Effects of Low-Lipid Diets on Growth, Haematology, Histology and Immune Responses of Parr-Stage Atlantic Salmon (Salmo salar).

Simple Summary: This study investigated the effects of three artificial diets used in aquaculture, two for rainbow trout and one for Atlantic salmon, on the growth rate, organ responses, gene expression and immune responses of parr juvenile Atlantic salmon. The diets differed with respect to their carbohydrate, protein and lipid levels, with the lipid levels being the main focus of this study. We found no significant differences in the growth, feed efficiency, immunity, histology or gene expression of Atlantic salmon parr between the rainbow trout and salmon feed groups. Therefore, rainbow trout feed, which has a relatively low lipid content, could be used as a rearing feed for juvenile parr-stage Atlantic salmon, bringing potential economic benefits. Lipids in fish diets provide energy and play important roles in immunity and metabolism. Atlantic salmon, a species that migrates from freshwater to seawater, requires high energy, especially during smoltification. Juvenile teleosts have low lipid requirements, and a high dietary lipid content is known to have negative effects on their growth and digestion. Therefore, this study evaluated the effect of two commercial rainbow trout feeds (low-lipid, 13.41% and 14.6%) on the growth and immune responses of early parr-stage Atlantic salmon compared to commercial salmon feed (high-lipid, 29.52%). Atlantic salmon parr (weight: 14.56 ± 2.1 g; length: 11.23 ± 0.44 cm) were randomly divided into three groups and fed either one of two commercial rainbow trout feeds (RTF1 and RTF2) or the commercial salmon feed (ASF) for 12 weeks. At the end of the feeding trial, growth, haematology, histology and gene expression analyses were performed. There were no significant differences in weight gain rates or feed efficiency between the groups (p > 0.05). Superoxidate dismutase, glutathione peroxidase, lysozyme and immunoglobulin M activities were not different among the experimental groups (p > 0.05). A histological examination of the liver and intestinal tissues showed no pathological symptoms of inflammatory response or lipid accumulation in any of the groups. In an intestinal transcriptome analysis using RNA-seq, the expression levels of several genes linked to lipids, immune-related proteins, cytokines and chemokines did not differ significantly between the groups (p > 0.05). Commercial rainbow trout feed with low lipid content has no clear negative impact on the development of Atlantic salmon during the early parr stage (14.5 to 39.6 g). This study provides basic information for the development of economical feed for early parr-stage Atlantic salmon. [ABSTRACT FROM AUTHOR]