Graduate Thesis Or Dissertation
 

Stress, osmoregulation, and the hormone cortisol in yearling coho salmon, Oncorhynchus kisutch

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  • Freshwater (FW) and seawater (SW) acclimated yearling coho salmon, Oncorhynchus kisutch, were subjected to severe confinement stress in FW, SW, or a medium (1/3 SW) that was approximately isosmotic to the fish's blood. Chronic stress caused osmotic imbalances in FW and SW, but not in 1/3 SW. In SW, blood osmolarity and electrolyte concentrations increased, while in FW they generally decreased. Acclimation conditions (FW or SW) before stress influenced the severity and duration of the osmotic imbalance. Confinement stress greatly amplified the osmotic imbalance following transfer from FW to SW compared to that in unconfined fish whose water supply was switched from FW to SW. Plasma cortisol levels during stress were also affected by acclimation conditions and ambient salinity. Plasma cortisol levels increased during acclimation to SW. Maxmimal concentrations of approximately 220 ng/ml occurred within 1.5 h after the water source was switched from FW to SW. After 21 d in SW, cortisol levels were still slightly elevated (23 ng/ml) compared to those in FW control fish (4 ng/ml). Chronic treatment with cortisol lowered gill Na+-K+-ATPase levels in FW fish but did not affect plasma osmolarity, Na, K, Ca, or Mg levels in fish in FW or during acclimation to SW. Thyroxine (T) and triiodothyronine (T3) levels in plasma increased significantly after ambient water was switched from FW to SW. Maximal levels of T3 (8.0 ng/ml) occurred within 12 h after the initial exposure to SW, followed by a return to FW basal levels (4.0 ng/mi) within 24 h. Plasma T levels were higher than FW control levels (4.2 ng/ml) for at least 120 h after exposure to SW; peak levels (14.3 ng/ml) occurred at 12 and 72 h. Chronic treatment with cortisol significantly lowered plasma T3 levels in FW and during acclimation to SW; but it had no significant effect on T concentrations. The metabolic clearance rate of corticosteroids determined after a single injection of 3H-cortisol was higher in SW- than in FW-acclimated fish. Uptake and retention of corticosteroids in liver, gill filaments, and gall bladder bile was greater in SW than in FW fish. The stress of long-term (5 d), but not short-term (12 h), continuous confinement apparently increased the clearance rate of corticosteroids in both FW and SW fish. Chronic, but not acute, administration of exogenous cortisol at physiological levels appeared to increase the clearance rate of corticosteroids in FW fish.
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