Implications of varying dietary protein, lipid and 17α-methyltestosterone content on growth and utilization of protein and energy in juvenile coho salmon ( Oncorhynchus kisutch)

Abstract

Duplicate groups of juvenile coho salmon ( Oncorhynchus kisutch) were fed daily to satiation one of four “dry” diets. Diets contained either 35 (LP) or 51% (HP) protein and either 12 (LL) or 17% (HL) lipid on a dry matter basis. Additional groups received the same diets supplemented with 1 ppm 17α-methyltestosterone (MT). Most of the eight treatments were administered for 294 days (July to April). Fish fed HP diets grew faster in weight than those fed LP diets. MT supplementation significantly enhanced growth regardless of diet composition. Appetite was greatest in fish fed LP diets; MT supplementation increased appetite in July and August. Protein efficiency ratios (PER) during these months were inversely related to dietary protein and directly related to lipid content. These trends in PER were, in most instances, reversed during September and October, but a distinct trend was not evident in subsequent months. Groups fed MT diets had increased PERs between July and October. Dietary protein content did not affect proximate composition of fish sampled in November; however, high dietary lipid content of MT-free diets was associated with low body moisture, protein and ash and with high lipid content. These effects on body composition were usually reduced when the diets contained MT. In April, body protein content of fish fed non-supplemented diets was directly related and lipid content inversely related to dietary protein-calorie ratio. MT supplementation of the LP-LL diet increased body protein and ash content, number of β cells in the pituitary, and thyroid activity (histological assessment). Lipid deposition in omentum, liver and whole body was decreased. In April, osmoregulatory ability of fish fed LP diets with MT was impaired. However, hormone withdrawal for 14 days restored normal ability. It is concluded that manipulation of dietary protein, lipid and MT content can significantly improve protein utilization and thus offers a means to lower food costs in coho culture.