Farming of fish in ocean
cages is fundamentally harmful to wild fish, according to an
essay in this week's Conservation Biology.
physics, professor Neil Frazer of the Department of Geology
and Geophysics at the University of Hawaii at Manoa explains
how farm fish cause nearby wild fish to decline. The
foundation of his paper is that higher density of fish
promotes infection, and infection lowers the fitness of the
For wild fish, lowered fitness means 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
The above paradigm explains recently documented declines of
wild fish in areas with sea-cage farm fish.
"Sea lice are an important example of disease transfer in
ocean fish farming," said Frazer. "Sea lice are tiny crabs
that attach to marine fishes, eating their skin and sometimes
deeper tissue. Skin is important to fish because they need to
keep their tissues less salty than the ocean. Also, when lice
puncture the skin they create an entry point for other
infections. So wild fish weakened by lice have more difficulty
finding food and escaping predators."
A female sea louse can produce over a thousand larvae
during her life. Larvae drift in the ocean and a lucky few of
them drift close enough to a fish to attach. Most larvae die
without ever finding a fish. In a fish farm environment, a
larva's chance of finding a fish increases, so more larvae
survive to become lice, and those lice put more larvae into
the water. With more larvae in the water, more wild fish
become infected and die as a result.
Larger numbers of lice are especially dire for salmon
because juvenile salmon must transit coastal areas where
salmon farms are located. Juvenile pink and chum salmon suffer
most because they spend much of their early life in coastal
waters, and they are so small at ocean-entry that infection by
even one or two lice can be fatal.
The calculations in the paper show that 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.