Pulse fishing of a walleye population : response, recovery and management implications
Spencer, Stephen Cameron
Master of Science
SubjectWalleye fishing Ontario Henderson Lake
Fish populations Ontario Henderson Lake
Walleye (Fish) Ontario Henderson Lake
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The response of the Henderson Lake boreal fish community to a large scale exploitation of walleye was studied. The pulse fishery removed approximately 89 percent (3226) of adult walleye (1980-82) from Henderson lake during a period when recruitment was negligible. Following the exploitation, the ninespine stickleback, a major forage item, rapidly declined and by 1984 had apparently disappeared. By 1986 northern pike and yellow perch populations were increasing, two large yearclasses of young o f the year walleye (in 1985-86) were also produced. I expected that such an excessive harvest would have collapsed the walleye population. This would allow northern pike and yellow perch to increase in size and number with northern pike eventually becoming the sole top predator. Unsuccessful attempts to estimate adult walleye abundance in 1986, 1988 and 1991 seemingly supported the hypothesis that the population would not recover. However, 1994 and 1995 population estimates showed that the walleye population had recovered to about 25 percent (871 fish) of pre-harvest numbers. In addition, the ninespine stickleback were once again caught in seine hauls. Northern pike numbers have declined from pre-harvest levels. Multimesh gillnet estimates o f yellow perch catch per unit effort (CUE) indicated a decrease in abundance from levels measured in 1991. Good indices for measuring population response (harvest effects and recovery) were: average length at age for younger walleye and northern pike, as well as changes in length distributions for both populations. Condition factor and length-weight data were poor indices and were not correlated to fish abundance. Schumacher-Eschmeyer estimates were not good indicators of population abundance during the harvest phase of the study where as six foot trapnet CUE followed actual adult walleye numbers throughout the study period. A model predicting walleye abundance and time to recovery was employed. It included: 1984 population numbers, fecundity at age, pre-harvest estimates of mortality, weather data and age of recruitment to the gear to predict current abundance and time to recovery. Population numbers for walleye in years 1995 and 2003 were predicted at 1450 and 3280 fish, respectively. From this model I estimated it would take at least 20 years to mitigate the effects of a 90% removal of the adult stock. If pulse fishing is to be employed as a management technique, removal rates would have to be much lower.