| dc.description.abstract | 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. | |
