Ocean fish farming puts wild stocks at risk...for lice

The farming of fish in net cages is taking yet another hit for its impact on wild fish.

(Image: Salmon in the wild. U.S. Fish and Wildlife Service image.)

Predictably, keeping living things in crowded circumstances creates issues—whether it's fungal attacks on monoculture crops or recurrent colds among kids in pre-school.

On fish farming, the tight confines of the netted pens promote parasites, which then can infest wild fish, said University of Hawai'i professor L. Neil Frazer, of the Department of Geology and Geophysics, School of Ocean Science and Technology. He wrote an essay in the journal Conservation Biology.

And in some ways, he argues, this form of marine farming creates issues that land-based agriculture doesn't. But he also suggests that there are ways to respond to the problem.

“Farm fish...share water with wild fish, which enables transmission of parasites, such as sea lice, from wild to farm and farm to wild fishes. Sea lice epidemics, together with recently documented population-level declines of wild salmon in areas of sea-cage farming, are a reminder that sea-cage aquaculture is fundamentally different from terrestrial animal culture,” Frazer wrote.

His work was done on Mainland salmon farming, not on any of the local fish farming projects. But Frazer said his work helps explain the phenomenon of declines in wild fish populations around salmon sea cage fish farms.

One issue: In the wild, a sick fish might quickly weaken from inability to feed or be eaten by a passing predator. But in cages, managers use medications and adequate feeding to keep them alive.

“The difference is that sea cages protect farm fish from the usual pathogen-control mechanisms of nature, such as predators, but not from the pathogens themselves. A sea cage thus becomes an unintended pathogen factory,” Frazer wrote.

Sea lice are crab-like creatures that infest fish, eating skin and other tissue and creating injuries that can be opportunities for infection. The prevalence of the parasite can create in increase in the overall number of parasites in the environment, increasing the chance that they will infest wild fish.

Frazer said his work shows that wild fish in the environment around fish farms can be reduced in number and may even disappear.

How to respond? Frazer does not argue that fish farming must be halted. To respond to the issues he raises, he has several recommendations:

“Declines of wild fish can be reduced by short growing cycles for farm fish, medicating farm fish, and keeping farm stocking levels low.

“Declines can be avoided only by ensuring that wild fish do not share water with farmed fish, either by locating sea cages very far from wild fish or through the use of closed-containment aquaculture systems. These principles are likely to govern any aquaculture system where cage-protected farm hosts and sympatric wild hosts have a common parasite with a direct life cycle,” he wrote.

For more information, see http://www3.interscience.wiley.com/journal/120122721/issue.

© 2008 Jan W. TenBruggencate