Fishing in the Dark

The shock boat leaves Hasler Lab’s pier around 8 pm, heading out for a night of electrofishing on Lake Mendota.

A few weeks ago, a team of students and staff headed out on Lake Mendota well after sundown. The group was taking a yearly census of fish populations in Lake Mendota, one of several Wisconsin lakes that make up a study area of the National Science Foundation-funded Long-Term Ecological Research program, or LTER. There are 26 LTER sites across the U.S. and the Center for Limnology houses the project called North Temperate Lakes.
Fish crew members stand poised at the bow of the boat waiting for stunned fish to float into range of their nets along the dark northern shoreline of Lake Mendota.

Besides Mendota, the southern Wisconsin lakes represented are Monona, Wingra and Fish Lake near Sauk City. Up north, the crew monitors Allequash, Sparkling, Trout, Big Muskellunge and Crystal Lakes as well as two bogs all near the CFL’s Trout Lake Research Station in Vilas County.
While all of these lakes are monitored year round for water chemistry, temperature and other parameters, the fish census only happens once each summer. And part of that census involves heading out at night when predatory fish move inshore to hunt.
Matt Klos holds his net near the electrified wires dangling in the water, waiting for a stunned fish to surface.

Electrofishing involves dangling electrified wires into the water and giving the water in front of the boat a continuous jolt. Fish are momentarily stunned, float to the surface and are then netted out of the water by crew members standing at the bow.
Fish are kept in a large live well until they can be transferred to a “work up” boat where researchers record information like species ID, length and weight before putting fish back into the water.
Nora Hickey displays one of  several large carp caught during the night.

The information is added to three decades’ worth of data, says senior LTER research specialist, Ted Bier. With data like this, Bier says, fisheries researchers can explore things like relative abundance and growth rates of fish or use it as a predictor of population cycles.
Up north, LTER fish crew data helped scientists examine the effect of invasive rainbow smelt on Crystal Lake fish populations. Since data on native populations of perch and walleye had been collected well before smelt made it to the lake, it was easy to chart the decline in their catch rates with each passing year’s post-invasion fish census.
Nora Hickey displays a catch of sheepshead, perch and ….

On this particular night, crew members pulled up some large walleye, a handful of freshwater drum, bullhead and bluegill, several large carp and lots and lots of yellow perch.
The crew electrofished for half-hour stretches at three research sites that have been the same spots designated for electrofishing since the LTER project began monitoring southern Wisconsin lakes.
Team members aboard the “work up” boat sort fish into different tanks and prepare to take measurements and collect other data.

At the end of each run, the “work up” boat pulled alongside the shock boat and transferred fish into their own live wells for sorting and data collection.
The crew worked well into the night counting and cataloging fish, finally calling it a night as the clock passed from late Wednesday into early Thursday morning.
Fish were sorted into bins by species and held until they could be weighed, measured, ID’ed and then returned to the lake.

At the end of the night, the crew had cataloged hundreds of fish and added a little more data to the big picture of the long-term composition of Lake Mendota (and Southern Wisconsin’s) fisheries.
Fish crew worked up the last catch of the night on the Hasler Lab boat slip.