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Tanks teach cod to fend for themselves

A stimulating environment could help farmed fish to bolster wild stocks.

Some plants and pebbles in the tank can help cod deal better with predators and prey. Credit: © Alamy

Hatchery-reared cod are being taught 'life skills' in a bid to help them survive in the wild. The organizers of the project hope that by raising cod in more stimulating enclosures, the fish will fare better in the open ocean and contribute to ailing natural stocks.

In a study carried out in Bergen, Norway, researchers have discovered that Atlantic cod (Gadus morhua) are bolder and more inquisitive if they are raised in tanks containing stones and plastic plants, and fed at varying times and locations.

Raising them in these conditions, rather than in unfurnished tanks with predictable mealtimes, means the cod may stand a better chance when released in the wild, say Victoria Braithwaite of the University of Edinburgh, UK, and Anne Salvanes of the University of Bergen, who carried out the study.

Fish have much more sophisticated cognitive abilities than they've been given credit for in the past. Victoria Braithwaite , University of Edinburgh

The idea of replenishing wild stocks with farmed fish is controversial. This is partly because farmed fish tend to grow larger than their wild counterparts, which leads some to fear that they will compete with natural fish over food and actually drive down stock numbers. And if the raised fish are inbred, and they outcompete natural fish, the resulting lack of genetic diversity would adversely affect the population.

On the other side of the coin, farmed fish seem to have problems adapting to life in the wild. In some cases, fish have been spotted trying to eat pebbles that resemble the food pellets with which they were previously fed. This may mean that introduced fish do not thrive or breed, so stock numbers will not be improved.

After considering all these factors, Braithwaite argues that a responsible release campaign could help struggling species, including salmon and trout as well as cod. "I'm not advocating we take any old cod and release them," she says. "We would prefer to breed from wild stocks every time, and then release those."

Born to be wild

In their study, Braithwaite and Salvanes caught wild cod and bred their offspring in hatchery tanks for eight weeks before dividing them into four groups. One group was raised in normal farm conditions; the others were given either an unpredictable food regime, a tank furnished with stones and plants, or both.

After several weeks, the researchers tested the fish's ability to deal with predators, prey and new surroundings. Fish raised in the most stimulating environment were more likely to venture out to meet a dummy fish. Those given furnished tanks recovered from signs of stress more quickly after being chased with a net, which is a rough simulation of a predator attack. And fish given unpredictable food were more likely go after live prey. The results are reported in Proceedings of the Royal Society 1.

The results show the benefits of living in an enriched environment. Similar effects have been shown in birds and in mammals such as rodents, Braithwaite says. "Fish have much more sophisticated cognitive abilities than they've been given credit for in the past," she adds.

The researchers are now setting up a larger project on the west coast of Ireland, and they hope that commercial investors will be attracted by the idea. Previous attempts to equip fish for life in the outside world have been labour-intensive, and involved exposing them to real prey and predators. "We're advocating a simpler approach," Braithwaite says.

References

  1. BraithwaiteV. A. & SalvanesA. G. V. Proc. R. Soc. Lond. B, published online, doi:10.1098/rspb.2005.3062 (2005).

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Hopkin, M. Tanks teach cod to fend for themselves. Nature (2005). https://doi.org/10.1038/news050531-3

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