Effects of dietary replacement of fish oil with different lipid sources on growth, fatty acid composition, mineral content and expression levels desaturase and elongase genes in rainbow trout (Oncorhynchus mykiss)

In this study, the effects of alternative lipid sources on growth, mineral content, proximate composition, fatty acid profile and gene expression of rainbow trout were investigated. Four different diets were prepared with isonitrogenous and isolipid casein-gelatin based formulations, consisting of cod liver oil (CO), goose fat (GF), soybean oil (SO) and mixture of cod liver oil, goose fat and soybean oil (BLEND). Rainbow trout were randomly distributed in three replicate research tanks with 25 fish per tank for each diet group, and the feeding trial lasted for 56 days. Dietary lipid sources did not significantly affect growth performance, feed conversion ratio, survival rate and whole-body mineral composition. The crude lipid and neutral lipid contents were found to be highest in the fish fed GF diet and lowest in the CO group. MUFA content was increased by GF, while it was decreased by SO. Diets generally increased the amount of n-6 PUFA while it decreased the amount of n-3 PUFA and the ratio of n-3/n-6 PUFAs compared to control. The GF diet decreased whole body α-linolenic acid (ALA) and linolenic acid (LA) levels, while SO and BLEND increased these levels. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) levels were decreased generally in three diets compared to the control group. mRNA expression levels of delta-6-desaturase enzyme were increased in SO and BLEND groups with highest values in SO in both muscle and liver tissues. Elongase expression levels were increased in GF and BLEND groups with highest values in GF in both muscle and liver tissues. It was concluded that synthesis of EPA and DHA from LA and ALA is limited in fish fed all diets. Goose fat can be promising dietary source due to its potential of increasing the n-6 content, decreasing the moisture holding capacity and positive effect on flesh quality.