Poor water quality suppresses the cortisol response of salmonid fish to handling and confinement

Confinement of brown trout in small troughs of static water for 1 h at a density of six fish per 25L stimulated the hypothalamic-pituitary-interrenal axis and resulted in an elevation of plasma cortisol from basal levels (less than 2 ng/ml) to about 100 ng/ml, the degree of stimulation being dependent upon water temperature. Confinement at a density of 30 fish per 25L resulted in a 50% suppression of this response. It is demonstrated that this effect is mediated by changes in water chemistry and not by crowding per se. Experimental manipulation of the water chemistry showed that reduced pH (7.1 to 6.3), elevated free CO2 (63 to 520 μmol/L) or elevated ammonia (8 to 1300 μg/L as total ammonia nitrogen) had no individual effects on the interrenal response to acute confinement. Elevated ammonia in combination with reduced pH significantly increased the plasma cortisol levels in response to acute confinement, whereas a combination of reduced oxygen (100 to 20% saturation), elevated free CO2 and elevated ammonia markedly suppressed (∼ 50%) the cortisol response of both brown trout and rainbow trout to acute confinement in a manner similar to that observed with trout at high densities. A compensatory increase in plasma cortisol levels was observed during the subsequent recovery of fish which had been confined for 1 h in water of poor quality. These findings are discussed in relation to the exposure of fish to multiple stresses and to the role of corticosteroids in the stress response.