Predator-prey trophic relationships in temperate pelagic ecosystems.

Supervisors: Maite Louzao(AZTI), Joan Giménez Verdugo (MaREI-UCC)
Marine ecosystems hold an entangled framework of relationships between ecosystemcomponents. When analysing community dynamics some ecosystem componentsand/or species could play a major role than others in structuring ecosystems, howeverthe strength of interactions must be considered, as drastic changes in one abundantprey is expected to have consequences on the integrity of the ecosystem. Analysis ofcarbon (δ13C) and nitrogen (δ15N) stable isotopes has proved to be a powerful tool todescribe the trophic ecology and trophic relationships between marine organisms, asthe isotopic signature of a consumers’ tissues reflects the isotopic signatures of foodsources, proportional to their dietary contribution integrated over time.The Bay of Biscay (BoB, North-East Atlantic) is a highly harvested area by fisheries,that exploit the same habitats as several marine megafauna species such ascetaceans and seabirds. The BoB represents a relatively mature and stableecosystem, with a large proportion of its energy flow originating from detritus (Lassalleet al., 2011). Furthermore, forage fish have shown to have a key role in marine pelagicecosystems in the BoB, as they conform the main pathway by which energy istransported from lower trophic levels to apex predators.In this master’s thesis, we have developed an integrative ecological study to improveour knowledge on the existing trophic relationships between different pelagiccomponents. Firstly, we reunited all stable isotope published data in the BoB and weidentified trophic gaps in the characterization of pelagic ecosystems based on aliterature review. Secondly, we studied the trophic ecology of seabirds, one of the lessstudied marine megafauna functional group in the BoB. Specifically, northern gannets(Morusbassanus) diet was analysed through stable isotope analysis, which has neverbeen determined before in the BoB. Mixing models indicated that northern gannetsprimarily feed on blue whiting, and identified an additional important feeding resourcecomposed by the cluster of juvenile European hake, Atlantic mackerel and Atlantichorse mackerel. Finally, we assessed the trophic overlap of the northern gannet withother taxonomic groups of marine megafauna such as seabirds, cetaceans and largepredatory fishes. Trophic niche overlap was found with the common guillemot andother cetaceans (i.e. the harbour porpoise, the short-beaked common dolphin and thelong-finned pilot whale), indicating trophic similarities between these species.