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BEHAVIOR AND PHYSIOLOGY OF MARKED FISH should lead to more accurate population esti- mates. Our goal was to evaluate the use of electrofish- ing for population estimation. We investigated the possible effects of capture, handling, marking, and multiple electroshocks on the normal behavior and physiology of coastal cutthroat trout Oncorhyn- chus clarki. Although there are variations in the methodologies for mark-recapture and depletion experiments, we used fairly representative pro- cedures. Specifically, our objectives were to de- termine the effects of (1) electroshocking plus marking and multiple electroshocks on cutthroat trout behavior in a natural stream; (2) electro- shocking plus marking on the integrity of social hierarchies, frequency of aggressive behavior, and feeding rate of wild and hatchery fish in an arti- ficial stream; and (3) a single electroshock, mul- tiple electroshocks, and electroshock plus marking on selected physiological indicators of stress. 645 TABLE 1.--Selected physical characteristics of 12 sec- tions of Mill Creek, Oregon, used for electrofishing ex- periments on cutthroat trout during the summer and autumn 1987-1988. mr = mark-recapture; dep = de- pletion method. Boul- Electro- Mean ders fishing Mean Mean canopy (num- treat- Length width depth cover her > ment Section (m) (m) (cm) (%) 50 em) used t 53 7.7 30.6 43 1 mr, dep 2 42 6.3 18.7 28 7 mr, dep 3 34 5.5 32.0 5 3 mr 4 43 8.2 22.4 96 19 mr 5 62 5.3 19.6 27 10 mr, dep 6 63 7.3 27.0 48 16 mr 7 86 6.7 22.0 43 11 mr, dep 8 60 5.4 17.5 90 mr 9 55 9.5 27.8 70 15 dep 10 58 4.8 31.5 63 5 depe 11 75 8.1 23.5 50 9 mr 12 50 40 4 mr a Data were unusable because of equipment failure Methods Natural stream observations. -Electrofishing experiments were conducted at Mill Creek, a third- order tributary of the Yamhill River, which in turn flows into the Willamette River in Polk County, Oregon. Twelve creek sections were se- lected for experiments on the basis of their suit- ability for effective electroshocking and snorkeling and the number of cutthroat trout present. Con- ductivity ranged from 59 to 80 µS/cm, average water velocity from 3 to 10 cm/s, and water tem- peratures from 11 to 13°C during the study. Other physical characteristics of the study sections and the electrofishing treatment used in each section are listed in Table 1. Experiments were conducted during summer and autumn in 1987 and 1988. A preliminary dive in each section enabled us to observe the behavior of normal, undisturbed cutthroat trout qualitatively. This information, along with observations on cutthroat trout behav- ior in other streams, enabled us to describe char- acteristics of normal fish behavior and was the basis for assessing any changes in behavior caused by electrofishing treatments. To evaluate the effects of capture by electrofish- ing and subsequent marking, we subjected 10 of the 12 experimental sections to the following pro- tocol (section 8 also was treated in 1988). Block seines were placed at the upstream and down- stream ends of the section to prevent fish emigra- tion or immigration. A single upstream pass fol- lowed by a relatively quick downstream pass was made by three or four people; two were fitted with backpack electroshockers (Coffelt model II-A3 used to deliver 600 V DC), and either one or two (de- pending on the size of the section) were netters. Cutthroat trout longer than 100 mm (total length) were anesthetized in unbuffered tricaine (MS-222), measured to the nearest mm, and weighed to the nearest g. Fish were then marked with colored fingerling tags on monocord thread inserted by needle through the dorsal musculature behind the dorsal fin so that the tag hung about 2.5 cm from the point of insertion. These tags had no apparent short-term deleterious effects on the fish and were highly visible under water. Fish were allowed to recover (to the point of swimming upright) in buckets of fresh water and released one or two at a time throughout the length of the section. It re- quired 20-30 min to complete the electrofishing runs and less than I min to weigh, measure, and tag each fish. Upon release and at 1-6, 24, and 168 h after release, we snorkeled in the stream section (av- erage time 15 min) to observe the behavior of marked and unmarked fish qualitatively. Some sections were not snorkeled at all times. The diver followed each marked fish after release to record the immediate behavioral response in terms of choice of specific location within the section. At 3 Mention of trade names or manufacturers does not imply endorsement by the U.S. Government or any oth- ers involved with this study.