Diet composition of the hooknose (Agonus cataphractus, L.) in different habitats of coastal waters in the German Wadden Sea (southern North Sea)

The Wadden Sea is an important habitat for a large number of fish species, supporting functions such as reproduction, breeding and feeding. The hooknose (Agonus cataphractus, Linnaeus, 1758) is a resident demersal fish species of the Wadden Sea, but due to its non-commercial importance it is rarely studied. In the present study, the feeding strategy and prey selection of the hooknose related to the benthic in- and epifauna in the field were investigated in six different habitat types (circalittoral sand, circalittoral mud, circalittoral mixed sediments, circalittoral coarse and gravel sediments, sublittoral sandbanks and reefs) in coastal waters of the German Wadden sea between 2020 and 2021. The stomach contents of A. cataphractus were dominated in most of the habitat types by very mobile epibenthic prey species, mainly the brown shrimp (Crangon crangon, Linnaeus, 1758), amphipods (Ampelisca spp., Microprotopus maculatus, Norman 1867) and cumaceans (Diastylis bradyi, Norman 1879, Pseudocuma longicorne, Bate 1858), similar to the benthic communities in the field. Infauna species such as the tube-building polychaete Lanice conchilega (Pallas 1766) were rarely consumed and potentially avoided due to their ability to withdraw below the feeding depths. Our results further showed habitat-specific differences in the hooknose diet, especially in reefs, where skeleton shrimps (caprellids) dominated the diet. The habitat type also influences the condition of A. cataphractus being highest in reefs but lowest on sandbanks. Our results highlight the importance of knowledge functional relationships between smaller-sized Wadden Sea fish species and its typical habitats, which in turn is essential for applicable management measures of the whole Wadden Sea area.