A University of Alberta study shows, for the first time, that parasitic sea lice infestations caused by salmon farms are driving nearby populations of wild salmon toward extinction.

The results, appearing in the Dec. 14 issue of the journal Science, show that the affected pink salmon populations have been rapidly declining for four years. The scientists expect a 99 per cent collapse in another four years, or two salmon generations, if the infestations continue.

"The impact is so severe that the viability of the wild salmon populations is threatened," said lead author Martin Krkosek, a fisheries ecologist from the University of Alberta. Krkosek and his co-authors calculate that sea lice have killed more than 80 per cent of the annual pink salmon returns to British Columbia's Broughton Archipelago. "If nothing changes, we are going to lose these fish."

Previous peer-reviewed papers by Krkosek and others showed that sea lice from fish farms can infect and kill juvenile wild salmon. This, however, is the first study to examine the population-level effects on the wild salmon stocks.

"It shows there is a real danger to wild populations from the impact of farms," said Ray Hilborn, a fisheries biologist from the University of Washington who was not involved in the study. "The data for individual populations are highly variable. But there is so much of it, it is pretty persuasive that salmon populations affected by farms are rapidly declining."

According to experts, the study also raises serious concerns about large-scale proposals for net pen aquaculture of other species and the potential for pathogen transfer to wild populations.

"This paper is really about a lot more than salmon," said Hilborn. "It is about the impacts of net pen aquaculture on wild fish. This is the first study where we can evaluate these interactions and it certainly raises serious concerns about proposed aquaculture for other species such as cod, halibut and sablefish."

The data are from the Broughton Archipelago, a group of islands and channels about 260 miles northwest of Vancouver that is environmentally, culturally and economically dependent on wild salmon. To pinpoint the effect of salmon farms, the study used a large dataset collected by Fisheries and Ocean Canada that estimates how many adult salmon return from the ocean to British Columbia's rivers each year. Extending back to 1970, the data covers 14 populations of pink salmon that have been exposed to salmon farms, and 128 populations that have not.

Sea lice are naturally occurring parasites of wild salmon that latch onto the skin in the open ocean. The lice are transmitted by a tiny free-swimming larval stage. Open-net salmon farms are a haven for these parasites, which feed on fish skin and muscle tissue. Adult salmon can survive a small number of lice, but juveniles headed from the river to the sea are very small, thin-skinned and vulnerable.

In the Broughton Archipelago, the juvenile salmon must run an 80-kilometre gauntlet of fish farms before they reach the open ocean. "Salmon farming breaks a natural law," said co-author Alexandra Morton, director of the Salmon Coast Field Station, located in the Broughton. "In the natural system, the youngest salmon are not exposed to sea lice because the adult salmon that carry the parasite are offshore. But fish farms cause a deadly collision between the vulnerable young salmon and sea lice. They are not equipped to survive this, and they don't."

Salmon bring nutrients from the open ocean back to the coastal ecosystem. Killer whales, bears, wolves, birds and even trees depend on pink salmon. "If you lose wild salmon there's a lot you are going to lose with them - including other industries such as fishing and tourism," said Krkosek.

"An important finding of this paper is that the impact of the sea lice is so large that it exceeds that of the commercial fishery that used to exist here," said Jennifer Ford, a co-author and fisheries scientist. "Since the infestations began, the fishery has been closed and the salmon stocks have continued declining."

The researchers observed that when farms on a primary migration route were temporarily shut down, sea lice numbers dropped and salmon populations increased. "Even though they have complicated migration patterns they all have one thing in common - overall, the populations that are declining are the ones that are going past the farms," said Mark Lewis, a mathematical ecologist at the University of Alberta.