Sea Lice and Salmon

Crustacean Parasites Kill Wild and Farmed Fish Species

Dec 18, 2007

Sea lice are a problem in fish farming; now scientists fear that they will wipe out wild populations. They infest salmon, trout, charr, dace and other species.

What are Sea Lice?

Sea lice are parasitic crustaceans that live on the outside of fish. Two species, Lepeophtheirus salmonis and Caligus elongates, are particularly well known because of their effects on farmed fish: they feed on the skin and muscle tissue of salmon, trout, charr, dace and many other species, causing lesions and bleeding. Flattened and attached to the skin of fish, they look a bit like tadpoles with bushy tails.

Why are Sea Lice a Problem in Fish Farming?

A lot of fish farming (aquaculture), particularly salmon and trout farming is done in marine coastal areas in open net systems—that is, the fish are kept in netted areas in direct contact with marine water. These unnaturally large numbers of fish are susceptible to diseases that spread from fish to fish. (Wild fish are less likely to become infested because they do not stay so close together for such long periods and they range over large areas of the ocean.)

Wild fish were infested with sea lice before fish farming, but the parasites were present in relatively low numbers. The ready food supplies provided by fish farms have been a boon to sea lice—farmed fish can have thousands of times more sea lice than wild fish. A fish with many sea lice can suffer considerable physical damage and is thought to be more susceptible to bacterial infection. Many die.

While both L. salmonis and C. elongates range widely in the oceans, aquaculture in North America has been particularly affected by L. salmonis, while fish farming operations in northern Europe have suffered from C. elongates.

Why are Sea Lice a Problem for Wild Salmon?

While fish farmers used to worry about their fish becoming infected with sea lice, the problem has recently been turned on its head: scientific evidence now points the finger at coastal fish farms as being the source of sea lice that are killing so many young salmon that whole populations are critically endangered.

In the wild, adult salmon, many of which are infected with sea lice, are usually far out at sea when young salmon come down the rivers and enter the ocean—the young fish don’t become infested with the parasites until they are bigger and older.

Fish farms place a very significant source of infection close to shore, sometimes right in their path—they are becoming infected before they are strong enough to withstand the parasites, and they are dying.

Saving Wild Fish from Sea Lice

Scientists are urging that salmon aquaculture operations be moved immediately so that infested fish are not in coastal areas that young salmon travel through. This would do a lot to lessen the impact of the parasites on young wild salmon (Although the evidence with respect to farmed trout is not as solid, it’s likely that this approach would save young trout as well).

Another solution—and one that might be preferable for other environmental reasons—would be to farm fish in closed systems that are not in contact with marine water.