Effects of acute stress on plasma cortisol, sex steroid hormone and glucose levels in male and female sockeye salmon during the breeding season

Abstract

In the sockeye salmon ( Onchorynchus nerka), as well as a number of migratory salmonid species, the plasma level of cortisol, the main stress hormone in fish, increases during the spawning period, and then fish die after the spawning. The response of cultured individuals of this species to an artificial acute stress during the spawning period was investigated by measuring levels of cortisol, sex steroid hormones and glucose in blood plasma. In addition, physiological changes in hormone and glucose levels were studied in wild individuals caught in the lake at different spawning stages of the natural condition. The fish cultured and matured in the research pond were confined in a small net and their blood was collected at 3, 5, 15 or 30 min after the start of confinement. Wild sockeye salmon were caught when they gathered in the mouth of their maternal river and their blood was collected just before and after ovulation and also after the artificial removal of eggs and sperm in pre-ovulatory individuals. Acute stress increased cortisol and glucose levels in males. However, in females, the initial cortisol level in the plasma was higher than that in males and did not change significantly with the confinement time. Testosterone levels in both sexes, 11-ketotestosterone levels in males and plasma 17α-progesterone levels in females decreased with the confinement time. In the natural condition, cortisol levels were higher in females than in males and did not change until the last sampling. In males, the cortisol levels increased from the time of spawning until just before death. Glucose levels in males and females increased whereas cortisol levels in females did not change. It is concluded that during the breeding season, male sockeye salmon respond to confinement stress with elevated cortisol levels and decreased sex steroid levels, while female sockeye salmon show only the latter response. It is postulated that female sockeye salmon are not refractory to stress in the breeding period, because their sex steroid levels decrease in response to stress. In females, cortisol levels probably reach maximum levels before blood samples are taken.