Reconstruction of the past trophic position of lost top predators in a temperate coastal fish food web

Over the last century substantial changes have taken place in the fish community of the Dutch coastal North Sea zone with the loss of top predators, such as sharks and skates. Whether their disappearance has changed the trophic structure of these shallow waters, has not been properly investigated. In this study historical data of 9 species of sharks and skates, either being (near)-residents (one species), juvenile marine migrants (three species) and marine seasonal visitors (five species) of the Dutch coastal North Sea zone were analysed. (Near)-resident and juvenile marine migrants species had a demersal and all marine seasonal visitors species a pelagic lifestyle. Based on stomach content composition, trophic position of the various shark and skate species were reconstructed. Near-residents (lesser spotted dogfish) and the marine juvenile migrants (smooth hound, thornback ray and spur-dog) had a benthic/demersal diet (polychaetes, molluscs and crustaceans), while the pelagic marine seasonal visitors fed dominantly on cephalopods and fishes. Diet overlap occurred for fish (tope shark and lesser spotted dogfish), for hermit crabs (lesser spotted dogfish and smooth hound) and for shrimps (thornback ray, common stingray and smooth hound). Trophic position ranged from 3.2 for basking shark preying exclusively on crustaceans to 4.6 for the porbeagle consuming exclusively fishes. The calculated trophic positions of shark and skate species indicated that they were not at the top of the trophic pyramid but might have been on top of their own ecological niche.