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'. ,~ t .l \,' ~_' :.J..--r ;0" ~;, ,:/,,,,(..;(",/\ North American Journal of Fisheries Management 11:200-205, 1991 ,~ "-" Population Dynamics of a Lightly Exploited Channel Catfish Stock in the Powder River System, Wyoming-Montana ""' DAVID R. GERHARDT I AND WAYNE A. HUBERT u.s. Fish and Wildlife Service, Wyoming Cooperative Fish and Wildlife Research Unit2 University of Wyoming, Laramie, Wyoming 82071, USA Abstract. - The population dynamics of a lightly exploited stock of channel catfish Ictalurus punctatus were assessed in the Powder River system, Wyoming-Montana, with special emphasis on evaluation of changes that may occur if the harvest is increased. The samples contained a substantial proportion of large fish, but growth rate was similar to that in other areas at lower elevations and latitudes. Fish up to 21 years old were collected. Annual exploitation was estimated to be only 1.9-2.0%. Maximum yield was predicted to occur at 18-22% annual fishing mortality, but this level of exploitation would be expected to cause a shift toward smaller sizes of fish and virtually eliminate fish in the large size-classes (> 50 cm total length), which are the principal attraction for the current anglers. l Western U.S. states are best known for their coldwater fisheries, but warmwater fisheries are becoming increasingly popular among anglers in that region (Stewart 1988). As a result, more em- phasis is being placed on the management of warmwater species such as the channel catfish Ic- talurus punctatus. Few data on channel catfish populations are available to assist in developing management alternatives. Smith and Hubert (1988) identified a stock of channel catfish in the Powder River drainage of northeastern Wyoming with rel- atively high numbers of the large fish preferred by anglers. This channel catfish stock thus provides an attractive fishery that may require more inten- sive management. This study was designed to provide quantitative information about the channel catfish stock in the Powder River system. Of primary interest were various aspects of population dynamics, including age, growth, mortality, and potential yield. Be- cause these channel catfish live at relatively high elevation and latitude, the growing season is short- er than that in many other areas where channel catfish occur, and slow-growing fish were expected. We had three objectives relative to the descrip- tion of population dynamics of channel catfish in the Powder River: (1) to assess population struc- ture with respect to length and age of fish in the stock, (2) to estimate annual mortality and ex- 1 Present address: U.S. Forest Service, Arapaho-Roo- sevelt National Forest, 240 West Prospect Road, Fort Collins, Colorado 80526, USA. 2 The Unit is supported by the University of Wyo- ming, Wyoming Game and Fish Department, and U.S. Fish and Wildlife Service. ploitation rates, and (3) to assess the influence of increased exploitation on potential yield and pop- ulation structure. Study Area The Powder River system provides a unique opportunity to study channel catfish. It is in a remote area of northeastern Wyoming and south- eastern Montana, and has been relatively unaf- fected by channelization, extensive water devel- opment, high exploitation rates, or various human impacts common to other waters where channel catfish are native. The study area included the Powder River and two tributaries, Crazy Woman Creek and Clear Creek (Figure 1). The Powder River drainage basin is 34,300 km2, and the drain- age area within the study area (upstream from Moorhead, Montana) is 20,948 km2, most of which consists of semiarid plains grassland and sage- brush shrubland. Annual evaporation exceeds pre- cipitation in most ofthe basin (Hodson et al. 1973). Elevation ranges from 770 m at the confluence of the Powder River with the Yellowstone River in Montana to 3,950 m at the Powder River head- waters in the Bighorn Mountains of Wyoming. In general, the Powder River is a low-gradient stream with poorly developed riparian areas, high- 1y fluctuating flows, and unstable substrates. In- tense storms in the headwaters during summer often result in sudden increases in discharge. Tur- bidity can exceed 500 Jackson turbidity units, and total dIssolved solids sometimes exceed 1,300 mglL (Smith and Hubert 1988). In contrast, Crazy Woman Creek and Clear Creek have well-developed riparian zones, confined banks, stable substrates, and relatively low tur- 200