Restoring Lake Trout: Signs of Hope at the Mid-lake Reef

by Casey Twanow

It’s a chilly afternoon out on Lake Michigan. Scientists John Janssen and Rob Paddock, from UWM’s Great Lakes WATER Institute, are 40 miles and four hours offshore on the research vessel Neeskay. In the onboard laboratory, they are glued to a monitor showing an eerie, green-tinged landscape.

They are viewing the Mid-lake Reef, an area where rocky plains slope up from the lake bottom and drop off in steep cliffs, through the eyes of an ROV, a remotely operated vehicle. With a large, PlayStation-like handset, Paddock guides the ROV over rocks carpeted with invasive quagga mussels, 130 feet below.

The ROV, armed with electrodes and a suction tube, roams the reef as the scientists scan the monitors. They wait for a hint of motion, then Janssen squeezes a remote to trigger an electric shock near the ROV and Paddock sucks the briefly stunned fish into the ROV’s collection cylinder.

After three hours exploring the reef, the scientists have what they came for – three small lake trout, called fry, which Janssen will preserve for genetic analysis. One is a newly-hatched “sacfry,” still carrying a nourishing yolk; the others are just weeks old. An excited Janssen scoops them into a cooler, saying, “This is the deepest anybody’s ever documented lake trout producing fry.”

Will Life Find a Way?

Great numbers of lake trout once spawned at the Mid-lake Reef, but native lake trout were wiped out 60 years ago by over-fishing and invasive sea lamprey predation. Since then, despite annual stocking by the U.S. Fish and Wildlife Service (USFWS), natural reproduction in Lake Michigan has been negligible.

There is evidence, however, that lake trout stocked on the Mid-lake Reef 15 years ago have matured and are successfully reproducing at low levels. Since 2003 Janssen has collected about 3,000 lake trout eggs on the reef. About half of them have hatched in his laboratory, which means the eggs were fertilized and viable. When he dragged a trawl net over the reef, although he caught only four lake trout fry, one had food in its belly and another had filled its swim bladder, a buoyancy organ. These milestones—first feeding and swim bladder inflation—are keys to survival and growth.

Scientists and fisheries managers believe the reef holds promise as a nursery grounds to restore lake trout populations. The reef is within an 1,100 square mile refuge, so trout are protected from fishing. It is also less hospitable to invasive species like the round goby, which feeds on lake trout eggs, than nearshore areas where trout are typically stocked. The USFWS plans to focus lake trout stocking on the reef, which should increase the trout that return there to spawn.

Research like Janssen’s will reveal which strains of lake trout are spawning successfully on the Mid-lake Reef. Preliminary analysis of eggs he has collected suggests most trout spawning on the reef are a strain native to New York’s Seneca Lake. Further genetic analysis of eggs and fry will help USFWS stock strains well-adapted to reproduce on the reef, where conditions differ from shallow spawning sites.

October 25, 2006M

It’s now spawning season, a crisp, sunny day on the Neeskay. The coming “gales of November” can prevent regular fieldwork, so Janssen and Paddock head for the Mid-lake Reef whenever the weather offers a window of calm, day or night.

The scientists and crew launch the ROV and within minutes the monitors show lake trout patrolling the reef. The scientists are here to collect eggs deposited by these spawning trout. The ROV is equipped with a rigid tube that Paddock uses to prod among the rocks, periodically vacuuming up mussels, cloudy sediment, and hopefully eggs.

The weather holds, and the scientists run the ROV past sunset. Then, working by flashlight on the cold deck, Janssen carefully sorts the day’s catch. Among the quagga mussels he finds several dozen lake trout eggs, pearl-sized, yellow blobs. He drops them into a cooler; back in the lab he’ll incubate them to assess their survival and send some away for genetic analysis.

Spawning season will soon be over, and the reef below will quietly incubate eggs through the winter. No one knows if the Mid-lake Reef can anchor a lake trout comeback in Lake Michigan. But come spring, Janssen and Paddock will return to the reef to search again for tiny fry that signal hope for their species’ recovery.

The Great Lakes WATER (Wisconsin Aquatic Technology and Environmental Research) Institute is the largest freshwater academic research institute in the Great Lakes region. More information:

John Janssen is a senior scientist at the Great Lakes WATER Institute. His major research areas are fisheries biology and Great Lakes fisheries.

Rob Paddock a researcher at the Great Lakes WATER Institute. His major research areas are water chemistry and underwater instrumentation. Casey Twanow joined scientists John Janssen and Rob Paddock on two of their many trips to the Mid-lake Reef to provide firsthand accounts of their ongoing research at the site.

Riverwest Currents online edition – March, 2007