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Trawling the seabed doesn’t just remove some of the fishes living there; it also makes some of the survivors thinner and less healthy by forcing them to use more energy finding less nutritious food.
That’s the conclusion of a new paper published in Proceedings of the Royal Society B, based on the work Dr Andrew Frederick Johnson undertook while studying for his PhD at Bangor University.
‘We already knew that some species of bottom-dwelling fish in trawled areas were skinnier than those elsewhere, but until now it was assumed this was because they couldn’t find enough food and went hungry,’ he says.
Johnson’s work sampling fish in the Irish Sea on Bangor University’s research vessel Prince Madog shows that’s not true; the stomachs of fish in trawled areas are as full as elsewhere, but they’re full of different and less nutritious prey that the fish have to put more energy into finding. For instance, a fish with a bellyful of shellfish may feel full, but a lot of their meal is shell with no nutritional value. Much better to fill up on juicy worms – but these suffer more from the effects of trawling.
‘It’s not that the fish are hungry, exactly – it’s that after trawling they are having to spend more energy finding prey that’s much less energy-rich,’ says Johnson, now at Scripps Institution of Oceanography, San Diego. A thinner fish with less energy in reserve will find it harder to cope with stressful situations or to invest in reproduction, which may have long-terms consequences for affected populations.
Trawling involves pulling a net along near the seabed, with chains attached that churn up the sediments and disturb bottom-dwelling fish so they swim up to be caught. It’s a controversial fishing method that makes it much easier to catch fish that live on the sea floor. But at the same time it does serious damage to seabed ecosystems and can kill about a third of the invertebrates that live in the trawl path; recovery sometimes takes years. And this latest study shows the practice doesn’t just affect flatfish directly by catching them; it also kills many of the small invertebrates they eat, so it indirectly affects the health of other fish in the area.
Continue reading at Planet Earth Online.
Read the study at Royal Society Publishing.
Boat image via Shutterstock.
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