BACKGROUND
This study addressed nutritional strategies for Atlantic salmon during the freshwater pre-transfer period, a stage relevant to later robustness in seawater. The authors note that mortality in Atlantic salmon in Norway ranged from 15 to 16% from 2017 to 2021, and approximately 35% of sea cage mortality occurred in the first 0–3 months at sea for the 2010–11 salmon generations in Norwegian farming. Because phospholipids can support growth, survival, and organ health in early fish life, the investigators tested whether different phospholipid sources in freshwater diets would influence subsequent growth and tissue health. Krill meal was of particular interest because it supplies phospholipids, phosphatidylcholine, EPA + DHA, protein, water-soluble nitrogenous compounds, and astaxanthin. The main comparison was krill meal against soy lecithin and marine phospholipids from fishmeal, with diets designed so that the 12% krill meal, 2.7% fluid soy lecithin, and 4.2% marine phospholipid diets all provided the same added 1.3% phospholipid in the freshwater phase.
METHODS
Atlantic salmon with an initial weight of ca. 67 g were pit-tagged and randomized into 24 freshwater flow-through tanks, with 40 fish per tank. After 15 days of acclimation, fish were 74 ± 12 g and then entered a 53-day freshwater feeding period. Six diets were tested: Control, KM4, KM8, KM12, VegPL, and MarPL. The freshwater feeds were formulated to have similar digestible energy, reported as 22.1–23.6 MJ/kg gross energy, protein in the 46–49% range, fat in the 22–24% range, similar calculated 1.1% EPA + DHA in diet, 15–16% saturated fatty acids in total fatty acids, and a 1.3 n-6/n-3 fatty acid ratio. Water temperature during freshwater averaged 14.3 °C, with a 13.3–15.3 °C range; oxygen saturation averaged 107% at the inlet and 90% at the outlet. After the freshwater phase, 17–20 fish per tank, totaling 459 fish of ca. 167 g, were transferred without acclimation from 0 ppt freshwater to 28.5 ppt seawater into a common 21.6 m3 tank. Transfer included a temperature drop from ca. 14 to 9 °C and crowding stress by lowering water level to ca. 20 cm for one hour, with supplemental oxygen, within a ca. 0 to 30 h period after transfer. Fish then received the same commercial diet for 98 days in seawater. Daily seawater temperature averaged 9.4 °C, with an 8.5–11.1 °C range. Growth was assessed by individual weighing on Days 0, 15, 33, and 68 in freshwater and on Days 35, 73, and 98 after seawater transfer. Hepatosomatic index was measured in 10 fish per tank, giving 40 fish per diet at the end of freshwater and again 40 fish per diet at the end of seawater. Histology was performed at the end of freshwater on liver and gill samples from five fish per tank, totaling 20 liver and 20 gill tissues per diet group, for Control, KM12, VegPL, and MarPL. Statistical analysis used hierarchical generalized additive models, with 95% credible intervals estimated by parametric bootstrap with 10,000 random draws per parameter.
KEY RESULTS
Salmon started the feeding phase at 74 g and reached 158 g overall tank average by the end of the freshwater period, representing 2.1-times the initial fish weight. In freshwater, there was no clear trend for increased feed intake with krill meal dose, although the Control and KM12 diets showed a trend toward increased feed intake compared with MarPL and VegPL; the authors emphasized overall high variability. For growth, there was a trend for increased fish weight gain with increased krill meal dose during the freshwater phase, but this did not persist across the whole trial after all fish were switched to the same seawater commercial diet. In the phospholipid source comparison, fish fed KM12 had increased weight gain compared with VegPL during freshwater, while MarPL and Control were intermediate. Over the full trial, weight gain was similar for KM12, MarPL, and Control, with a trend toward higher weight gain than VegPL. For hepatosomatic index, increased krill meal inclusion from 0 to 12% of diet was associated with a trend toward lower hepatosomatic index at the end of freshwater. KM12 also showed a lower hepatosomatic index than MarPL, VegPL, and Control at the end of the freshwater phase. However, there was no decrease in hepatosomatic index with krill meal dose at the end of the whole trial after the shared seawater feeding period. Histologically, no major liver differences were observed between Control, KM12, VegPL, and MarPL, including no major difference in lipid droplet accumulation; the liver histology was described as normal. In gills, an increased probability of very mild to mild lamella inflammation and hyperplasia scores was indicated in salmon fed VegPL and MarPL compared with Control and KM12. Other gill outcomes, including vascular lesions, filament inflammation, necrosis of respiratory epithelium, necrosis affecting deeper tissues, fusion of lamella, and other lesions, did not show major diet-related differences.
CLINICAL IMPLICATIONS
This was not a human clinical study, but it has practical significance for aquaculture health management. The findings suggest that phospholipid source in the 53-day freshwater pre-transfer period can influence short-term growth and hepatosomatic index in salmon weighing from 74 g to 158 g. Krill meal at up to 12% of diet showed a favorable growth trend during freshwater and a lower hepatosomatic index, while soy lecithin tended to perform worse for growth. However, once fish entered seawater and all received the same commercial feed for 98 days, these growth differences were not clearly sustained. The absence of major liver histology differences reduces concern that the lower hepatosomatic index reflected obvious liver pathology, and the modestly better gill histology pattern with KM12 and Control may be relevant because gill health is critical after seawater transfer. Overall, the data support only cautious conclusions: benefits of krill meal were trends with high variability, only one dose each of soy lecithin and marine phospholipids was tested, and the study cannot isolate phospholipid effects from other krill meal components such as choline, EPA + DHA, astaxanthin, and water-soluble compounds.