The Effects of Increased Individual Growth Rates on Depressed Population Size

Abstract

There are many examples of aquatic populations that have shifted to a lower equilibrium size after a period of intense exploitation. Holling (1973) discussed the history of several populations in the Great Lakes that were heavily fished, collapsed to a low level, and remained at that level even though fishing pressure decreased. Le Cren (1958) and Le Cren et al. (1972) described the decline of the Eurasian perch (Perca fluviatilis) population in the north basin of Lake Windermere following a period of intense exploitation that began in 1939. After exploitation stopped in 1948, the population remained at a low level until the early 1960's when biomass, but not numbers, achieved its preexploitation level (Bagenal 1977). Yellow perch (Perca flavescens) populations in Lake Michigan declined in the early and mid-1960's and have remained low since then (Wells 1977). The Pacific sardine (Sardinops caerulea) fishery experienced high catches during the 1930's followed by a precipitous decline during the late 1940's and early 1950's (Murphy 1966, 1977). Fishing has continued until the present but landings have remained at a low level. Similar declines have been observed in other clupeoid populations (Murphy 1977). The population of pink salmon that spawns on odd years on the Atnarko River in British Columbia dropped from a high of 25 million to about 80,000 in 1967 (Peterman 1977). Although exploitation has diminished, it has remained at a low level since then. The annual central California Dungeness crab (Cancer magister) catch declined from an average level of approximately 2 million kg through the 1940's and 1950's to a level less than 500,000 kg through the 1960's and 1970's (fig. 1). Exploitation has continued throughout this period but its intensity has decreased since the decline. One or more causes have been ascribed to each of these declines. Explanations of declines in the Great Lakes have involved fishing pressure, changes in the environment, changes in predators, and changes in competitors (Holling 1973; Wells 1977). The decline of perch in Lake Windermere has been attributed to increased predation due to a change in age structure of the pike population caused by fishing pike (McCormack 1970). Murphy (1966, 1977) attributed the continuing