Dynamics of an Exploited Population of Lake Whitefish (Coregonus clupeaformis)

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Abstract
The life history of the exploited lake whitefish (C. clupeaformis) population of Lesser Slave lake, Alberta [Canada], is described. Mean age is high, having increased under continued exploitation during the last 30 yr. A method of estimating the population age structure is described. The apparent annual rate of survival of adult fish has fluctuated markedly, but on average seems to lie in the interval 0.40-0.50; the annual rate of natural survival is estimated to be 0.53. Juveniles survive better than adults. Rate of survival during the 1st yr of life was estimated to be about 0.0002. Maturity was late, the median age at 1st reproduction being 6-6 1/2 yr. The regressions of fecundity on age and length are described; length now contributes almost all the attributable variance to maturity and fecundity. The sex ratio fluctuates in time; this is caused by the fluctuations in age structure, since sex ratio varies with age. The relationship between sex ratio and age is used to calculate the relative rates of survival of male and female fish. These data are used to construct a life table and to compute population parameters. The ability of the population to respond to exploitation eroded during the last 30 yr. In some respects, the life history of the Lesser Slave Lake stock appears to be unusual. After about 30 yr of large-amplitude oscillation the whitefish population collapsed in 1965. This was not caused by chronic overfishing. In the 1st place, the rate of fishing mortality is no more than moderate. Secondly, a multiple regression equation describing whitefish catch in 48 other Alberta lakes in which there was no overall decline in catch successfully predicts the observed mean catch at Lesser Slave lake. The oscillations in catch are claimed to reflect a limit cycle in the abundance of the whitefish, driven by a lagged relationship between a predator (the fishermen) and its prey (the whitefish). Whether or not this cycle was deterministically stable, such behavior will inevitably put the population in risk of extinction during troughs in the cycle. Current management policies may encourage the destabilization of whitefish populations, and a remedy is proposed.

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