Scavenging on trawled seabeds can modify trophic size structure of bottom-dwelling fish

Disturbance by towed bottom-fishing gears often kills larger sensitive benthos, leading to changes in the abundance, size, and species composition of benthic communities. Short-term availability of trawl-damaged prey, and longer-term shifts in benthic prey community composition, both have the potential to affect feeding opportunities and realized dietary preferences of bottom-feeding (benthivorous) fish. To investigate these effects of bottom-fishing activity (by otter trawls, beam trawls, and dredges) on the feeding of benthivorous fish, we compared the trophic level at body size and diets of four species in areas of the Celtic Sea subject to low, intermediate, and high fishing activity. Trophic level was estimated using nitrogen stable isotope analysis, and fishing activity was quantified with vessel monitoring system (VMS) data. For whiting (Merlangius merlangus) of all sizes, trophic level was slightly lower in areas of higher fishing activity. After accounting for the results of the diet analysis, we concluded that this reflected increased scavenging of benthic invertebrates in more intensively fished areas. For megrim (Lepidorhombus whiffiagonis), the rate of increase in trophic level with size was lower with increasing fishing activity, implying that megrim may also substitute fish with lower-trophic invertebrates that can be scavenged in more intensively fished areas. For plaice (Pleuronectes platessa) and lemon sole (Microstomus kitt), no significant effects of fishing activity on trophic level were detected. We conclude that differences in the intensity of fishing activity with towed bottom gears had small but variable effects of the trophic size structure of the four species, and that this primarily reflected scavenging rather than diet changes following longer-term shifts in composition of the prey community.