Adult Walleye Population Estimates
Fisheries assessment crews conduct mark-recapture spawning adult walleye population estimates in the spring of each year. Soon after the ice goes out, marking surveys are conducted, during which walleye are captured, checked to determine their sex, measured, given a fin clip to identify the fish as being marked, and released back into the lake. On GLIFWC's long-term study lakes, where population estimates are conducted annually, walleye are also given an individually numbered floy tag. Spines are collected from a subsample of fish in order to determine age.
Walleye are usually captured by electrofishing, which temporarily stuns fish so they can be netted by assessment crews, but fyke nets are sometimes used as an additional capture method. Typically, more male walleye are captured than female, due to differing behavior of the sexes during spawning time. During marking surveys, crews focus on areas with greater concentrations of spawning walleye. Marking surveys are conducted until a pre-determined target number of fish have been marked, or until the percentage of walleye captured with the mark is sufficiently large.
Between one and three nights after completing the marking surveys, a recapture survey is conducted with electrofishing gear, during which the entire shoreline of the lake is sampled. Walleye captured in the recapture survey are sexed, measured, and examined for the presence of the fin clip given during the marking surveys. The ratio of marked to unmarked walleye is the same in the recapture sample is assumed to be the same as in the entire lake. This information can then be used to estimate the total number of adult walleye in the lake. If M is the number of fish marked during the marking surveys, C is the total number of fish captured during the recapture survey, and R is the number of marked fish captured during the recapture survey, then the population is estimated by (M+1) x (C+1)/(R+1), where the 1s are incorporated into the formula to correct for bias.
Population estimates are calculated for all sexable walleye and walleye of unknown sex greater than 15", and are stratified into four length groups. When tribal spearing harvest occurs before the recapture survey, an adjustment to the population estimate calculations is made. Since tribal spearing harvest is completely monitored, the number of marked fish harvested is subtracted from the number of marked fish released by the assessment crews, and the total number of fish speared is added to the population estimate, except for walleye of unknown sex less than 15", so that the pre-harvest number of walleye is estimated.
Tribal and state biologists have identified a population density of 3.0 per acre for spawning adult walleye as a benchmark for a healthy, naturally reproducing walleye population. Walleye densities are typically lower in lakes where the population is sustained primarily or entirely by stocking than in lakes where the population is sustained by natural reproduction.