Fisheries - Electrofishing
It’s a quiet evening, and you’re enjoying the solitude at your lake home. Suddenly you’re aware of what sounds like the noise of a generator. “But at 11:00 in the evening?” From around the point comes a large boat, with two extremely bright floodlights. “What’s going on? Is someone spearing?”
This is the story we hear every spring and fall: concerned homeowners contact State Conservation Officers to report spearing on their lake. It is actually one of our crews of biologists and technicians, performing standardized electrofishing to collect data on the walleye populations throughout northeastern and central Minnesota.
The Fond du Lac Resource Management Division, together with the 1854 Authority, meets each year with local Department of Natural Resources Fisheries personnel to identify lakes where additional information on walleye populations is needed. As a result of those discussions, the Fond du Lac Resource Management Division and the 1854 Authority, in cooperation with the DNR, schedule electrofishing assessments for adult and young-of-the-year (age-0) walleye populations for the upcoming sampling season.
Electrofishing assessments begin shortly after sunset. At that time our boat, equipped with a large generator, bright flood lamps, and a transformer that regulates and controls electrical current, moves slowly along shorelines where walleyes are concentrated. The generator and transformer provide controlled power to two electrodes mounted on the front of the boat. When walleyes (or other fish) enter the electrical field, they undergo a reaction known as galvanotaxis. The electrical current causes involuntary muscular contractions and the resulting movements actually draw the fish toward the front-mounted electrodes. At that time crewmembers net the stunned fish and transfer them to an aerated tank in the boat. After being placed in the tank, fish recover rapidly, usually within one to two minutes.
After each sampling station has been completed, electrofishing stops and the crew will begin to “work-up” the samples and to collect data from each fish. Each walleye will be measured, and scale or spine samples may be taken for age and growth analysis. All fish are released unharmed close to the point of capture.
Electrofishing differs depending upon the season. Spring electrofishing targets adult walleye. The objective is to obtain a population estimate for each lake, and to compare that estimate with gill net data collected by the MN DNR that same year. The goal is to correlate gill net catch data with spring electrofishing catch data. MN DNR staff identifies lakes that are scheduled for standard trap and gill net assessments by DNR personnel for that summer. These lakes are then targeted for spring electrofishing surveys. Spring surveys continue for 3to 5 nights, as our crews attempt to mark as many individual walleye as possible. Each sampled walleye receives a mark, typically the removal of the second or third dorsal fin spine. As sampling continues into the third and fourth nights, typically a significant number of adult walleye will have been marked in previous evenings. A population estimate is calculated based upon the number of walleye marked during previous evenings, the total number of walleye sampled during the last evening, and the number of marked walleye sampled on the last night.
The dorsal fin spines that are removed for marking the walleye are also used to age these fish. Scales and spines are prepared and aged much like a forester counting rings on a tree. Information on growth rates and age composition will be calculated and compared between years and lakes to better understand the dynamics of walleye populations.
Two to 3 months following our sampling, the MN DNR performs their standardized gill and trap net assessments in the same lakes. We will have informed the DNR what mark to look for, as well as the total number of marked walleye at large. We ignore fishing mortality, assuming that the same proportion of marked and unmarked walleye will be harvested. The DNR then gets a second population estimate from the gill nets, which can be compared to our estimate.
Fall electrofishing assessments differ from spring surveys on a number of key points. First of all, surveys are conducted for a single night only. Sampling stations are established on all lakes surveyed. These same stations are sampled every year, and the number of age-0 and age-1 walleye per station and per lake are determined. Scales collected from each individual are used to determine the ages of these small walleyes.
Assessments of young-of-the-year (age-0) walleye are not intended to derive a population estimate. Rather, they are intended to develop an index of the success of this year’s reproduction that can be compared to other years and other lakes. The index is simply the catch rate of age-0 walleye, reported as the number sampled per hour of shocking time. Our goal is to be able to identify what the catch rate index needs to be to lead to a “strong” year class. A “strong” year class is one that leads to a significant number of older walleye, which translates into better fishing opportunities.
Currently, we are working on models to predict the number of age-1 walleye next year based upon the number of age-0 walleye sampled this year. While we’re still working on these models, the results look very promising. In the future, we will be looking to correlate our age-0 electrofishing indices with MN DNR gill net data, to build models that will predict the number of adult walleye in the future based upon the number of age-0 walleye now. This will help us identify lakes that should have good walleye fishing in the future, as well as identify lakes that deserve additional management attention.