How benthic habitats and bottom trawling affect trait composition in the diet of European plaice (Pleuronectes platessa) in the North Sea

Flatfish diet depends on prey availability in the benthic habitats. Prey availability can in turn also depend on bottom trawling disturbances. Overall, bottom trawling shifts the prey diversity in favour of small, opportunistic and short-lived species. Modelling exercises suggest that abundance of prey items with these trait characteristics increases with low trawling effort, but decrease at high effort. To date, little empirical evidence has shown the effect of trawling onto the diet of flatfish, particularly when it concerns functional traits rather than taxonomic metrics. Our study examines the diet of European plaice (Pleuronectes platessa) in the Southern and Central North Sea and hypothesizes that bottom trawling significantly affects the trait composition of its diets. A database with stomach content data was compiled and converted from taxon records into a functional trait information. A suite of ten traits were selected reflecting prey susceptibility to trawling (e.g. size and living habitat) and their abilities to recover (e.g. longevity). The stomach content data were related by location to benthic habitats and high-resolution bottom trawling effort. Benthic habitats were defined using EUSeaMap and regrouped into three main habitat clusters. The diet of plaice in habitat cluster 1 (shallow coastal areas and the Dogger bank) was dominated by deposit-feeding prey that live in the sediment while the trait composition was more diverse in deeper habitats (cluster 2 and 3). Bottom trawling did not change the trait composition of the diet in habitat cluster 2, but altered the diet in habitat cluster 1 significantly. Our findings provide empirical evidence at the scale of fisheries management that bottom trawling may shift plaice diet towards opportunistic species, but that trawling effects are not unequivocal across benthic habitats.