Fish Population Structure in Arctic Lakes

Abstract

Existing theories attempting to account for the predominance of older, larger individuals and the often observed bimodal character of the length-frequency distribution of unexploited northern fish populations were evaluated and rejected. The error in interpretation appears to result from problems in age determination of northern lake trout (Salvelinus namaycush) and lake whitefish (Coregonus clupeaformis). An alternative explanation of the observed structure of arctic lake fish populations is proposed. Growth of fish in these populations is relatively rapid until first maturity, when it becomes slow through a life span that may exceed 50 yr. Mortality declines rapidly through early life and stabilizes at a very low level through much of the life span. The combination of these growth and mortality patterns produces a population containing many small, few intermediate, and many large fish. Selection by gill nets provides a unimodal sample of large fish or a bimodal sample of large and smaller fish from such populations. A model is developed to demonstrate this and compared with results obtained from several northern Quebec lakes. Key words: Salvelinus namaycush, Coregonus clupeaformis, age, population structure, arctic lakes