Effects of feeding level and sexual maturation on carcass and fillet characteristics and indices of protein degradation in rainbow trout (Oncorhynchus mykiss)

Abstract

Sexual maturation in salmonids requires mobilization of proteins from muscle tissue, which is evidenced by increased expression of proteolysis-related genes and decreased muscle protein content. However, it is unknown how ration level affects this proteolytic response. In the current study, female diploid rainbow trout (Oncorhynchus mykiss) approaching ovulation were fed for 12weeks at 0.25% and 0.50% tank biomass, and to apparent satiation. Triploid trout, which exhibit little ovarian growth, were included at the 0.50% ration level. Gonad somatic index increased in diploids from 3.98% to approximately 13% and was unaffected by ration. Reduced feed intake and maturation negatively affected protein and lipid deposition in skeletal muscle, as indicated by reduced fillet weight and reduced fillet protein and lipid content. During mid-vitellogenesis, expression of genes involved in proteolytic pathways was higher in diploids compared to triploids and the majority of the differentially expressed transcripts were cathepsin and autophagy-related genes. These differences increased with maturation and expanded to include multiple components of the proteasome, ubiquitin ligases, calpastatins, and caspase 9. Expression patterns of multiple proteolysis-related genes suggest that fish consuming the moderate 0.50% ration had the lowest capacity for protein degradation. In summary, these results suggest that maturation increases protein degradation and that higher levels of feed intake are unable to alleviate these effects. However, higher levels of feed intake prevent a net loss of muscle protein, suggesting that dietary nutrients are able to replace endogenous nutrients mobilized from skeletal muscle in support of gonad growth.