A. D. Pickering

Fish stress and health in aquaculture

Preface 1. Stress in finfish: past, present and future - a historical perspective B. A. Barton 2. Effects of rearing conditions on the health and physiological quality of fish in intensive culture G. A. Wedermeyer 3. Effects of stress on reproduction and growth of fish N. W. Pankhurst, and G. van der Kraak 4. The endocrinology of stress J. P. Sumpter 5. Ionic, osmotic and acid-base regulation in stress G. McDonald, and L. Milligan 6. Behavioural response to stress C. B. Schreck 7. Genetic basis to the stress response: selective breeding for stress-tolerant fish T. G. Pottinger and A. D. Pickering 8. Immune-endocrine interactions P. Balm 9. Dietary effects on stress and health T. C. Fletcher 10. Measurements of stressed states in the field J. D. Morgan, and G. K. Iwama Index.

Stress responses and disease resistance in salmonid fish: Effects of chronic elevation of plasma cortisol

Basal levels of plasma cortisol in unstressed salmonid fish are normally in the range 0–5 ng ml−1. An acute stress such as handling or 1 h confinement caused a temporary elevation of the plasma cortisol levels of both brown trout,Salmo trutta L., and rainbow trout,Salmo gairdneri Richardson, in the range 40–200 ng ml−1 with a return to basal levels within 24–48 h. The extent of the cortisol elevation in response to an acute stress was dependent upon both the species and strain of trout. Chronic stresses, such as prolonged confinement or crowding, resulted in an elevation of plasma cortisol levels to approximately 10 ng ml−1. Under these circumstances, blood cortisol levels remained elevated for periods of up to 4 weeks before acclimation finally occurred.It is shown, by means of intraperitoneal implantation of cortisol, that chronic elevation of plasma cortisol levels in the brown trout results in a dose-dependent increase in mortality due to common bacterial and fungal diseases. This effect is apparent at plasma cortisol levels as low as 10 ng ml−1, levels below those often reported as being representative of ‘unstressed’ fish. These findings are discussed in relation to the known immunosuppressive effects of corticosteroids in teleost fish.

The effects of acute and chronic stress on the levels of reproductive hormones in the plasma of mature male brown trout, Salmo trutta L

Chronic confinement for 1 month caused a significant elevation of plasma cortisol but suppressed the levels of plasma testosterone and 11-ketotestosterone in sexually mature male brown trout. An acute handling stress for 1 hr elevated blood cortisol and ACTH levels and also suppressed circulating androgens. This androgen suppression in response to acute stress was accompanied by an elevation of plasma gonadotropin levels. These findings are discussed in relation to stress-induced suppression of reproductive function in mammals and the possible biological consequences of such a suppression in fish are outlined.

The deleterious effects of cortisol implantation on reproductive function in two species of trout, Salmo trutta L. and Salmo gairdneri Richardson

Implantation of a cortisol-releasing pellet (60 mg kg-1 fish) into the peritoneal cavity of brown trout, Salmo trutta L. (sexually maturing males and females), and rainbow trout, Salmo gairdneri Richardson (maturing males and immature fish of both sexes), significantly elevated their plasma cortisol level. At 18 days postimplantation, cortisol-implanted sexually maturing male brown trout had smaller gonads, a lower plasma testosterone level, and less gonadotropin in their pituitary gland than control fish. Plasma levels of 11-ketotestosterone and gonadotropin were not significantly affected. Cortisol-implanted sexually maturing female brown trout had smaller gonads, reduced plasma levels of 17 beta-oestradiol, testosterone, and vitellogenin, and a lower pituitary gland gonadotropin content than control fish. The plasma gonadotropin level was unaffected. At 36 days post-implantation, cortisol treatment of maturing male rainbow trout significantly suppressed plasma gonadotropin levels. Plasma levels of testosterone, 11-ketotestosterone, and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one, pituitary gonadotropin content, and gonad size were not significantly affected. In sexually immature female rainbow trout, cortisol administration suppressed the level of vitellogenin in the plasma, compared to control-implanted fish. The 17 beta-oestradiol level was not affected. Cortisol implantation did not affect the plasma testosterone level in sexually immature male trout. These results suggest that prolonged elevation of plasma cortisol, to levels well within physiological range, can affect a wide range of reproductive parameters in both brown and rainbow trout. Further, some effects are manifest in immature as well as in mature fish. These findings are discussed in relation to the effects of cortisol treatment on the state of health of the treated fish.

Effects of acute and chronic stress on the levels of circulating growth hormone in the rainbow trout, Oncorhynchus mykiss.

