Salmon are on the run. According to a recent paper, farmed salmon escaping from Norwegian aquaculture facilities are mating with wild salmon frequently enough to dilute their genetic stock. Due to living in abnormal salmon conditions, the escaped fish-farmed salmon have decreased genetic variability – in a survival of the fittest, low genetic variability can make a species more susceptible to disease or even extinction.

In 2013, the world’s largest producer of farmed salmon, Marine Harvest, were offering a S$120 bounty for every recaptured fish after thousands of farmed-fish jailbroke from their 127,000-fish cage in Norway. With a rush to try and mitigate the consequences of the breakout, it appears that little has been done to fully resolve the issue. As wild salmon are growing scarce – one report suggests that divided among the world’s population, wild salmon could provide only a single serving for each person per year – the concern of fish-farm escapees killing off the salmon species is building, just like the number of escapees.

“The annual number of spawning salmon in Norwegian rivers is around 500,000 and a guesstimate can be that the number of escaped farmed salmon a year is approximately the same,” statistician Ola Diserud, one of the study’s first authors told UW360. For comparison, Norway’s marine farms hold close to 400 million fish. 

“The really large escape events (more than 100,000) caused by storms is luckily not that many anymore. These are usually reported. But a large proportion of the escapes are not reported, those occurring during handling or the fish finding a way out on their own (e.g. a small rift in the net) so we can only guess on the total numbers.”

"Offspring from farmed salmon in nature have lower survival chances and fitness. Ongoing escapes of farmed salmon and interbreeding with wild salmon is therefore expected to lower the survival and viability of wild salmon populations." © pixabay.com

“Offspring from farmed salmon in nature have lower survival chances and fitness. Ongoing escapes of farmed salmon and interbreeding with wild salmon is therefore expected to lower the survival and viability of wild salmon populations.” © pixabay.com

With such a large number of salmon being farmed, the odd escape is always going to be likely. But putting the finger of how exactly they escape is not so easy, in fact there are many possibilities.

“Storms are probably the major cause for escapes, if a giant offshore net-pen is breached more than 200,000 farm salmon can escape in one event. But they can also escape during handling in the pens (e.g. net torn open by a boat) or you can have continuous “trickle escapes” from when they are put out as small smolts (Smoltification: salmon transition from living in freshwater as a juvenile to migrating to sea to grow and mature).”

There are major differences between wild and fish-farmed salmon, part of the reason why this interbreeding is giving those involved such a headache. “Farmed salmon have changed genetically from selective breeding in many commercially important traits, such as growth rate and age at maturation,” Diserud states.

“These traits are not favourable in nature and offspring from farmed salmon in nature have lower survival chances and fitness. Ongoing escapes of farmed salmon and interbreeding with wild salmon is therefore expected to lower the survival and viability of wild salmon populations.

“Each river has its specific habitat (discharge, temperature, acidity etc.) so each wild population has adapted to survive in this habitat. When farmed fish interbreed the offspring will not be as fit to live in this environment.”

"You can have continuous “trickle escapes” from when they are put out as small smolts (Smoltification: salmon transition from living in freshwater as a juvenile to migrating to sea to grow and mature).” © Wikimedia Commons

“You can have continuous “trickle escapes” from when they are put out as small smolts (Smoltification: salmon transition from living in freshwater as a juvenile to migrating to sea to grow and mature).” © Wikimedia Commons

Escapees interbreeding with wild salmon is expected to reduce the viability and adaptability of wild salmon populations. It appears a stark reminder of the warm-water fish native to Africa, the Tilapia, which escaped from a fish farm in Lake Apoyo in Nicaragua between 1995 and 2000. Consequently, their pollution and feeding reduced the lake’s quantity of an aquatic plant called charra, which was an important source of food for the lake’s native fish populations and effectively “screwed up the entire lake.”

There are other cases, including Pacific oysters being introduced in the UK in the 1960s and in turn forcing out native oysters and altering the entire marine environment. With history supposedly repeating itself, it’s no surprise that conservationists are concerned of the effects.

“A ‘weakened salmon’ will have the largest impact in wild populations already under pressure by other factors such as overfishing, and hydropower regulated rivers,” Diserud states, but suggests that “interbreeding is probably less of an issue when the wild population is healthy and has plenty of wild fish.” 

If it’s happening to salmon, then there is a strong possibility that other farmed species that are farmed in the same area as their wild conspecifics will probably be experiencing the same problems – with farm-fish escaping and interbreeding. But the escapees can also hurt other native species by competition and predation.

“Farmed salmon is kept at extreme densities (up to several hundred thousand in one net pen) and they are fed regularly, two conditions that are very far from their natural life,” Diserud indicates.

 

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Salmon fish farms in Norway © pixabay.com

When these farmed salmon escape the critical factor in how they will effect the wild population is whether they can survive until spawning.

“The chances for a farmed salmon to survive from the escape to spawning depends on several factors, such as how old it is when escaping (time to maturation) and when in the season it escapes (conditions the fish meets in the wild).”

Much is needed to be done to mitigate the consequences of escaped farmed salmon, and more is needed to be carried out to help prevent it.

Diserud suggests new escapes can be prevented by new farm designs (e.g. land-based farms or closed-containment facilities) and farming practices.

“Another alternative under investigation is to produce sterile salmon.”