Séminaire (présentation en Français) au LOG par Carolina Giraldo (Post Doc au LOG) intitulé :
Le titre du séminaire :
"Influence of depth on the food web structure and trophic niche overlap among fishes from the English Channel"
Date : le 14 octobre 2016 à 11h
Lieu : LOG salle de conférences à la Station Marine de Wimereux
Depth is one of the main environmental variables influencing the structure of marine food webs by either directly or indirectly influencing benthic-pelagic coupling and predator-prey relationships. In shallow coastal environments the high degree of connectivity between pelagic and benthic networks results in complex systems with multiple interactions that can be difficult to understand. We used stomach contents (SC) and stable isotopes analysis (SIA) to investigate depth-related changes in feeding patterns for 33 fish species from the eastern English Channel (EEC) collected between 5 and 80m depth. Species were first aggregated into functional groups (i.e. trophic groups) depending on trophic and habitat similarities. Stomach contents were used to determine the general topology of the food web (ie. Trophic links) and SIA was used to estimate the proportions of different sources to fish-groups’ diet (Bayesian mixing model IsoWeb). For example, benthic-suspension feeders (mainly bivalves) were the main food source for endobenthic-feeding fish (29% of the diet) and were as important as pelagic subsidies in planktivorous fish (31%). The EEC aggregated food web was then used as a template to explore the influence of depth on the resource partitioning among predatory fish. Mixing models including depth as a continuous covariate successfully untangled and identified different feeding strategies among functional groups. In general, species could benefit from both, pelagic and benthic prey in shallow waters ( 25m). In deeper waters, species feed predominatly in either benthic or pelagic sources depending on the specie’s habitat preferences. Our results support the hypothesis of a stronger benthic-pelagic coupling in shallow waters and highlight the importance of including environmental factors such as depth as a proxy of habitat variation to fully understand resource use and food web structure in highly exploited epicontinental seas. Furthermore, we analyzed the effect of depth on niche width (Bayesian standard ellipse areas, SEAb) and overlap at the species’ level of the 10 main commercial fish species from the English Channel belonging to diverse functional groups : pelagic piscivorous (e.g., Atlantic mackerel), demersal piscivorous (e.g., European seabass), benthic feeders (e.g., Small-spotted catshark) and endobenthic feeders (e.g., Common sole). At the specie’s level our results highlight the importance of accounting for feeding strategy and habitat when comparing isotopic niches across species at the population level.