Salmon producers have spent a lot of time and resources on developing salmon farming that is both economically and environmentally sustainable, based on a fast-growing and large juvenile fish (smolt) that does not have to be in the sea for so long. A new study suggests that fish that have grown very fast before being transferred to the sea may face challenges as they become less robust and cope less with stress in the seawater phase.
Like humans, farmed salmon can suffer from heart disease. For example, many fish get Cardiomyopathy Syndrome (CMS) which is initiated by a virus and, in the worst case, can cause the heart to rupture, especially if the fish is stressed. “We now see increased mortality related to heart problems in Norwegian salmon farming, despite decades of research on salmon health and welfare. It is especially problematic when large and apparently fresh fish die before slaughter. This mortality represents significant welfare and financial problem,” said Ida Beitnes Johansen, associate professor at the Department of Preclinical Sciences and Pathology at NMBU Veterinary College.
Researchers at the Veterinary College and the Faculty of Medicine at the University of Oslo found that intensive production of smolt in freshwater gives salmon heart problems later in the seawater stage.
In the hatchery, the conditions are created to get a smolt ready to be transferred to seawater in less than a year with high feed supply and temperature. “This is much faster than in nature where the salmon can spend 2-5 years in an often cold and nutrient-poor river before life in the sea begins. In the study, we show that rapid production in freshwater has consequences for how the salmon's heart develops. The heart becomes rounder and less symmetrical and we found that such changes are associated with mortality in CMS outbreaks,” said Johansen.
The study was conducted in collaboration with Ellingsen Seafood, one of a few companies that focus on alternative, slow production of juvenile fish, so-called “old smolt”. Studies also showed that slower and more naturally produced salmon have a heart development that is more similar to that of wild salmon. “They get better heart health later in life and in fact, also grow much faster than “normal” smolts after release into the sea. In other words, our findings suggest that a slightly longer production time in the freshwater phase leads to faster growth and fresher fish later in life. Heart health and reduced mortality can thus be achieved by promoting rapid growth in the sea rather than in freshwater in Norwegian salmon farming,” concluded Johansen.
Download the study here.
