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l DYNAMICS OF LIGHTLY EXPLOITED CHANNEL CATFISH 201 'J{ . '! WYOMING Broadus . .~O~T~N~._._._._.__._._._._._._ WYOMING o FiGURE I. - Study area and sampling sites in the Pow- der River system, 1987 and 1988. bidity. Biotic productivity is higher in the tribu- taries than in the Powder River. Crazy Woman Creek and Clear Creek are the only tributaries in the Wyoming portion of the system that could be considered perennial streams. Channel catfish hav~ access to the entire river system c:xcept for a seq- tion of Clear Creek upstream from a diversio~ dam 11 km from the creek's confluence with the Powder River. Methods During 1987, eight sampling sites were estab- lished from the upper reaches of Crazy Woman Creek near Interstate 90 to the lower reaches of the Powder River near Moorhead, Montana, a dis- tance of290 km (Figure 1). Sites were selected on the basis of water depth (I-I. 5 m) and accessibility by vehicle. Areas of unstable substrate were avoid- ed, when possible, to reduce the probability of nets being inundated with silt. Three sites (1-3) were in Crazy Woman Creek, four (4, 5, 7, 8) in the Powder River, and one (6) in Clear Creek, near its mouth. Sites I, 4, and 7 were not sampled in 1988, and site 6 was abandoned early in the 1988 field season due to frequent vandalism; however, an additional site (9) was established in Clear Creek, 11.5 km upstream from the mouth. Fish were captured with hoop nets of two de- signs, D-shaped metal-framed nets and circular wood-framed nets. Metal-framed nets consisted of seven hoops, the first of which was 1 m in diameter and D-shaped. Netting was 3.8 cm (bar measure) at the open end and decreased to 2.5 cm at the cod end. Wood-framed nets consisted of seven circular hoops: the open end was 76 cm in di- ameter. Netting was 5-cm-square mesh at the open end and decreased to 2.5 cm at the cod end. Both types of net had square throats attached to the second hoop and a finger throat on the fourth hoop. Thirty-two hoop nets were set at the eight sam- pling sites: 3 each at sites 1,2,6, and 7; 4 at sites 3 and 5; and 6 at sites 4 and 8. The same number of nets was set at the sites sampled in 1988, and 2 nets were set at site 9. Total length, weight, and age were determined for all channel catfish collected during the two years. Total length was measured to the nearest milli- meter. Fish weighing less than 2 kg were weighed to the nearest gram, all those weighing 2 kg or more were weighed to the nearest 45 g. The right pectoral spine was removed for aging. Fish were tagged with numbered Floy anchor tags (FD 67C) and released at the site of capture. Tags were in- serted with a Floy tagging gun under the dorsal fin on the left side, where the anchor "T" would lock behind the interneural bones. Tags were sequen- tially numbered, and markings included the amount of the reward (US$5) and the name and address of the Wyoming Game and Fish Depart- ment in Cheyenne, Wyoming. We used the incremental approach described by Gabelhouse (1984) to describe length-frequency distributions and derive relative stock density (RSD) indices. We expressed RSDs as the per- centages of stock -length and larger fish (:;:: 30 1 mm) in each offive size categories: stock (301-400 mm), quality (401-500 mm), preferred (501-600 mm), memorable (601-700 mm), and trophy (>700 mm). Mean length at a given age was computed from the observed lengths of fish captured within each group. Spines were air-dried, fixed in formalin, decalcified with a hydrochloric-formic acid solu- tion, dehydrated with alcohol, and permeated with histological-grade paraffin. The tissue was then embedded in paraffin blocks and sectioned at the distal end of the basal groove with a microtome.