VULNERABILITY TO PREDATION AND PHYSIOLOGICAL STRESS RESPONSES OF EXPERIMENTALLY DESCALED JUVENILE CHINOOK SALMON, ONCORHYNCHUS-TSHAWYTSCHA

Juvenile salmonids, Oncorhynchus spp., commonly encounter conditions ( e.g., during hatchery release and dam passage) that result in damage t o the skin, scale, and slime complex. We conducted laboratory experime nts to determine if descaling of juvenile chinook salmon, O. tshawytsc ha, increased their vulnerability to predation, and to assess the phys iological stress responses elicited by descaling. Salmon were experime ntally descaled on either 10% or 20% of their total body area. When of fered equal numbers of control and descaled juvenile chinook salmon, n orthern squawfish, Ptychocheilus oregonensis, did not consume signific antly more of either prey type (48-60% of consumed prey were descaled) . Juvenile chinook salmon descaled on 10% of their body area did show significant physiological stress responses, however. Mean concentratio ns of plasma cortisol peaked 1 h after descaling, and returned to cont rol levels by 12 h. Plasma glucose peaked 3 h posttreatment and remain ed elevated for 24 h. Plasma lactate increased immediately following t reatment and returned to undisturbed control levels by 3 h. The osmore gulatory response of plasma potassium was highly variable, but plasma sodium decreased immediately and remained low for 24 h. The observed p hysiological responses suggest that descaling of juvenile chinook salm on could result in decreased resistance to disease and other stressors encountered in the field, possibly leading to reduced performance cap acity and lowered survival.