A Model for Kelt Studies: Effects of Energy Restriction on Metabolic Factors and Reproductive Development in Post Spawning Rainbow Trout

Migration and reproduction in salmonids are energetically costly processes; energy reserves during hypothesized critical periods are an important consideration in decisions to initiate and continue sexual maturation. To determine how energy balance affects the onset of a new reproductive cycle in iteroparous salmonids, we conducted a study on post-spawned female rainbow trout (Oncorhynchus mykiss). We restricted food availability, for four months after spawning in a group of females (0.1% bodyweight per day, 3 tank replicates), and compared these fish with post-spawned fish in a control group that was fed a standard brood stock ration (0.5% bodyweight per day, 3 tank replicates). Bodyweight, length, and muscle lipid content were determined, and blood was collected from all fish at intervals of four weeks. Five fish from each group were terminated at each sampling time for the collection of brain, pituitary, liver, and ovary samples. Plasma levels of reproductive hormones were quantified by radioimmunoassay and enzyme-linked immuno-sorbent assay, and tissue gene expression levels of factors associated with energy balance and reproductive maturation were analyzed using quantitative reverse-transcriptase PCR. Food restriction significantly (p<0.05) reduced Fulton’s condition factor (k), muscle lipid content, and specific growth rate in weight from 2 months onward, and significantly (p<0.05) reduced the hepatosomatic index from 3 months onward. In the liver, insulin-like growth factor (IGF-1 and IGF-2) gene expression was significantly (p<0.05) reduced in food-restricted fish within two months, whereas IGF binding protein-1 gene expression significantly (p<0.05) decreased over time in both treatment groups. Liver leptin gene expression was significantly (p<0.05) lower in food-restricted fish at four months. These results show that this feed restriction regime affected factors associated with energy balance within two to four months. In the brain, gonadotropin releasing hormone-2 (GnRH-2) gene expression was significantly lower (p<0.05) in food-restricted fish at four months, but GnRH-3 was unaffected. In the pituitary, follicle stimulating hormone gene expression was significantly lower (p<0.05) in food-restricted fish at four months. These results show that this feed restriction regime affected the reproductive endocrine axis within four months. While it is unknown whether feed restriction would have resulted in failure to complete ovarian development, this study shows that energy availability in the immediate months after spawning slows reproductive development. Further experimentation will be necessary to determine how energy restriction could arrest, or prevent initiation of, ovarian development. The implications of these findings for steelhead kelt reconditioning programs will be discussed.