Bottom trawling leads to hungry fish

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One unintended consequence of bottom trawling is skinnier bottom-feeding fish, according to a study to be published in a forthcoming issue of the Proceedings of the Royal Society B: Biological Sciences.

Samuel Shephard and coauthors used survey data and statistical models to study the effects of bottom trawling on the populations of six species of fishes in the Celtic Sea: lemon sole (Microstomus kitt), plaice (Pleuronectes platessa), haddock (Melanogrammus aeglefinus), megrim (Lepidorhombus whiffiagonis), cod (Gadus morhua) and whiting (Merlangius merlangus).

Using the weight of an average-length fish as a measure of its body condition, the authors found that bottom trawling had negatively affected body conditions of small and large lemon sole, small megrim and large cod, but positively affected body conditions of large whiting.

The results of the study support the hypothesis that trawling reduces the availability of benthic prey such as crustaceans, echinoderms and polychaete worms. 

This in turn compromises the fitness of fishes dependent on the availability of such prey.  The authors predicted reductions in fish condition by as much as 20 per cent, with such reductions being associated with lower growth, reduced survival and decreased fecundity.

For commercially important species, harvested fish of a given length might be expected to produce lighter fillets of lower value.

Most significantly, a reduction in condition (feeding success) implies that bottom trawling can reduce habitat carrying capacity. This is likely to further diminish fisheries productivity, and impair the recovery of threatened stocks and ecosystems.

For more information, see the paper: Shephard, S, JL Thorley, D Brophy, JG Hiddink, D Stokes and DG Reid (2011) Benthivorous fish may go hungry on trawled seabed. Proceedings of the Royal Society B: Biological Sciences. doi: 10.1098/rspb.2010.2713