Size-related trophodynamic changes in three target fish species recovering from intensive trawling

Abstract.

The consequences for food-web structure of protecting marine communities from trawling have been little explored. We investigated the Gulf of Castellammare, northwest Sicily, where a trawl ban initiated in 1989 has led to great increases in the biomass of some target species. Using stable isotopes of carbon and nitrogen, our objectives were to see if mean trophic level and omnivory had increased after the ban in three of the most common fishery-target species, namely, the anglerfish Lophius budegassa, Mediterranean hake Merluccius merluccius, and red mullet Mullus barbatus. We compared size data from before and after the fishery closure, but we also compared recently derived data from outside and inside the closed area. In all three species δ¹³C and δ¹⁵N were found to differ markedly depending on fish size. δ¹³C was found to decrease in L. budegassa and Merluccius merluccius but increase in Mullus barbatus with increasing fish length. δ¹⁵N increased in all three species, and this was thought to reflect feeding at increasingly higher trophic levels during the animals' lives. Mean length, and therefore δ¹⁵N-derived trophic level, increased after the trawl ban only in the anglerfish L. budegassa. Based on δ¹³C and δ¹⁵N data, none of the species became more or less omnivorous after the ban. After 9 years of no trawling, increases in numerical abundance were not accompanied by substantial size-related trophodynamic shifts in any of the three species of fish studied.