No abstract

No abstract

Yearling coho salmon Oncorhynchus kisutch acclimated to fresh water or seawater were subjected to severe chronic confinement stress in fresh water, seawater, or a medium (one‐third‐strength seawater) that was approximately isosmotic to the fishˈs blood. Plasma osmolarity, Na, K, Ca, Mg, and cortisol and blood hematocrit were measured in fish sampled at 1, 7, 24, and 48 hours after stress began. Plasma osmolarity and electrolyte concentrations increased during stress in seawater, but generally decreased during stress in fresh water. Osmotic imbalance did not occur in fish that were stressed in one‐third seawater. Hematocrit increased in fish stressed in fresh water and decreased in fish stressed in seawater. Fish acclimated to fresh water tended to regain osmotic balance within 48 hours when stressed in fresh water, but in seawater they died. Fish acclimated to seawater tended to regain osmotic balance within 48 hours when stressed in seawater, but in fresh water a compensatory trend was not evident. Confinement stress greatly amplified the osmotic imbalance after fish were transferred from fresh water to seawater compared with that in unconfined fish. Plasma cortisol concentrations increased during stress for all groups; however, fish stressed in one‐third seawater had lower concentrations of cortisol than the other groups within 7 hours after stress began. Received February 8, 1983 Accepted August 7, 1983

Yearling coho salmon Oncorhynchus kisutch and steelhead Salmo gairdneri were exposed to high (2–3 g/L) or low (0.4–0.6 g/L) concentrations of three kinds of suspended solids (topsoil, kaolin clay, and volcanic ash) as long as 7–8 d. Such exposure did not cause mortality, but plasma cortisol concentrations were temporarily elevated in both species after exposure to 2–3 g/L of suspended topsoil, indicating that such exposure may have been stressful to the fish. Feeding rates of both species were reduced at high exposure concentrations. Exposure of yearling steelhead for 2 d to high or low concentrations of suspended topsoil, kaolin clay, or volcanic ash induced similar elevations of plasma cortisol levels, and, in groups exposed to high concentrations, blood hematocrits were increased. Osmoregulatory performance in fresh water and after transfer to 26‰ seawater was unaffected, and gill tissue appeared normal, after exposure to suspended solids. Exposure of yearling steelhead to high concentrations of suspended topsoil reduced the fishesˈ tolerance of subsequent infection by the bacterial pathogen Vibrio anguillarum. These results suggest that coho salmon and steelhead can survive exposure to high concentrations of suspended solids, but may undergo sublethal physiological stress that reduces their performance capacity.