A new study in shows why and how ocean fish farms can hurt wild fish populations.
Done by prof. Neil frazer of the university of hawaii at manoa, the basic premise is that the higher density of fish in the farms promotes infection, and that infection lowers the fitness of surrounding fish. For the wild fish this means:
Lower fitness doesn’t benefit fish any more than humans
…More difficulty finding food and escaping predators, causing higher death rates. But farmed fish are not only fed, they are also protected from predators by their cage, so infected farm fish live on, shedding pathogen into the water. The higher levels of pathogen in the water cause the death rates of wild fish to rise.
Even if fish farms are medicated, the effect isn’t good
After describing how sea lice attack salmon the report sums up the impact on ocean fish farms:
…Even if lice levels on farm fish are controlled by medication, local wild fish still decline. Also, there is a critical stocking level of farmed fish. If a sea-cage system is stocked above the critical level, local wild fish decline to extinction. Long story short — growing farm fish in sea cages can’t save wild fish, but it can easily destroy them.
– by Matthew McDermott
via Science Codex