The Dynamics of Exploited Lake Trout Populations and Implications for Management

Abstract

This report examines mortality, growth, reproduction, and potential yield of lake trout (Salvelinus namaycush) with a view to understanding the dynamics of exploited trout populations. Natural mortality rates were low (10-37%) except in the presence of lamprey. Total mortality rate of exploited populations ranged up to 92 percent, but populations with mortality in excess of 50 percent were declining. Growth in length varied twofold between populations and exploitation appeared to elicit more rapid growth. Size at first maturity ranged from 330 mm to more than 600 mm, and age of first maturity from 5 yr to 19 yr. Age and size at maturity were not correlated. Five years was the minimum age of maturity even in populations declining due to high mortality. Intermittent spawning occurred and was more common in northern populations. Fecundity ranged from 840 eggs/kg to 1,700 eggs/kg, and the relationship between length and fecundity varied between populations and between years within populations but not necessarily in relation to exploitation. Population size averaged 6/ha for most populations. Most production was confined to the youngest age-classes and production and biomass of mature trout was very low. Estimated yields were less than 0.5 kg/ha for all populations. Although mortality, growth, and maturity may be used as indicators of fishery potential and fishery development they are not certain indicators in trout populations. Available data indicate, however, that exploitation should not exceed 0.5 kg/ha and mortality should not be increased beyond 50 percent. J. WILDL. MANAGE. 42(2):307-328 The lake trout is one of the principal recreational and commercial species in Canada's fresh waters. Although commercial yield has declined dramatically over the last 20 yr (Anon. 1973), this has, to some extent, been compensated by an expansion of recreational fisheries for trout (Martin and Fry 1973, Rawson 1961, R. P. Johnson, Sask. Dept. Nat. Resources, pers. comm.). Some lakes have shown high and stable commercial yields of trout for long periods of time, in particular the Laurentian Great Lakes before the appearance of the lamprey (Wells and McLain 1972, Hile 1949, Hile et al. 1950, Lawrie and Rahrer 1972) but also, for example, Lac La Ronge (Rawson 1961). The 3 largest of the Laurentian Great Lakes each produced between 1.8 and 3.6 million kg of trout annually for more than 50 yr before lamprey attacks annihilated the stocks. Lac La Ronge had an average annual yield of over 45,500 kg for 30 yr until 1950 when emphasis was shifted from commercial to recreational use of trout. The lake has continued to be a good producer of trout by angling, although total yield dropped to about half that taken in the commercial fishery. By contrast the commercial production of trout from Great Slave Lake declined from a peak of 1.8 million kg in 1949 (the fourth year of active commercial fishing on the lake) to virtually nothing in the late 1960's (Keleher 1972, Bond and Turnbull 1973), particularly in the large western basin of the lake. Overexploitation has been implicated in the early extermination of the small lake trout population in Lake Erie (Hartman 1972), and the collapse of trout in Lake Ontario at the turn of the century (Christie 1972). Local overexploitation is suspected for the recreational trout fisheries in Great Slave and Great Bear lakes (Falk et al. 1973). The purpose of this report is to review important features of the biology of lake trout with a view to understanding J. Wildl. Manage. 42(2):1978 307 This content downloaded from 157.55.39.177 on Fri, 18 Nov 2016 04:14:52 UTC All use subject to http://about.jstor.org/terms 308 DYNAMICS OF EXPLOITED LAKE TROUTHealey how this species responds to exploitation. In particular, I shall discuss mortality rate, growth rate, age and size at maturity, and fecundity. A consideration of these parameters in exploited and unexploited lake trout populations, together with estimates of potential production and yield, will form a basis for statements about management. The stimulus to prepare this report came from discussion with colleagues at the Freshwater Institute, Winnipeg and Fisheries Operations, Vancouver, on the problems of managing lake trout. I am indebted to these colleagues for helpful discussions on the general topic of trout management. W. A. Kennedy, L. J. Johnson, and R. LeBrasseur criticized a draft of the manuscript.