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Fishing WisconsinCeded Territory walleye management
The walleye fishery in the Ceded Territory of Wisconsin is very productive and contributes greatly to the enjoyment of our resources by both state residents and tourists. However, as with many other fisheries across the United States, over-exploitation of this resource is a very real concern. Therefore, population estimates are critical to effectively manage walleye populations in the Ceded Territory. Accurate population estimates allow fisheries biologists to calculate the number of fish that can be safely harvested from a given lake based on knowledge of the fishery and the biology of individual species of interest. This allows utilization of the resource without jeopardizing future presence or abundance of walleyes.
The Wisconsin Department of Natural Resources (WDNR) assesses walleye populations using three primary methods: spring adult and total population estimates, fall young of the year relative density estimates, and creel surveys of angler harvest. The Great Lakes Indian Fish and Wildlife Commission (GLIFWC) and the U.S. Fish and Wildlife Service conduct population estimates and young of the year surveys on additional lakes each year. In addition, GLIFWC monitors all tribal harvest that occurs. These methods provide information on the current harvestable population, an indication of the future harvestable population, and the amount of exploitation occurring.
Spring Adult and Total Population Estimates
One important method the Wisconsin DNR uses to assess walleye populations is spring adult and total population estimates. Walleyes are captured with fyke nets each spring shortly after ice out. Each fish is then measured (total length to the nearest tenth of an inch) and receives a permanent mark such as a fin clip. All sexable fish and those of unknown sex that are at least 15 inches in length are considered to be part of the adult population and are given a specific mark unique to that lake. Fish of unknown sex that are less than 15 inches in length are considered juveniles and given a different lake-specific fin clip. The goal is to mark at least 10% of the total spawning population, and marking continues until this target is reached or spawned out females begin to appear in the fyke nets.
To estimate adult abundance in each lake, walleyes are recaptured 1-2 days after netting with the use of electrofishing equipment. The entire shoreline of each lake, including islands, is electrofished, and this effort is used to calculate an adult walleye population estimate. All walleyes in this recapture run are measured and examined for marks. In addition, all unmarked walleyes are given an appropriate mark so that a total population estimate can be calculated. This is done via a second electrofishing effort approximately two weeks later using methods similar to those used to obtain adult population estimates.
Population estimates are calculated by multiplying the number of marked fish known to be in the lake by the total number of fish captured, then dividing by the number of marked fish captured. The abundance of walleyes in each of four length classes (<11.9", 12-14.9", 15-19.9", and >20") is estimated, and these are summed to estimate the adult and total abundance of walleyes in each lake. If spearing occurs after the start of the marking period, the number of marked walleyes speared is subtracted from the number of marked fish at large during the recapture period. These fish are then added back to the estimated number of fish present at the time of marking for the appropriate population (adult or total).
Fish populations in general, and walleye populations in particular, are extremely variable and can change drastically from year to year, so a continuing survey of lakes provides critical information on trends occurring in these populations. However, it is logistically impossible to obtain accurate population estimates from all harvested lakes in the Ceded Territory each year. Therefore, random subsamples of lakes are selected each year for walleye population estimates.
Fall Young of the Year Estimates
Young-of-the-year (YOY) surveys provide an index of the abundance and survival of young walleyes from the time they are hatched or stocked until their first fall. Since young fish form the basis of future adult populations, YOY surveys provide fisheries managers with insight into potential adult population changes in the near future. Early indications of these potential changes allow fisheries biologists to develop management strategies that can accommodate expected changes in adult populations.
Electrofishing for YOY walleyes is done during the early fall, ideally when the water temperature is between 50-65º F. The entire shoreline of a lake is electrofished and all walleyes are examined and measured. The number of YOY walleyes caught per mile of shoreline is used to calculate the density of YOY walleyes per acre, which in turn is multiplied by the number of acres in a given lake to estimate the total abundance of YOY walleyes for that lake.
Creel Surveys of Angler Effort, Catch and Harvest
Creel surveys provide important information used to gauge sport angler impacts on fisheries, including number of hours fished, the total number of fish caught or harvested, and the percent of the total fish in a population that are being harvested. Based on data collected from creel surveys, fisheries managers can determine trends in total catch and harvest, as well as the number of hours it takes anglers to catch fish of a particular species.
Creel surveys in the Ceded Territory are generally conducted May-October and December-March on a random subsample of 15-20 lakes for which population estimates have been calculated so that managers can assure a balance between the number of fish in a population and the rate at which they are being harvested. Surveys are conducted by creel clerks, who visit the lakes at random times to count the number of anglers on the lake and collect information on the species targeted, harvest, catch, lengths and marks of harvested fish, and fishing effort. Information from these interviews is then used to provide an estimate of total fishing effort, harvest rates, and release rates of each species in each lake for the year.