Trawling makes for skinny flatfish

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 the Proceedings of the Royal Society of London 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, based on earlier work by Dr Jan Geert Hiddink (2011, Journal of Applied Ecology), but until now it was assumed this was because they couldn’t find enough food and went hungry”.

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 means it’s harder to cope with stressful situations or to invest in reproduction which may have long terms consequences for effected populations.

This is just the latest evidence of the negative consequences trawling has on marine ecosystems. The controversial fishing method involves pulling a net along near the seabed, with chains attached to its bottom that churn up the seabed and disturb bottom-dwelling fish so they swim up to be caught.

Trawling makes it much easier to catch fish like plaice or dab that live on the sea floor. At the same time it does serious damage to seabed ecosystems and can kill about 1/3 of the invertebrates that live in the trawl path, and recovery from this can take years in some areas. And this latest study shows the practice doesn’t just affect flatfish populations directly by catching them; it also kills a large fraction of the small invertebrates they eat, having an indirect effect on the health of some of the fish remaining in the area.

The effect varies from species to species. Johnson found that populations of bottom-dwelling flatfish with a specialised diet, like plaice, were more seriously affected, while there was no effect on generalist feeders like dab.

This means that if trawl fishing carries on with no restrictions or attempts to mitigate the impacts, in areas that are heavily trawled you’d expect the seabed to be increasingly dominated by generalists, at the expense of specialists. That matches what’s happened fairly closely – plaice stocks struggled for many years (until trawling effort was reduced substantially), while dab remained plentiful whether in heavily or lightly trawled areas.

‘Dab are better at catching energy-rich prey, so they can easily live in trawled areas, whereas plaice rely on a more specific diet so they have much more trouble adapting to changing prey environments,’ Johnson says. ‘This is the first time anyone’s managed to use the feeding ecology of different fish species to explain the impact of trawling on their condition.’ While the study looked only at these two species, its conclusions are likely to apply to any fish that spends all or some of its life on the seabed.

To ensure the sustainability of flatfish stocks, we may need new rules to make fishermen use less harmful gear, or trawl only over particular areas of the seabed.

Johnson suggests that fishing boats could reduce the effects of trawling by using new, innovative fishing gears that reduce the impact on the invertebrates that live on the seabed. Trawlermen and women in the Netherlands and Belgium have been developing electrical trawls to replace the heavy chains, where electrical pulses are used to drive the fish off the bottom into the nets. Preliminary studies show that these trawls may have less of an effect on the food of flatfish but more work in this area is still needed.

He also says the introduction of ‘trawling lanes’ could help – this way, fishing boats would be allowed to trawl only in designated areas of ocean. This would mean that trawl damage to the seabed would be confined to these areas, but the boats could still hope to catch fish there as they’d move in from neighbouring untouched areas. But he acknowledges that this kind of measure would be a challenge to enforce and would require a careful selection of the lanes in collaboration with the fishers, because if unproductive trawling lanes are selected this approach may end up being counter-productive.

Marine ecologist and Lecturer, Dr Jan Hiddink, of Bangor University’s School of Ocean Sciences commented:

“This work shows that fish are very flexible predators. Even though ten times fewer prey were available in frequently trawled areas, the fish still managed to fill their stomachs, although with lower quality prey. It is therefore worth exploring if it is beneficial to separate fishing grounds into areas where the fish can feed, and fill up with high quality prey without spending a lot of effort, and other areas where the fish are caught when they move on.”

Publication date: 11 December 2014