawning area and the historical coastal nursery ground (north of 30°S). In order to understand the oceanographic processes that drive this larval transport, we inferred circulation patterns from two data sources: mesoscale eddy trajectories identified by applying the Okubo-Weiss parameter to satellite geostrophic currents, and 20 years of satellite tracking data of drifters. Our results showed that eddy trajectories lead to net northwestward offshore transport (the opposite direction of larval connectivity). In addition, mean circulation associated with the subtropical anticyclonic gyre and recurrent energetic meandering structures seem to be the major mechanisms driving the spatial dynamics of the early jack mackerel life history, determining a net transport to nursery grounds. These mechanisms could play a key role in recruitment, which supports the continuity of the jack mackerel population.

Nursery habitat availability limits adult stock sizes of predatory coastal fish

AbstractHabitat protection is a strategy often proposed in fisheries management to help maintain viable populations of exploited species. Yet, quantifying the importance of habitat availability for population sizes is difficult, as the precise distribution of essential habitats is poorly known. To quantify the contribution from coastal nursery habitats to exploited fish population sizes, we related adult density to the amount of nursery habitat available for 12 populations of the two dominant predatory fish species in a 40 000-km2 archipelago area of the Baltic Sea. Habitat distribution was mapped using three conceptually different techniques, Maxent, generalized additive models, and random forest, using spawning and 0-group point samples. Adult densities were estimated from gillnet surveys. Regressions demonstrated no evident effect from fishing, whereas habitat availability had a positive effect, explaining almost half of the variation in population sizes of both species. This result shows that a substantial proportion of the potential production of adult fish can be estimated by mapping essential nursery habitats distribution. Responses were non-linear, indicating that habitat protection has largest effects where there is little available habitat. By demonstrating the importance of habitat limitation of two exploited fish species, we provide quantitative support to the benefits of habitat protection for fisheries.

Changes in potential North Sea spawning grounds of plaice (Pleuronectes platessa L.) based on early life stage connectivity to nursery habitats

We explored the hypothesis that spawning ground locations of North Sea plaice reflect the locations of nursery grounds using drift scenarios based on a baroclinic, shallow-water circulation model (HAMSOM). The transport of pelagic eggs and larvae was simulated each year from 1975 to 2006 using in situ forcing, temperature-dependent development and stage-specific behaviour of eggs and larvae. This long-term simulation period also allowed us to explore climate effects. A release position was considered a potential and suitable spawning site if larvae from that area reached coastal nurseries after the onset of metamorphosis. In general, larvae were transported in an anti-clockwise direction and settled in nurseries that were relatively close to the release positions. Spawning locations that were offshore were poorly connected to nursery grounds while those closer to the shore had higher connectivity. Simulated suitable spawning locations broadly agreed with the main centres of egg production (English Channel, Southern Bight, German Bight), except for the known spawning grounds south of Dogger Bank. Over the 31-year simulation period, positive and negative trends in transport success were found for the western and eastern parts of the North Sea, respectively. Changes in the west (Flamborough Head) were mainly due to changes in water circulation patterns whereas those in the east (northern German Bight) were induced by changes in both currents and water temperature. The implications of these findings, and the significant correlation between changes in drift and recruitment, suggest that climate-driven changes in the suitability of nursery grounds will directly affect the distribution and productivity of plaice in the North Sea.

Comparison of the reproductive ecology of two sympatric blacktip sharks (Carcharhinus limbatus and Carcharhinus tilstoni) off north‐eastern Australia with species identification inferred from vertebral counts

Precaudal vertebral counts were used to distinguish between 237 morphologically similar Carcharhinus limbatus and Carcharhinus tilstoni and were congruent with differences in reproductive ecology between the species. In addition to differing lengths at maturity and adult body size, the two species had asynchronous parturition, were born at different sizes and the relative frequencies of neonates differed in two coastal nursery areas. Despite evidence that hybridization can occur, these differences suggest the species are largely reproductively isolated.

The role of structuring benthos for juvenile flatfish

Within coastal nurseries, the distribution of juvenile flatfish may depend on small-scale habitat variability. The presence of ecosystem engineers is known to have important impacts in coastal sediments. Lanice conchilega is a well-known marine ecosystem engineer of shallow soft bottom ecosystems, shaping the macrobenthic community and attracting flatfish. The present study examines the relation between juvenile flatfish and L. conchilega reefs through two experiments. In a field experiment in the Dutch part of the North Sea, the benthic habitat is evaluated by comparing relative differences in numbers of juvenile flatfish between ecosystem engineered habitats and adjacent bare sand (i.e. non-ecosystem engineered) habitats. The hypothetical shelter seeking behaviour was further examined using stomach content analyses. Results show that juvenile plaice Pleuronectes platessa was the dominant species within the tube worm habitat and the species selects specifically for this biogenic habitat. This selection was explained as feeding behaviour. In a complementary laboratory study, food was excluded and the shelter function of the ecosystem engineered habitat was investigated. This experiment quantifies the selection for this habitat by juveniles of the common sole Solea solea. Results from the flume experiment, manipulating the number of tube worms, show that distribution of sole was not random when current velocities are high. The selected habitat is the one with low density tube worm aggregations. Overall, we conclude that structuring benthos plays an important role for juvenile flatfish, both as refuge and as feeding ground.

Sources of organic matter for flatfish juveniles in coastal and estuarine nursery grounds: A meta-analysis for the common sole (Solea solea) in contrasted systems of Western Europe

Coastal and estuarine nursery grounds are essential habitats for sustaining flatfish stocks since only these shallow and productive areas provide the high food supply that allows maximizing juvenile growth and survival in most flatfish species. However, the main organic matter sources at the basis of benthic food webs might differ drastically between estuarine nursery grounds under strong freshwater influences, where food webs are mainly supported by continental organic matter, and coastal ecosystems under limited freshwater influence, where the local marine primary production is the main source of carbon for the benthos. To better understand the links between continental inputs to the coastal zone and stock maintenance in the highly prized common sole, Solea solea (L.), we investigated the variability in the organic matter sources supporting the growth of its young-of-the-year (YoY) in five contrasted estuarine and coastal nursery grounds under varying freshwater influence. Stable isotopes of carbon and nitrogen allowed tracing the origin of the organic matter exploited by YoY soles in the very first months following their benthic settlement, i.e. when most of the juvenile mortality occurs in the species. A mixing model was run to unravel and quantify the contribution of all major potential sources of organic matter to sole food webs, with a sensitivity analysis allowing assessment of the impact of various trophic enrichment factors on model outputs. This meta-analysis demonstrated a relative robustness of the estimation of the respective contributions of the various organic matter sources. At the nursery scale, the upstream increase in freshwater organic matter exploitation by YoY soles and its positive correlation with inter-annual variations in the river flow confirmed previous conclusions about the importance of organic matter from continental origin for juvenile production. However, inter-site differences in the organic matter sources exploited for growth showed that, although freshwater organic matter use is significant in all nursery sites, it is never dominant, with especially high contributions of local primary production by microphytobenthos or saltmarsh macrophytes to juvenile sole growth in tidal nursery ecosystems. These patterns stress the need for maintaining both the intensity of freshwater inputs to the coastal zone and of local autochthonous primary production (especially that of the intertidal microphytobenthos) to preserve the nursery function of coastal and estuarine ecosystems.