Deepwater Sharks at Their Northern Limits—Distribution, Diet and Trophic Relations

Abstract

Deepwater areas and continental slope regions south and west off Iceland are characterized by a relatively high species diversity and abundance of sharks that likely play a critical ecological role within this ecosystem. The study investigates trophic ecology of the sharks employing stomach content analysis and stable isotope analysis of nitrogen and carbon. In a standardized groundfish survey in Icelandic waters spanning 28 years, sharks were exclusively caught south of the Greenland–Iceland ridge and west of the Iceland–Faroe ridge, likely due to significantly lower seawater temperatures prevailing north of the ridges. The sharks exhibited a diverse diet primarily comprising secondary, and tertiary consumers, placing them within the lower spectrum of the fourth trophic level with estimated trophic levels ranging from 4.1 to 4.5 depending on the species or method employed. Three distinct feeding categories emerged: the first characterized by a main predation on various fish species, the second involving a specialized predation on mesopelagic fish and the third strategy encompassing a significant feeding on crustaceans. Resource portioning was evident through low to medium diet overlap indices among some of the shark species. Ontogenetic changes were observed in two of the most abundant and data-rich species, namely the black dogfish (Centroscyllium fabricii) and great lanternshark (Etmopterus princeps). As these sharks increased in size, there was a dietary shift from crustaceans to fish, reflected in higher trophic levels based on δ¹⁵N values. Given the scarcity of sharks in stomachs of demersal fish within the study area, coupled with low fishing pressure on sharks off Iceland, it is plausible that their abundance is currently regulated primarily by bottom-up processes or environmental factors.