Historical trophic ecology of some divergent shark and skate species in the Dutch coastal North Sea zone

Abstract

Over the last century the fish community of the Dutch coastal North Sea zone has lost most its shark and skate species.Whether their disappearance has changed the trophic structure of these shallow waters has not been properly investigated. In this study historical dietary data of sharks and skates, being in the past (near)-residents, juvenile marine migrants and marine seasonal visitors of the Dutch coastal North Sea zone were analyzed for the period 1946-1954. Near-resident and juvenile marine migrant species were demersal while all marine seasonal visitors species were pelagic. Based on stomach content composition, the trophic position of four of the various shark and skate species could be reconstructed. The (near)-resident species, the lesser spotted dogfish, the marine juvenile migrant, the starry smooth hound, and the benthopelagic marine seasonal visitor, the thornback ray, had a benthic/demersal diet (polychaetes, molluscs and crustaceans), while the pelagic marine seasonal visitor, the tope shark, fed dominantly on cephalopods and fishes. Diet overlap occurred for fish (tope shark and lesser spotted dogfish), for hermit crabs (lesser spotted dogfish and starry smooth hound) and for shrimps (thornback ray and starry smooth hound). Trophic position ranged from 3.2 for thornback ray preying exclusively on crustaceans to 4.6 for the tope shark consuming higher trophic prey (crustaceans and fish). The analysis indicates that most of the shark and skate species were generalist predators. The calculated trophic positions of shark and skate species indicate that those species were not necessarily at the top of the marine ecosystem food web, but they might have been the top predators of their particular ecological assemblage.

Trophic structure and resource utilization of the coastal fish community in the western Wadden Sea: evidence from stable isotope data analysis

“…Whereas skates and sharks used to be common in the North Sea and surrounding coastal areas, nowadays they are either absent or occurring in low densities (Wolff 2005). Predatory shark and skate species had trophic positions (based on historical archive dietary data) in a range of 3.2 to 4.6 (Poiesz et al 2021). Another explanation might be due to trophic downgrading, where food webs are losing complexity and trophic biodiversity due to changing environmental conditions (changing temperatures, eutrophication) and competition (Saleem 2015, Edwards & Konar 2020, Yan et al 2020.…”