The object of this research project was to determine the effects of Manitoba Hydro's Selkirk Thermal Generating Station (STGS) discharge of cooling water into Cooks Creek on the overwintering behaviour of Red River channel catfish (Ictalurus punctatus). In December, 1987, and January, 1988, large numbers of channel catfish were killed by cold shock in Cooks Creek when the STGS shutdown. Catfish catch per unit effort (CPUE) in Cooks Creek was found to be 7579 catfish/100 m2/24 hr. Because channel catfish are a warmwater species, catfish were assumed to enter Cooks Creek whenever the STGS operated in the winter months. From October, 1991 to June, 1993, 29 channel catfish were radio-tagged and released at the confluence of Cooks Creek and the Red River. Cooks Creek and the Red River adjacent to the creek mouth were regularly sampled using standardized gill net gangs. When the STGS was not operating the annual catfish CPUE in Cooks creek peaked at 16.51 catfish/100 m2/24 hr. The CPUE averaged 4.27 catfish/100 m2/24 hr in the creek during the warmwater (above 5 oC) non-STGS operating periods. When the STGS operated in the fall of 1991, catfish CPUE peaked at 3.51 catfish/100 m2/24hr and averaged 1.42 catfish/100 m2/24hr. When the STGS operated during the winter of 1992-1993, no catfish were caught in Cooks Creek. However, the CPUE in the Red River adjacent to Cooks Creek during STGS operation in 1992-1993 peaked at 5.73 catfish/lO0 m2/24 hr and averaged 2.10 catfish/100 m2/24hr. Through radio telemetry it was found that above 5 oC, channel catfish were active and using the shallow river areas and tributaries; below 5 oC, channel catfish leave tributaries such as Cooks Creek to overwinter in the Red River and Lake Winnipeg. A temperature based model for channel catfish behaviour was constructed. The model states: 'When the water temperature is above 5 oC, catfish are active, having a high degree of spontaneous activity; when the water temperature falls below 5 oC, the catfish enter a facultative dormancy characterized by reduced spontaneous activity; catfish will always select water temperatures which maximize surplus power. This model can be used to predict the effect of future STGS operation on channel catfish. If the STGS begins operating after the Red River drops below 5 oC, there will not be any channel catfish in Cooks Creek and channel catfish in the Lower Red River will not be susceptible to recruitment by the STGS thermal plume. If the station begins operating before the Red River temperature falls below 5 oC, then channel catfish will be recruited into the Cooks Creek thermal plume until the Red River temperature drops below 5 oC. The number of fish recruited into the thermal plume is dependent upon the length of time the station operates while the river is above 5 oC, and the proportion of Red River flow diverted to Cooks Creek by the STGS. Channel catfish entrained by the thermal plume will remain in Cooks Creek until the STGS shuts down and the creek returns to ambient Red River water temperatures. If minimum impact or intermediate impact operating schedules are used in combination with the modified shutdown procedure, installation of a fish fence in the mouth of Cooks Creek is not necessary to prevent channel catfish kills during winter STGS operation. This research has formed a basis for the STGS operating guidelines.