Frequently trawled grounds show low levels of species diversity, abundance and biomass of invertebrate animals compared to less frequently trawled areas. It has long been assumed, quite reasonably, that these reduced abundances likely means less food for fish that feed on such creatures.
In turn, more recent studies have also linked such reductions in food resources to declines in fish stock biomass and individual fish condition. Put simply, we predict that some fish living on frequently trawled grounds are skinnier than fish in un-fished areas, probably due to a lack of food.
A new study, published in the Proceedings of the Royal Society of London B, that links trawling, fish food and fish living in trawled areas says this may not always be the case. Although fish on trawled ground might be skinnier, it is not because they are hungry.
The study conducted by researchers from the School of Ocean Sciences, Bangor University (UK) used a detailed dietary analysis of commercial plaice and dab to see what is really happening on trawled fishing grounds.
The team’s field site was a nephrops fishing ground in the Irish Sea that shows a large range in trawl frequency from zero to 16 trawl passes per year. By analysing the stomach contents of plaice and dab from different sites, the team was able to explain why some fish were skinner in trawled fishing grounds than in other areas.
At the study site, as trawling increased there was a reduction in the amount of food available for the fish to eat, as expected. It was also noted that of the two species studied, plaice became thinner at trawled sites whereas dab showed no significant change in body condition. Strangely though, stomach fullness was uniform across all test sites.
Fish at heavily trawled sites were just as full as those at non-fished sites. The missing piece of the puzzle came when looking at the different organisms within the fishes’ stomachs.
Dab appeared to be a generalist feeder; able to eat most species it came across on the seafloor. The more specialised plaice, was unable to adjust to the altered prey communities found at more trawled sites, most likely having to search harder and longer for its few selected prey items.
By comparing the energy content of the different prey species eaten by each fish, it again became obvious that the energy rich diet of dab leaves it in a much better position to cope with the bottom trawl impacts than plaice.
Being such a fussy eater and restricted to specific prey items, large changes in what was available to plaice meant that instead of making different food choices, the plaice has to spend more time foraging for fewer, less energy rich prey. Plaice is not as resilient to trawl impacts as dab.
What does this mean in the grand scheme of things? It means long term, in heavily trawled areas, plaice catches may be poorer than those of dab. This is not only a consequence of plaice being skinnier. Body condition often translates to reproductive success and skinnier fish are likely to be less successful breeders. This has long term consequences for plaice populations in areas with limited or no management of seafloor trawl impacts.
Increased trawling has the potential to reduce future biomass and economic yields of species like plaice that are unable to adapt to changing sea floor conditions. Although fish may not go hungry, changes in foraging behaviors, lower energy gains and ultimately skinnier specialist feeders are likely to have important consequences for bottom trawl fisheries in our coastal seas.
