Paper: Johnson AF, Gorelli G, Jenkins SR, et al (2014) Effects of bottom trawling on fish foraging and feeding. Proceedings of the Royal Society B: Biological Sciences 282:20142336–20142336. doi: 10.1098/rspb.2014.2336
When the seafloor is trawled, more than what is targeted is captured. The overall effect is an anthropogenically altered food web decreased in total density. The remaining fishes now have a different buffet to choose from because the nets have also captured prey items. For some fishes there may be a reduction of their preferred prey, for others their ideal prey may not have been greatly affected. Some fishes may benefit if their competitors are reduced, or if their favorite dinner items have been injured and cannot easily flee. This also means that fishes that are picky are likely to suffer if their favorite food disappears, while fishes with diverse taste are less vulnerable to change. And like anyone who changes their diet knows, this all has an impact on body condition and possibly population levels.
Enter, Johnson et al.
Johnson et al. wanted to understand how bottom trawling in the Irish Sea, UK, indirectly affects fish populations by their impact on prey populations. They chose to analyze the stomach contents of two commercially important flat fish species, plaice (P. platessa) and dab (Limanda limanda), to see how heavy trawling affects their diets. Plaice uses a suction-like method to eat prey of limited mobility (i.e. polychaetes and bivalves), while dab are opportunistic feeders targeting highly mobile prey (i.e. crustaceans). The research team expected to see a reduction in biomass and lower energy food in plaice stomachs when trawling becomes more intense. Conversely, they expected dab stomachs to be relatively unaffected because of their opportunistic hunting tactics.
Johnson et al. found that trawling results in the decline of both fish and prey biomass, so the ratios of fish to prey remained unchanged. Significantly, they were able to show for the first time, that fish living in heavily trawled areas still found plenty of food to fill their stomachs despite the alterations to composition and quantity of their food choices.
At the end of the day, fishes didn’t go hungry. Dab showed no change in body condition and the observed reduction in body condition of plaice was not because there was less to eat overall. Johnson et al. attribute the worsening of the plaice condition to potential reductions in foraging efficiency. Plaice spend more energy (swimming, searching, and foraging) than before to find enough food to fill-up their bellies. Additionally, the food they find (primarily the bivalve, A. alba) is not as energy rich as the food they would find in lesser-trawled areas (primarily polychaeta, Nephtys spp.).
For plaice, rather than reduced feeding, the increased foraging effort is a potential mechanism that could affect the fish body condition. In contrast, dab are largely unaffected by trawling because they have different feeding strategies than plaice. Dab can readily adapt to trawling-induced disturbance because they target larger, more energy-rich food sources (crustaceans) meaning they can spend less time foraging and more time resting between meals. The team also suggested the possibility that the particular prey which dab targets are not reduced in density by the trawl, meaning they are more abundantly available for ingestion.
There is plenty more to investigate about the effects of bottom trawling on predator-prey relationships, but for the meantime, Johnson et al. have made an important contribution to the details surrounding mitigation measures for fisheries management.