Aquaculture North America

Stress-free smolt could save the industry millions

September 30, 2013
By Tom Walker

Salmon smolt should be sedated before being transported by sea, Martin Haugmo Iversen claims in his doctoral thesis at the Faculty of Biosciences and Aquaculture at Norway’s University of Nordland.

According to the thesis, this would lead to increased animal welfare, reduced mortality and minimize economic loss.

The thesis, Stress and its impact on animal welfare during commercial production of Atlantic salmon (Salmo salar L.), shows that stress is a significant factor in fish mortalities. Currently, a large number of salmon juveniles die during transport to sea cages.This is largely due to stress he maintains.

Stressed to death 

“My dissertation shows that these smolts are stressed to death,” Haugmo Iversen says in an interview published by the University of Nordland.

In Norway, the smolt mortality rate is around 16 -17% after transfer to sea cages. Haugmo Iversen has focused his investigation on the significance of good animal welfare for smolt physiology.

Salmon smolts become distressed. They show increased levels of the stress hormone cortisol – which can play a direct or indirect role in fish mortality, he points out.

He argues that stress-reduction measures, such as sedation of fish, can reduce the mortality rates among young salmon.

Time to rest

Young fish are transported to sea cages in well boats or tankers. “The transport time is usually six to eight hours,” says Haugmo Iversen. “In bad weather the processes of loading and unloading the salmon can be especially stressful for the fish. It is important that the fish are allowed time to rest between these processes.”


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          Sea-ready salmon are pumped in quantities of up to 400,000 fish from the smolt vessel into a boat for transport to the sea cages.

“Mortality is highest among the last 10-15,000 individuals, which have had the longest period in the smolt vessel. We found stress levels to be 3-6 times higher in the last group transferred then in the first group of fish,” he reveals.


Haugmo Iversen recommends using approved organic or synthetic sedatives to reduce the fish’s levels of plasma cortisol. Sedatives restrict the stress response and have a calming effect on the fish.

Healthy fish are also good news for the farming industry.

“Losses on the cost price of juvenile salmon in the farming industry are close to half a billion Norwegian Kroner each year,” he notes.

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