Atlantic salmon invade the Pacific. (Environmental Intelligence).

Large numbers of Atlantic salmon have escaped from fish farms in Pacific waters and are now thriving in 77 rivers in British Columbia, according to a recent report by the Canadian Parliament's Senate Committee on Fisheries. In introducing the fish, aquaculture proponents had claimed that Atlantic salmon would not be able to compete with native Pacific salmon, but the large number of these invasive fish living in Pacific waters may now pose a serious threat to wild salmon species.

Experts warn that Atlantic salmon could undermine the health of native species by competing for food, interfering with migrations and spawning, displacing juveniles, and digging up eggs. Farmed salmon could also spread diseases, which occur with regularity in aquaculture pens, due to the physical stress and confined conditions.

The report finds that between 1994 and 1998, about 44,000 farmed salmon escaped each year from Canadian aquaculture pens. Canada produces just 3 percent of the world's farmed salmon, and it is likely that the ecological problems are more intense in countries with larger operations. In Norway, the largest farmed-salmon producer, escapes have become so numerous that 87 percent of the 30 rivers currently monitored had farmed fish living in them. Escapees now make up almost one-third of the fish population in Norwegian rivers where they are found.

"Aquaculture-escaped Atlantic salmon are capable of spawning in B.C. streams," said John Volpe, an aquaculture expert at the University of Victoria. "We have shown this to be true beyond any shadow of a doubt." The effect that these escapees will have on wild populations is unknown, but one of the greatest risks is the potential loss of genetic integrity in native species.

There are over 5,000 genetically distinct strains of Pacific salmon in British Columbia alone. Interbreeding with the Atlantic salmon could homogenize this diversity and destabilize native salmon populations by, in effect, erasing traits essential for long-term survival.

Despite the potential risks of the spread of Atlantic salmon, the Canadian Senate Commission expects worldwide salmon farming to double to a yield of almost 2 million tons in the next 10 years.