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652 MESA AND SCHRECK 175 o-e electroshock+marking I o •- - • single electroshock 150 0. • • • o control rn °i C: 125 ° NO 100 I 0 75- 0 ? 1 U 50 tam!) 25 bl b \'Jb--_____ 1T b ? 'Qb .. 1 4 [L Q 0 1 3 6 12 24 168 TIME (h) FIGURE 5.-Mean plasma cortisol concentrations in unstressed (control) Alsea Hatchery cutthroat trout and in fish subjected to a single 4-s, 300-V-DC electroshock or electroshock plus marking. Means represent pooled data from two trials (N = 10). Means within a time interval with no letters in common are significantly different (P < 0.05); time intervals with no letters shown indicate no significant difference among the means. Vertical bars represent 1 SE. electroshocks (Figure 8, upper panel). Lactate con- centrations remained elevated for 3 h after the first shock and returned to control levels by 6 h. Al- though lactic acid in fish that received a single electroshock peaked at 1 h posttreatment, this concentration was not different from that in con- trol fish; however, lactic acid was significantly higher than control levels at 3 h and returned to control concentrations for the remaining sample periods. In fish that received the 30-s handling stress, lactic acid reached a peak at 0.5 h and re- turned to control levels by 6 h. In the second trial, lactic acid increased in fish that received multiple electroshocks, peaked immediately after the third shock, and gradually decreased to normal by 6 h (Figure 8, lower panel). Lactate increased only slightly by 0.5 h and returned to normal by 1 h posttreatment in fish that received only a single shock. Discussion Electroshock and the procedures involved in es- timating fish population size elicited a general stress response that was manifested not only physiolog- ically but also behaviorally. This response lasted for several hours, and wild cutthroat trout were affected more severely than hatchery cutthroat trout. In Mill Creek, behavioral changes occurred in fish that were captured by electrofishing and marked. Although variability among sections was high, we typically observed 50% or fewer of the marked fish behaving normally. Perhaps the most striking responses were general lethargy and cryp- tic behavior of the fish, which lasted for several hours. Such lethargic behavior in fish has been reported after various stresses (Bouck and Ball 1966; Sigismondi and Weber 1988). Most of our fish sought some type of cover immediately after release, and several were observed trying to dig into the substrate or wedge themselves between rocks. This cover-seeking behavior was in direct contrast to normal fish behavior in Mill Creek, where fish were seldom found in cover, behaved skittishly, and often fled in the presence of a diver. These behavioral changes might alter the catch- ability of marked fish during subsequent electro- fishing attempts; sick or fatigued fish do not react well to electric current (Halsband 1967). Fish may become so affected by electricity that they become physically unable to be drawn towards the anode (Cross and Stott 1975). Cobble and boulder sub- strates may provide a refuge for fish so that lower percentages offish enter the catch (Saul 1980). Our inability to locate a high percentage of marked fish in several sections of Mill Creek suggested that subsequent electrofishing effort would have been expended over a reduced population. This may have been due to fish emigration or mortality; however, marked fish were rarely seen outside the study sections. We believe that fish remained in the sections but were not located by divers because the fish were in uncharacteristically heavy cover. This cover-seeking response is one factor that