Spring Spawning Adult Walleye Population Estimates
Great Lakes Indian Fish and Wildlife Commission 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. By assuming that the ratio of marked to unmarked walleye is the same in the recapture sample as in the entire lake, this information can be used to estimate the total number of 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.
Fall Walleye Recruitment Surveys
Great Lakes Indian Fish and Wildlife Commission fisheries assessment crews conduct walleye recruitment surveys in the fall of each year. The primary objective of these surveys is to assess year class strength of stocked or naturally reproduced age 0 and age 1 walleye, or those "hatched" in the spring of the year, or in the spring of the previous year. Larger walleye and other game fish such as bass, northern pike and muskellunge are of secondary priority and are collected only if this effort does not detract from the collection of age 0 and age 1 walleye. Panfish and other species are collected as a third priority. Data from fall walleye recruitment surveys are used to determine whether lake recruitment code changes were needed, which are a way for tribal and state biologists to characterize whether the primary source of walleye recruitment in a lake is natural reproduction, stocking, or some combination of the two. Other uses included trend analysis of important mixed fishery lakes maintained by natural reproduction, and the development of a regional perspective of annual walleye year class strength.
During fall walleye recruitment surveys, assessment crews capture walleye by electrofishing, which temporarily stuns fish so they can be collected with nets. Electrofishing begins at dusk and continues until the entire shoreline or a designated portion of the shoreline has been surveyed. All fish collected are identified to species and their length. Scale samples are collected from a subset of walleye, which are later examined to determine what lengths of walleye are age 0, age 1, or older. After collecting these data, the fish are returned to the lake.
When the fall walleye recruitment survey data are summarized into reports, catch per effort (CPE) statistics are generated by dividing the numbers of age 0 and age 1 walleye captured by the length of the shoreline surveyed, which gives the number of age 0 and age 1 walleye captured per mile of shoreline surveyed. These CPE statistics can be easily compared from lake to lake in order to evaluate recruitment. On lakes sustained primarily or entirely by natural reproduction, age 0 CPEs typically average between 20 and 30 age 0 walleye per mile, and age 1 CPEs typically average between 6 and 10 per mile. On lakes sustained primarily or entirely by stocking, CPE values are usually lower. Tables and graphs are available showing mean and median CPEs from fall walleye recruitment surveys on lakes sustained primarily or entirely by natural reproduction across the Wisconsin ceded territory, and include data collected by the Wisconsin Department of Natural Resources.
Other Surveys
Other surveys conducted by GLIFWC inland fisheries assessment crews include spring walleye length frequency surveys to collect data on the average length of walleye in the lake. Post-spawning spring juvenile walleye surveys have been conducted on Mille Lacs Lake (Mille Lacs County, Minnesota); it is hoped that information on age 1, 2, and 3 walleye from these surveys will contribute to efforts to predict and model the status of the Mille Lacs Lake walleye population. Summer walleye recruitment surveys have been conducted on Kentuck Lake (Vilas County, Wisconsin) in order to assess survival of recent walleye year classes and to check for the presence of age 0 walleye that may have been produced through natural reproduction. Fish community assessment surveys have been conducted during the summer on Kentuck Lake using fyke nets in order to gather information on relative abundance of different fish species in the community.
