Natural lipid profile mimicking diets to enhance survival and fitness of restocked salmon parr in river ecosystems?

Abstract

Atlantic salmon parr (Salmo salar) reared in hatcheries for restocking in rivers are typically fed high-lipid commercial diets that diverge from natural prey profiles. This may have a detrimental effect on their ecological adaptation. This study evaluated whether diets mimicking natural lipid levels (12.7 % vs. 20 %) and fatty acid compositions (plant-derived α-linolenic acid ALA vs. marine eicosapentaenoic acid EPA), or with partial chironomid supplementation, enhance fitness traits essential for wild survival. Six dietary treatments were evaluated over a 47-day period, with parameters including growth, lipid metabolism, swimming performance, and immune responses pre- and post-bacterial challenge with Aeromonas salmonicida salmonicida. The results demonstrated that reducing dietary lipids to 12.7 % led to a significant decrease in fat accumulation while maintaining optimal growth rates. EPA12.7 % diets elevated citrate synthase activity, indicating enhanced aerobic capacity, whereas ALA20% diets increased lactate dehydrogenase activity, suggesting earlier anaerobic reliance during sustained swimming. Innate immune gene (c3, lysozyme, igm, mhc, cox2, mpo) expression remained unaffected post-infection across diets, with no mortality observed. Chironomid supplementation resulted in a reduction in lipid content compared to the commercial diet but also led to impaired feed conversion efficiency due to feed intake reduction. The findings demonstrate that 12.7 % lipid diets with EPA enrichment optimize metabolic capacity and reduce adiposity, aligning salmon parr physiology closer to wild counterparts. While plant-based oils did not compromise immunity, the inclusion of chironomid necessitated dosage adjustments to balance predation training and energy intake. This dietary strategy could enhance restocking success by mitigating the effects of feeding-related domestication.