The Lake Where You Live

Maybe a time or two you’ve seen an odd-looking boat on your lake, with people hauling in a big net, or with strips of metal dangling into the water.

That means you’ve observed biologists sample your lake’s fishery. At last month’s Wisconsin Lakes and Rivers Convention, Dan Dembkowski of the Wisconsin Cooperative Fisheries Unit and Josh Raabe of the DNR explained some of the goals and science behind sampling.

The aims of the various techniques, they said, include finding out how a given species population is structures (fish sizes, ages, grown and abundance), enabling comparison of a lake’s population with others, and evaluating how fish respond to environmental stress.

The aim is to collect a representative sample of the lake’s fishery, which means collecting samples in a variety of locations and from different habitats, depths, and near-shore and offshore sites. State fishery organizations generally adopt a set of standard sample methods so that one lake can be reliably compared to others.

There are two basic kinds of sampling methods: active (taking the gear to the fish) and passive (letting the fish come to the gear). Passive mainly involve gill netting and fyke netting. Gill nets, as the name implies, have mesh that traps fish by their gills as they try to swim through. 

Fyke nets generally are anchored in shallow water to capture fish that travel along shorelines. They are long and cylindrical and have a large opening makes it easy for fish to enter. Then they swim to a dead end and cannot escape. 

Active methods include seining with fine-mesh nets to trap juvenile fish and electrofishing to capture adults. Electrofishing is usually done from a boat equipped with batteries and a set of electrodes placed into the water. 

The operator can control the current, voltage and duration of the electrical pulse. The electricity stuns the fish, which float to the surface and are collected for study. They revive rather quickly and are released unharmed. 

Fish collected by any of these methods are counted, measured and weighed, and aged. The “gold standard” for determining age is to examine an otolith (ear bone), which annual has rings similar to those in the trunk of a tree. This measurement, though, requires killing the fish. Alternatively, researchers can count the growth rings in fishes’ scale or in a cross section of a dorsal spine.

Careful sampling can help biologists detect changes in fish populations, such as the relative abundance of species. At present a major area of interest is the shift in population between walleyes and largemouth bass.

The concern is that in many lakes bass are proliferating at the expense of walleyes, which for years have been the crown jewel of Northwoods lakes. One suspected cause is climate change (bass prosper in warmer water). Another is catch-and release, which again favors bass (my pet saying is that to many anglers, bass are for sport, and walleyes are for dinner).

In any case, careful and persistent sampling of these species can help biologists determine what is happening to these two species’ populations, and what management practices might be the most appropriate responses.

So when you see those strange-looking boats in your lake, know that they carry scientists doing essential work to help preserve and enhance our fisheries.

Ted Rulseh, a writer, author and advocate for lake protection, lives on Birch Lake in Oneida County. Visit him and his blog at https://thelakeguy.net.