Fisheries Lake Surveys

Lake Survey Program

The DNR Section of Fisheries is the lead agency responsible for fisheries management in the State of Minnesota. The primary tool that guides fish management is the lake survey. Lake surveys consist of periodic monitoring of fish populations, water chemistry, and fish habitat. Lake survey data is used to track fish population trends, evaluate the effectiveness of management actions such as stocking, and establish realistic management goals for a given lake. On average, the Section of Fisheries conducts 650 lake surveys each year. Most of the important fishing lakes are surveyed once every 3-5 years. The 11 largest lakes, including Mille Lacs, Superior, and Lake of the Woods, are surveyed annually. Smaller, more remote, or lightly used lakes may only be surveyed once every 10-20 years. Most of the lake survey fieldwork takes place from early June through late August, however, many areas also conduct more specialized sampling beginning right after ice-out and again in the fall prior to freeze-up.

Lake Survey Database

Once the summer field work is completed there is much to do with the data that is collected. Data must be entered into the lake survey database, checked for errors, and later analyzed and reported. Data files and reports are then sent to St. Paul where they are uploaded into the statewide database and transformed into the proper format for publishing on the DNR internet site. In general, it takes about 18 months from the time the nets are lifted during a lake survey until the results of that survey are published on the DNR Web site or available as printed reports from the DNR central office. The lake survey database contains information for about 4,000 different waters. If you cannot find a record for a lake that you are interested in, it is possible that the lake has not yet been surveyed, or it was surveyed many years ago and the data has not yet been entered into the database.


The glossary provided below is intended to help the user understand some of the terminology used in the survey reports. We are in the process of rewriting many of the "Status of the Fishery" narratives with the intent of removing some technical acronyms and jargon. We have also received a number of comments regarding the narratives that appear all in capital letters. This problem is a result of transferring some of the older data into newer data systems. Cleaning up the fonts and making the survey reports more user friendly will take some time. We ask that users bear with us during this transition.

Gill net: This is the main piece of equipment used for sampling walleye, northern pike, yellow perch, cisco, whitefish, trout, and salmon. The standard gill net is 6 feet tall by 250 feet long, with 5 different mesh sizes. Gill nets are generally set in off shore areas in water deeper than 9 feet. Nets are fished for a period of 24 hours. Fish are captured by swimming into the net and becoming entangled. Fisheries workers record length and weight data from each fish, determine the sex, look for parasites or disease, and remove several of the fishes scales for determining the fishes age. Most of the fish taken in gill nets are killed, but only a small portion of the lakes fish population is sampled during an individual survey event. The number of gill nets set during a survey is dependent on the lake acreage.

Trap net: This is the main piece of equipment used for sampling bluegill, crappie, and bullheads. The standard trap net is 4 feet tall by 6 feet wide with a 40 foot lead. Trap nets are generally set perpendicular to shore in water less than 8 feet in depth. Nets are fished for a period of 24 hours. Fish are captured by swimming into the lead and following it towards the trap. Most of the fish collected in trap nets are returned back to the water as soon as the necessary biological data is recorded. The number of trap net sets during a survey is dependent on the lake acreage.

Electrofishing: This is a specialized type of equipment that is most often used for sampling largemouth bass, smallmouth bass, and young of the year walleye. A boat-mounted generator is used to induce electrical current into the water that stuns the fish, allowing fisheries workers to net the fish for placement in live wells. Most of the fish caught by electrofishing recover rapidly and are promptly returned to the water after the necessary biological data is recorded.

Secchi disk: The secchi disk is a measure of water clarity. The standard secchi disk is a 8 inch diameter metal plate that is painted with an alternating white and black color pattern. The disk is lowered into the water until it disappears from view. The depth at which the disk can no longer be seen is the secchi depth that is recorded. Secchi disk readings can vary by season with the clearest water generally occurring in the spring, shortly after ice-out. Measurements are usually taken in the summer at the same time the fish sampling is occurring.

Littoral zone: This is defined as that portion of the lake that is less than 15 feet in depth. The littoral zone is where the majority of the aquatic plants are found and is a primary area used by young fish. This part of the lake also provides the essential spawning habitat for most warmwater fishes (e.g. bass, walleye, and panfish).

Dominant bottom substrate: These are the most common materials found in the shallow water areas of the lake (less than 4').

Abundance of aquatic plants: This is a general lakewide description of the amount of water plants found in and around the lake.

Number of fish caught: The catch is reported separately by gear type (gill net or trap net). The numbers presented are the average number of fish per net set. For example: if there were 10 nets set during a survey and 40 walleye were caught, the net catch would be reported as 4.0 fish/net.

Normal range: All of the lakes that have been surveyed have been grouped into 43 lake classes, or groups, based on similarities in chemical and physical characteristics. The normal range is the range of values for net catches or average fish size that could be considered normal for the respective lake class that the surveyed lake belongs to. For example: If the walleye gill net catch for a 1996 survey on Lake Wobegon was 6.0 fish/net and the normal range was reported as 2.0 - 4.5, one could interpret the current population as being higher than would be expected in other similar waters.

Historically, we used the statewide net catch average as the standard measure of comparison with current net catch statistics. We no longer use this statistic because it does not make sense to lump net catches for lakes like Mille Lacs and Lake of the Woods with that of shallow prairie lakes from southwest Minnesota (analogous to lumping apples and oranges). We feel the current system allows for more meaningful comparisons among groups of lakes that are more similar.

Average weight: This is the average weight, in pounds, of all fish caught in a particular gear type. Normal range data for the respective lake classes are also presented in the table.

Fish Stocking: Contrary to popular opinion, most of the fish caught in Minnesota each year are a result of natural reproduction and not a product of the state's stocking efforts. This is because Minnesota is blessed with overall outstanding water quality and excellent fish habitat. However, stocking fish is still an important part of fisheries management, especially where spawning habitat is not adequate to sustain a sport fishery. Approximately 25% of the state's 5,400 fishing lakes are on an established stocking schedule. The most commonly stocked species include walleye, trout, muskellunge, and northern pike. The lake survey reports contain only the last five years of stocking information. If there are no records listed it is likely that the lakes fish community is entirely supported by natural reproduction. Common sizes of fish stocked are as follows:

  • Fry: Fish stocked in lakes shortly after hatching from eggs.
  • Fingerling: Fish harvested from rearing ponds after one summer of growth.
  • Yearling: Fish that are a year old at the time of stocking.
  • Adult: Fish more than 1 year old, usually transferred from other waters.

Species codes: Some of the survey reports use three letter species codes instead of common names. Some of the common ones are as follows: WAE-walleye, NOP-northern pike, YEP-yellow perch, LMB- largemouth bass, MUE-muskellunge, TLC-tullibee, BLG-bluegill, PMK-pumpkinseed sunfish, BRB-brown bullhead, and WTS-white sucker.

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