Graduate Thesis Or Dissertation
 

Changes in plasma cortisol concentration of juvenile salmonids during stress

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  • I investigated changes in plasma cortisol concentration over time in juvenile salmonids subjected to various stressors that might be encountered in their normal life cycle. This work was directed at finding a general indicator for stress in fish that could be used be used to aid fisheries research and management. Plasma cortisol concentrations in juvenile chinook salmon (Oncorhynchus tshawytscha) netted and confined in a small live-cage rose from approximately 100 ng/ml to about 500 ng/ml in 24 h. Fish dip-netted into a bucket containing aerated water and sampled serially at 90-s intervals showed an increase in plasma cortisol concentration from less than 10 ng/ml to 100 ng/ml in 20 min. In juvenile cutthroat trout (Salmo clarki) acclimated to 13 C and subjected to a rapid increase in water temperature to 26 C plasma cortisol concentration increased from 20 ng/ml to 70 ng /ml in 25 min and remained elevated for more than 3 h, however, an increase in temperature from 12 C to 20 C elicited no change in cortisol. Fish acclimated to diurnal temperature cycles (13-23 C) had no substantial changes in plasma cortisol concentration throughout the cycles. No dramatic changes in basal levels of plasma cortisol were noted as fish grew from 7 to 14 cm over a period of 5 months. Fish acclimated to very warm water (22 or 23 C) had the same initial cortisol concentration as fish acclimated to cool water (9 or 12 C), but the trout in warm water had more erratic changes in cortisol during confinement. Mean plasma cortisol levels in juvenile chinook salmon increased from near 0 to about 200 ng/ml in response to 0.5 h of severe confinement, remained elevated for over 6 h after release, and returned to basal levels within 12.5 h, Fish subjected to severe, continuous confinement had increase in plasma cortisol to about 400 ng/ml during the first 1.5 h; little further increase occurred, and by 12.5 h mortality to the stressor reached 50%. In response to moderate confinement a definite but variable elevation in plasma cortisol occurred with a return to basal levels within 6 to 8 days as the fish acclimated to the stressor. A depression in gill Na+K ATPase was noted in juvenile salmon approximately 3 wk after acclimation to moderate confinement. Brief anesthetization with 50 mg/1 buffered tricaine methanesulfonate (MS-222) of yearling chinook salmon during mild handling resulted in plasma cortisol comparable to those in non-anesthetized controls. Prolonged exposure (180 min) to a depressing dose of MS-222 (25 mg/1) elevated cortisol more than an immobilizing dose (50 mg/1), while 100 mg/1 was lethal within 30 min Fish anesthetized (50 mg/1) during a severe 30 min handling stressor had substantially lower mortality to a second handling stressor after the fish were no longer anesthetized than untreated controls. Anesthetization during the first stressor also prevented the cortisol stress response evident in the control fish. Anesthetic (with or without buffer) administered before initial capture was most effective at increasing survival during a second stressor, while anesthetic supplied after capture was slightly less effective. A 0. 5% NaCl solution supplied after capture was less effective than any anesthetic treatment in increasing future survival, but was better than no treatment. The saline treatment did not attenuate the cortisol stress response. Introduction of seawater (25 to 30 g/1 dissolved solids) caused a slight, transient elevation of plasma cortisol in juvenile chinook salmon. When the fish were severely confined immediately after seawater introduction they had a significantly lower increase in cortisol and better survival than fish confined in fresh water.
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