From: Tom Marshall, Planet Earth Online
Published September 7, 2011 08:58 AM

Fish farms less harmful than thought

Coastal fish farms seem to do less harm to nearby plants and animals than previously believed, a new study reveals. And marine ecosystems can recover from this damage surprisingly fast.


But the analysis of a single trout farm in a Faroe Islands fjord over nearly a year also shows that these facilities need to be placed carefully, and that there's a limit to how many can operate in a particular area before its biodiversity suffers lasting harm.

In coastal farms, fish live in large cages hanging from pontoons on the surface. Fish faeces and uneaten food sink to the seabed, affecting its ecosystem. Badly-managed farms can also have serious effects on the surrounding water column.

The team monitored the cages, initially containing some 770,000 young trout. They measured the flow of the vital nutrients carbon and nitrogen through the system, also tracking the farmer's activities, changing water conditions and the quantity of waste deposited on the seabed below.

The results were better than they'd feared. 'We were surprised by how efficiently the food input is converted into fish biomass – particularly how much of the carbon is taken in,' says Professor Ronnie Glud, a marine biologist at Southern Danish University and one of the authors of the paper, published in Marine Ecology Progress Series. The work is part of Gunnvør á Norði's PhD thesis and was conducted in collaboration with researchers at the Scottish Association for Marine Science (SAMS).

'Earlier studies suggested the process was much less efficient,' Glud adds. 'This may be a sign of improvement in fish farming methods – farmers now know how to feed their fish much more efficiently so there's less waste and less environmental impact.'

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