Trophic ecology of large gadiforms in the food web of a continental shelf ecosystem

Abstract

Human activities affect continental shelves, especially due to the harvest of living marine resources. Understanding their functioning and dynamics has become a growing concern in recent decades, especially through use of trophic modelling approaches. Studying the feeding ecology of key component species also improves this understanding by providing accurate information on trophic processes, particularly the dependence on trophic pathways. This study focuses on the trophic ecology of four large gadiforms (cod, haddock, whiting and hake) found on the continental shelf of the Celtic Sea. The study combines information on recently ingested prey (gut content analysis) and a more integrated indicator of food sources (stable isotope analysis). Two size classes of each species were considered (small and large – split around 20 cm for haddock, whiting and hake and 60 cm for cod), as were shallow and deep zones. The four gadiforms show distinct feeding niches and strategies, which limit interspecific competition. Cod and haddock relied mainly on the benthic trophic pathway but differed in favouring piscivory and invertivory, respectively. Hake fed mainly on the pelagic trophic pathway, while whiting seemed to feed on both pathways. Ontogenetic shifts from invertivory to piscivory were observed for whiting in both zones and for hake and cod in the deep zone. An unexpected shift from the pelagic to the benthic trophic pathway was observed from small to large haddock in the deep zone. Taking into account the descriptions of diets made in adjacent ecosystems throughout the year, the four species show trophic plasticity towards one of the trophic pathways or the other. Differences between the shallow and deep zones were related to benthic and pelagic prey characteristics, which themselves are driven by the sedimentary and hydrological context, especially thermal stratification.