Stable isotopes reveal the effect of trawl fisheries on the diet of demersal target species

Bottom trawling can change food availability for benthivorous demersal target species by (i) changing benthic prey composition through physical impact and (ii) by reducing intra- and inter-specific competition for prey by removing other benthic consumers. Thus trawling may both negatively and positively influence the quantity and quality of food available. Using δ13C and δ15N we investigated potential diet changes of three commercial species across trawling gradients in the Kattegat [plaice, dab and Norway lobster (Nephrops)] and the Irish Sea (Nephrops). In the Kattegat trawling affected primarily the biomass of benthic consumers, lowering competition. Nephrops showed significant positive relationships for δ13C and a humped relationship for δ15N with trawling. In the Irish Sea intense trawling had a negative effect on benthic prey. δ13C and δ15N thus showed the inverse relationships to those observed in the Kattegat. Plaice from the Kattegat, showed a significant relationship with trawling intensity for δ13C, but not for δ15N. No relationship was found for dab. Changes of δ13C and δ15N correlated with changes in condition of species. The results show that the removal of demersal competitors and benthos by trawling can change the diets of target species, ultimately affecting their body condition.