Collapse of the Lake Whitefish (Coregonus clupeaformis) Fishery in Southern Indian Lake, Manitoba, following Lake Impoundment and River Diversion

Abstract

The lake whitefish (C. clupeaformis) fishery of Southern Indian Lake (SIL) underwent a general collapse after lake impoundment and Churchill River diversion in 1976. The fishery was substantial from its inception in 1941, with a mean annual whitefish catch of 333,500 kg over the 3 decades prior to lake impoundment. The whitefish catch prior to impoundment was composed almost exclusively (> 99%) of light colored, export (A) quality fish that were only slightly parasitized with the muscle cysts of Triaenophorus crassus. The market quality of the catch was maintained by selective fishing of certain lake basins and avoiding areas of the lake that were known to produce lower quality fish. Catch per unit of effort on traditional fishing grouds declined soon after impoundment to about 1/2 preflooding levels. Total catches were maintained at or near preimpoundment levels by major increases in total effort until 1982, when the whitefish catch fell to about 1/3 of its preimpoundment mean. Fishermen also responded to declines in catch per unit of effort on traditional fishing grounds with a major geographic shift of effort into areas formerly avoided. As a result, darker colored, continental (B) quality whitefish comprised from 12 to 72% of the summer catch in the 4 yr following impoundment. Dark whitefish are less marketable because of color and because of higher rates of Triaenophorus infestation. The mean Triaenophorus cyst count in shipments has increased markedly since impoundment and the lake was reclassified from export (A) to continental (B) classification in 1982. Catch declines on traditional fishing grounds apparently reflect the emigration of fish to other SIL basins and/or to adjoining water bodies. Prior to lake impoundment, there were significant differences in glycerol-3-phosphate dehydrogenase allele frequencies from whitefish samples taken from 4 basins of SIL and an adjacent unconnected lake, but these differences were absent after impoundment. These stock redistributions were probably triggered by physical changes in SIL after lake impoundment.