Aquaculture for all

Is rapid growth dangerous for juvenile farmed salmon?

Atlantic Salmon Health Welfare +5 more

Norwegian scientists are attempting to ascertain whether the rapid growth of juvenile salmon could lead to health issues when they are transferred to sea cages.

a tank of juvenile salmon
Nofima is attempting to establish the critical factors that effect the survival rate of smolts after transfer to sea cages

© Nofima

However, they report that their goal is made more difficult because of the variety of methods used salmon production in Norway.

“There is no shared basic knowledge. Things are done differently at each facility according to the lessons they’ve learnt,” Trine Ytrestøyl, a senior scientist at Nofima, explained.

Ytrestøyl is working to collect knowledge of how salmon can grow well, with low mortalities, after being released into salt water. The new project will focus on smolts – the juvenile salmon that are ready to make the transition from freshwater to salt water – and look at factors such as when they should be released into the sea, and how big they should be. In Norway, the answer differs from facility to facility and from region to region, according to the survey conducted by Ytrestøyl and her colleagues.

In the wild, salmon weigh 30 to 50 grams when they migrate downstream into the sea. Meanwhile, in the Norwegian aquaculture sector, the average weight of those released in the spring is 200 grams and 180 grams for those released in autumn.

“The time of year when the big smolt are released varies. Roughly half are released in summer, while 27 per cent of the fish are released in January and February. This seems to indicate that it may not be ideal for them,” she said.

Growing too rapidly

Ytrestøyl compares this to the aquaculture industry in the Faroe Islands, which has more extensive experience of releasing large smolt. The data indicate that the size of the smolts upon release does not have a significant impact on how well they survive or how much they grow in the sea. The crucial factors seem to be whether the smolt lived in warmer freshwater as juveniles and thus grew more rapidly before being released into the sea.

“Fish that have grown rapidly during the land phase grew less and had poorer survival rates in the sea,” she says.

An overview of a similar nature doesn’t exist in Norway. In order to obtain this type of data, researchers must collect data from each individual fish farming company. This can be difficult to obtain due to competition considerations. Relatively few fish farmers produce large smolt in Norway, so this data might be sensitive.

Even so, the fish farmers want larger smolt. Roughly 20 per cent of fish farmers in Norway release salmon that are larger than 250 grams. More than one in three of those who are not currently doing this are considering whether to produce smolt of this same size. Of those already in the process, two out of three are considering whether to increase the proportion of big smolt.

Salmon that spend more time in freshwater and less in the sea may have fewer problems with sea lice. In order words, better growth, better fish welfare and lower mortality. In addition, this enables fish farmers to exploit their licences better.

On the other hand, large smolt can have other health problems. The fish farmers report on fish with more gill problems, wider size deviation, poorer appetite and higher mortality rate after being released into the sea.

In the Faroe Islands, the average size of the smolt released into the sea is now more than 400 grams. But this Faroese data cannot simply be transferred to Norwegian conditions.

“In the Faroe Islands, they use a different production method during the freshwater phase,” Ytrestøyl explained. In the islands, the salmon receive slightly different signals that tell them when it’s time to prepare for salt water. They experience a period of short days making the fish think that winter has arrived. In Norway, this winter signal is shorter. Faroe Islands salmon are also kept in freshwater until release, whereas the use of brackish water is common in Norway.

Finding knowledge gaps

Part of Ytrestøyl’s research project has been to identify where knowledge is lacking.

“We’ve seen a dearth of literature on large smolt, as well as the fact that fish farmers are reportedly uncertain about how to carry out this process. They try out different things,” she said.

Seeing that production is done in such a wide variety of ways also makes it difficult to pinpoint the factors affecting salmon survival after being released into the sea.

“In order to draw any conclusions about an optimal solution, we need more data from those fish farmers who are doing things differently. This shows how crucial it is to collaborate with major operators,” Ytrestøyl added.

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