Abstract
The loss of fish in the seawater (SW) phase of Atlantic salmon farming is high, and a major proportion of this loss occurs in the period just after SW transfer. In the current study, we hypothesize that improvements made to the diet during the freshwater (FW) stage affect fish growth, survival and robustness later in the SW stage. To test this, salmon parr were fed five experimental diets in FW at 12 °C. In addition to a commercial-like control diet, fish were fed a diet with changed FA composition aimed to be more like the natural feed of salmon in FW, a diet with increased concentrations of selected AA/N-compounds (methionine, lysine, threonine and taurine), a diet with increased concentrations of methionine and certain B-vitamins (folate, B12 and B6) and a final diet combining all of these potential improvements. At the time of SW transfer, the robustness of fish fed the different diets was tested by direct transfer to SW at three different temperatures (8, 12 and 16 °C, without prior acclimation), as well as transfer into open net pens, while fed on a common commercial diet. Growth and proximate composition of the fish did not differ between the diet groups. All diet groups seemed to handle transfer to SW well, and while SW transfer elicited a stress response in the fish, this was not significantly different between diet groups. Fish transferred to SW at 8 °C had higher mortality, reduced mucus layer and increased prevalence of scale loss and wounds, but this applied to all diet groups. Hence, direct transfer to SW at a lower temperature than the fish has been acclimated to cannot be recommended. At the two highest temperatures, there were some differences between the groups in the severity of cataracts. Apart from this, none of the health- or welfare related parameters measured showed any difference between the diet groups, indicating that the control diet was already sufficient.
Highlights
Atlantic salmon (Salmo salar L.) has a life-history pattern with spawning and juvenile stages in freshwater (FW), followed by the parrsmolt transformation and migration to sea
In addition to a commercial-like control diet, fish were fed a diet with changed fatty acids (FA) composition aimed to be more like the natural feed of salmon in FW, a diet with increased concentrations of selected amino acid (AA)/N-compounds, a diet with increased concentrations of methionine and certain B-vitamins and a final diet combining all of these potential improvements
Salmon parr were fed five experimental diets; a commercial-like control diet, a diet with changed FA composition aimed to be more like the natural feed of salmon in FW, a diet with increased levels of selected AA, a diet with increased levels of methionine and certain B-vitamins and a final diet combining all of these potential improvements compared to the com mercial diet
Summary
Atlantic salmon (Salmo salar L.) has a life-history pattern with spawning and juvenile stages in freshwater (FW), followed by the parrsmolt transformation and migration to sea. In Norwegian salmon aquaculture, the total loss during the seawater (SW) phase is about 16–17% (Sommerset et al, 2020). A high proportion of this loss occurs just after SW transfer, indicating poor smolt quality and sub-optimal conditions in the FW phase (Hjeltnes et al, 2016). Improving the nutritional status of the smolt by optimizing the feed in the FW phase could result in a more robust smolt that handles the transfer to SW better. Compared to SW, relatively small amounts of feed are required in the FW phase. Minor investments made in enhancing the feed in the FW phase could potentially result in improved fish welfare, reflected in increased growth and survival in later production stages
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