Wild-Farm Salmon Hybrids Not Reaching Spawning Grounds?

James Owen in England
for National Geographic News
October 28, 2003
The Atlantic salmon is much admired. Flavorsome, high in protein and a
rich source of vitamins and omega-3 oils, it ranks among the world's
most popular foods. As a sport fish the Atlantic salmon's reputation
is unmatched. Then there's its incredible spawning run, when the fish
navigates across 1,500 miles (2,400 kilometers) of ocean, before
fighting up rapids and falls, to reach the very same waters in which
it was born.

But scientists say Salmo salar, the "leaper" in Latin, is now losing this ability, which could lead to the extinction of wild populations throughout Europe and North America. The reason, they say, is the creation of a new race of mongrel salmon—the product of escaped farmed salmon breeding with wild fish.

The warning follows a ten-year experiment in County Mayo, western Ireland, where researchers monitored successive generations of hybrid offspring produced by wild and farmed salmon. Reporting their findings earlier this month in the scientific journal Royal Society Proceedings B, they found these hybrid fish have poor survival rates at sea and are unable to find their way back to freshwater to spawn.

While it has been known for some time that escaped farmed salmon are breeding in rivers with native fish, scientists say the research provides the first scientific evidence that long-term interbreeding could lead to the extinction of wild populations.

"As repeated escapes [of farmed salmon] are now a common occurrence in some areas, a cumulative effect is produced generation on generation, which could lead to extinction of endangered wild populations," said Andy Ferguson, from the School of Biology and Biochemistry, Queen's University, Belfast, who was joint leader of the study.

Leading fish geneticist, Kjetil Hindar, from the Norwegian Institute for Nature Research, describes the research as "seminal," adding: "A long-term study of the fitness of cultured salmon in the wild is unique and the results will be of major importance to those dealing with the biology and management of salmonid fishes throughout the world."

The experiment, carried out at Ireland's Marine Institute research facility on the Burrishoole River System, was designed to simulate the impacts of fish farm escapees now present in rivers throughout northwest Europe and northeast North America.

Two Million Escapees

An estimated two million farm salmon are escaping from marine cages in the North Atlantic each year, equivalent to about 50 percent of all wild adult salmon in the sea. The worst single incident occurred last year, when 600,000 fish were lost during a storm in the Faroe Islands. Research suggests as many as one-third of adult salmon entering Norwegian rivers are farmed fish, while it's estimated they now outnumber native salmon by ten to one in some North American rivers.

The Burrishoole research team planted known numbers of farm and wild-origin salmon eggs on spawning beds. Young fish were also reared in hatchery tanks and released as smolts, the stage at which salmon head to sea before traveling to northerly feeding grounds. The team then monitored returning adults, caught in traps, coastal nets and by anglers, using DNA profiling to identify parentage.

They found that farm salmon showed an estimated lifetime success of just two percent compared with native stock. The survival rate of adult hybrid salmon was between 27 and 89 percent compared with wild fish.

Even more startling was the finding that 70 percent of second-generation, wild-farm hybrids died within a few weeks of hatching due to outbreeding depression. Ferguson says this is caused by genetic incompatibilities between parents but does not occur until the second generation when recombination of the parental genes takes place.

With potential young wild salmon instead being converted into hybrids because of the presence of cultivated fish, the study suggests the resulting decline in adult survival could eventually wipe out a population.

Ferguson says the influence of escapee salmon also removes significant genetic differences between wild populations. For instance, this may affect their ability to home in on native breeding rivers when returning as adults to spawn.

He added: "Maintaining genetic variation is important for any species to continue to adapt to changing environmental conditions such as global warming. However, even if all wild populations were genetically the same there would still be a reduction in fitness due to interaction with domesticated salmon."

This is because farm salmon have been bred selectively to grow quickly. The ability to hunt at sea, jump waterfalls and locate a specific river aren't attributes fish farmers look for.

Reduced Survival

"It can be concluded that genetic changes leading to reduced survival in the wild is a feature of all domesticated salmon and consequently hybrids between farm and wild fish also have reduced survival," Ferguson said.

Conservationists say measures that would reduce or even halt harmful genetic impacts caused by fish farm escapes include closed-contained marine farms (instead of sea cages) and the use of sterile fish known as triploids for salmon aquaculture.

Richie Flynn, of the Irish Salmon Growers Association (ISGA), responded to the study by saying, "The industry will take note of the report in adhering to the strict protocols agreed by ISGA members to prevent farm escapes."

He added: "It is hoped that those who manage river systems will also take note of the consequences of their actions in restocking those systems."

Flynn refers here to another conclusion of the Burrishoole study—that deliberate stocking of young cultivated salmon to boost depleted wild populations is likely have the opposite effect to that intended.

While monitoring farm salmon parr in the river, Ferguson and his team found that these larger, more aggressive juveniles chased away 57 percent of native parr. Ferguson adds that deliberate stocking means large numbers of pure farm juveniles are introduced in the first generation, causing even lower overall survival than hybrid fish.

He added: "Some have argued that stocking does good by introducing new genetic variation. Our study and similar studies on salmonids in North America shows this to be completely false."

There are also those who thought farming of Atlantic salmon would help reduce pressure on overfished wild stocks. Now it seems the industry has set the species on a very different course. Not on that incredible migration from ocean to river, but one that puts its very survival in doubt.

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