Salmon crisis in Norway

SALMON stocks in Norway are under threat from a deadly parasite being spread by record numbers of a trout-salmon hybrid. The fear is that the hybrid fish, which carry the parasite but remain healthy, could spread the worm to hundreds of unaffected rivers or streams.

The discovery has prompted Janne Sollie, the director of the Norwegian Directorate for Nature Management, to call for increased efforts to sterilise the nation’s 23 parasite-infested rivers with rotenone - a poison that kills the parasite, Gyrodactylus salaris, along with most other living creatures. The idea is to restock with hatchery-reared wild fish.

Norway is home to the world’s largest Atlantic salmon stocks, with 453 wild salmon streams etched into the country’s coast. The fish generate an industry worth an estimated 1.2 billion kroner a year ($170 million).

Acid rain, habitat destruction and escapees from fish farms also threaten Norwegian salmon stocks, but it is Gyrodactylus biologists fear the most. Poisoning a river with rotenone is a drastic but effective fix. Norway has treated 27 rivers since 1981, eight of which have become re-infested.

The burgeoning population of hybrids was discovered in the Vefsna river, which was thick with wild salmon until infested by the parasite in 1978. Biologists monitoring the waterway were surprised to notice a sudden increase in the number of young salmon in 1999 - virtually unheard of in a river with such a long history of infestation. What is more, the young fish carried relatively few of the millimetre-long helminth parasites, suggesting the salmon had developed a natural immunity to the pest.

But the results of genetic studies completed late last year show that the fish were not salmon at all, but a brown trout-salmon hybrid able to survive Gyrodactylus infestation.

At least a third of the fish in the river were hybrids - an astounding number compared to the average hybridisation rate of about 0.3 per cent, said Torbj0rn Forseth, research coordinator at the Norwegian government’s Institute for Nature Research, whose team released the results in a report earlier this month. The hybrids were likely fathered by salmon escaping from fish farms or by returning wild salmon that had mistakenly entered the Vefsna.

lan Fleming, a salmon expert at Oregon State University in Newport who worked for 10 years with Norwegian salmon stocks, says that if the hybrid fish inherit the brown trout’s wanderlust and go from stream to stream, they could prove a potent vector for transmitting the parasite.

Nancy Bazilchuk •