Stomach content and stable isotope analyses provide complementary insights into the trophic ecology of coastal temperate bentho-demersal assemblages under environmental and anthropogenic pressures.

Assessing organic matter fluxes and species interactions in food webs is of main interest to understand the ecological functioning in bays and estuaries characterised by a wide diversity of primary producers and consumers. Demersal fish and cephalopod assemblages were studied across a network of 24 shallow subtidal stations in the bay of Saint-Brieuc for their diversity, stable isotope compositions and stomach contents. The community was composed of 21 taxa, eight species accounting for 94.4% of the total abundance. Three different assemblages were identified along bathymetric gradient and spatial patterns in fish dredging. Marine POM and SOM were the most likely bases of food webs regarding δ13C range displayed by fish and cephalopod without differences among assemblages. Amphipoda was the main prey item in stomachs leading to significant diet overlaps among fish species, with some variations in additional items. Sepia officinalis was characterised by a singular diet and very low dietary overlap with other species. Contrasted stable isotope values and niche overlaps among species were evidenced in the δ13C/δ15N space. Callionymus lyra and Buglossidium luteum, characterised by the widest isotopic niches, encompassed those of other species, except the singular 13C-depleted Spondyliosoma cantharus. Coupling taxonomic assemblages, stomach contents and stable isotope analyses help disentangling the resources uses and evidencing trophic pathways. Contrasts in fish and cephalopod demersal assemblages occurring at different depths not necessarily imply differences in the trophic resources uses in such complex shallow coastal ecosystems under anthropogenic influences.

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.

Development of a fish-based index to assess the ecological quality of transitional waters: the case of French estuaries.

The Water Framework Directive requires the assessment of the ecological status of transitional waters considering the fish component. An original methodology, based on a pressure-impact approach, was established to develop a multimetric fish-based index to characterize the ecological quality of French estuaries. An index of contamination, based on the chemical pollution affecting aquatic systems, was used as a proxy of anthropogenic pressure. The fish metric selection was based on their response to disturbances tested via statistical models (generalized linear models) taking into account sampling strategy and estuarine features. Four metrics, for which discriminating responses to level of pressure were demonstrated, were retained to constitute the estuarine multimetric fish index. This new tool appeared particularly relevant to detect the contamination effects on fish communities in estuaries. It could help managers to take decisions in order to maintain or reach the good status required by the Water Framework Directive for 2015.

Estimates of the mortality and the duration of the trans-Atlantic migration of European eel Anguilla anguilla leptocephali using a particle tracking model.

Using Lagrangian simulations, based on circulation models over three different hydroclimatic periods in the last 45 years in the North Atlantic Ocean, the trans-Atlantic migration of the European eel Anguilla anguilla leptocephali was simulated via the passive drift of particles released in the spawning area. Three different behaviours were modelled: drifting at fixed depth, undergoing a vertical migration or choosing the fastest currents. Simulations included mortality hypotheses to estimate a realistic mean migration duration and relative survival of A. anguilla larvae. The mean migration duration was estimated as 21 months and the mortality rate as 3.8 per year, i.e. < 0.2% of A. anguilla larvae may typically survive the trans-Atlantic migration.