The acute stress of handling followed by confinement for a period of 1 or 24 hr caused a typical stress response in rainbow trout (elevation of plasma ACTH and cortisol) and a significant reduction in the concentration of circulating growth hormone. The chronic stress of low oxygen levels in both crowded and uncrowded tanks of fish caused a significant elevation of circulating GH levels, an effect which was abolished by the provision of additional aeration to the rearing tanks. This chronic elevation of GH levels was closely correlated with an elevation of plasma cortisol in the same fish. These findings are discussed in relation to stress-induced growth suppression and to the links between the hypothalamic-pituitary-interrenal axis and somatotrope activity.

The effect of starvation on growth and plasma growth hormone concentrations of rainbow trout, Oncorhynchus mykiss

Two experiments, one using 0+ the other 1+ rainbow trout, were conducted to investigate the effect of prolonged starvation on plasma growth hormone levels. The results from both experiments were essentially the same. As expected, starvation resulted in cessation of growth and in a lower coefficient of condition, whereas fed fish continued to grow and remained in good condition. Starvation had relatively little effect on the plasma cortisol level; in one experiment levels were elevated temporarily in starved fish, although by the end of the experiment there was no longer any difference between starved and fed fish, and in the other experiment plasma cortisol levels remained very low throughout the course of the experiment in both starved and fed fish. In contrast, in both experiments starvation had a pronounced effect on the plasma growth hormone level, which rose steadily during both experiments, such that it was six times higher after 1 month of starvation in 0+ fish, and five times higher after 6 weeks of starvation in 1+ fish. Thus, paradoxically, fed fish had very low plasma growth hormone levels and grew rapidly, whereas starved fish had elevated plasma growth hormone levels but did not grow. In both experiments a strong negative correlation was observed between the plasma growth hormone level and the coefficient of condition of the fish. The results are discussed with regard to the well-established metabolic changes that occur during starvation, and it is suggested that a major role of growth hormone during starvation is to aid in the mobilisation of fatty acids and glycerol from adipose stores.

Seasonal and diel changes in plasma cortisol levels of the brown trout, Salmo trutta L

A marked diel variation in plasma cortisol concentration was demonstrated in the brown trout, Salmo trutta L. For most of the year this variation took the form of an elevation of cortisol levels during the hours of darkness. There was some evidence of a phase shift from a peak at 2400 hr during the spring and early summer to a peak at 0400 hr during the late summer and early autumn. During the winter months a nocturnal elevation of plasma cortisol was not evident. A shorter, episodic elevation of plasma cortisol was associated with feeding in the brown trout. These findings are discussed in relation to the effects of stress and intensive cultivation on teleost fish.

Changes in the concentrations of plasma cortisol and thyroxine during sexual maturation of the hatchery-reared brown trout, Salmo trutta L

Abstract The concentrations of plasma cortisol and thyroxine were markedly elevated in mature, ovulated, female brown trout, Salmo trutta L. In mature male fish, cortisol levels were significantly elevated towards the end of the spawning season (although not to the same extent as the ovulated female fish) but there were no changes in circulating thyroxine levels. Immature fish of either sex showed no such changes. Saprolegnia infection caused a massive increase in plasma cortisol concentration regardless of sex or state of maturity but other ectoparasitic infestations did not affect the levels of either hormone.

Cortisol-induced lymphocytopenia in brown trout, Salmo trutta L

The administration of cortisol in the food of brown trout produced peak plasma cortisol titres of about 140 ng ml-1 at 12 hr and markedly reduced the number of circulating lymphocytes, which reached their lowest count at 36 hr. Cortisol did not affect the erythrocyte, thrombocyte, or neutrophil count. Sexual maturation of the male fish was associated with an increase in erythrocytes and a decrease in lymphocytes in the blood. These findings are discussed in relation to the increased susceptibility of the brown trout to a variety of infections during sexual maturation and under conditions of stress.

Stress-induced elevation of plasma α-MSH and endorphin in brown trout, Salmo trutta L

Abstract Handling and confinement caused a pronounced elevation in the plasma cortisol levels of brown trout. This response was more rapid, and the elevation greater, at 13.4 than at 5° although basal cortisol levels were also higher at the warmer water temperature. The large increase in plasma cortisol caused by handling and confinement was not accompanied by any changes in the plasma levels of either α-MSH or endorphin. However, when handling and confinement was combined with a thermal shock, not only was there a rapid and pronounced elevation in plasma cortisol, but there were also concomitant and sustained rises in the plasma levels of both α-MSH and endorphin. The levels of a α-MSH and endorphin induced by the thermal shock were considerably higher than those recorded in long-term, black-adapted brown trout, the only other circumstance in fish known to cause an elevation of the plasma levels of these two peptides. These results indicate that handling and confinement only activated the corticotrophs of the pars distalis, not the melanotrophs of the neurointermediate lobe, whereas when combined with a thermal shock, both cell types were activated.