Abundance and origin of lake whitefish, Coregonus clupeaformis (Mitchill) congregating downstream of the Missi Falls control dam, Southern Indian Lake, Manitoba
Barnes, Nicholas Edward
Large congregations of lake whitefish (Coregonus clupeaformis) accumulating below the Missi Falls control structure at the outlet of Southern Indian Lake (SIL), Manitoba, were studied in the open water seasons of 1986 and 1987 to determine the abundance and origin of these fish. The null hypothesis being tested was that all fish found below the Missi Falls control dam originated from downstream areas. The alternate hypothesis was that fish from Southern Indian Lake were present. A Jolly-Seber multiple mark-recapture model was utilized to provide estimates of population size, births, deaths and survival rates. The origin of the Missi Falls fish was estimated through morphological comparisons of the Missi Falls fish with fish caught in seven locations surrounding the control structure. Abundance increased gradually to a peak of 88,764 (+- 21,415) fish in the autumn of 1986. Abundance in 1987 peaked in the summer to numbers less than one third of the 1986 autumn estimate. Fish caught in 1986 at Missi Falls were in much better condition with fuller digestive tracts than those caught in 1987. Morphological comparisons of the Missi Falls fish indicated differences between fish caught in 1986 and 1987. Fish caught in the autumn of 1986 closely resembled fish caught in the locations upstream of the dam. Fish caught in the summer of 1987 most resembled those caught in the near-downstream lakes. There appeared to be gradual changes in the origin of Missi Falls fish between these two times. High discharges over Missi Falls in 1986 were similar to the discharge levels which existed prior to the impoundment of SIL. The presence of large numbers of healthy upstream-like fish congregating below Missi Falls in 1986 was evidence to reject the null hypothesis. It is thought that these fish represented an upstream stock whose migratory movements upstream over Missi Falls from downstream feeding areas had been interrupted by the control structure. These fish were probably following Churchill River cues and became trapped below the dam on a movement back to spawn and overwinter in SIL. Lower discharges over Missi Falls in the open water season of 1987 were from local upstream inflows only, since, at this time, the Churchill River was diverted to another outlet in SIL. Any migratory cues from the Churchill River would therefore not affect fish at Missi Falls at this time. These lower discharges in 1987 resulted in lower downstream lake levels and summer water temperatures which exceeded those preferred by lake whitefish. The congregation of physiologically-stressed, downstream-like fish below Missi Falls in 1987 was evidence to accept the null hypothesis; however some upstream fish were probably also present at this time. It is thought that the increased water temperatures in the downstream lakes in 1987 prompted fish from the downstream lakes to follow a temperature gradient upstream towards the cooler water at Missi Falls. It would appear that the composition of lake whitefish congregating below Missi Falls can change over time. The relative proportion of both upstream and downstream fish of the Missi Falls population appears to be dependent on environmental factors such as discharge.