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U.S. GEOLOGICAL SURVEY Life sciences technician Henry Thompson deploys an underwater receiver moored in front of the Michigan Department of Natural Resources research vessel Lake Char on the north side of Isle Royale in September 2018. (Andrew Muir, Great Lakes Fishery Commission)

ISLE ROYALE — Fisheries researchers go to great lengths, or sometimes deep waters, to find answers to their questions.

Recently, a multidisciplinary team of scientists aboard the Michigan Department of Natural Resources’ research vessel Lake Char discovered the deepest occurrence of arctic char — also known as lake trout — which spawn in the Great Lakes at depths of over 400 feet off the north side of the Isle Royale in Lake Superior.

Until this study, the spawning site and habitat of deep-sea lake trout was a mystery. Lake Superior is the last lake to have preserved the deepwater subspecies of lake trout, the siscowet, which is also the most abundant species here due to the lake’s enormous depth. This die-hard beast can be found at depths ranging from about 130 feet to the deepest point in the entire Great Lakes — 1,320 feet — first documented by this research team in 2006.

However, where these fish spawn and lay their eggs was a mystery until this recent discovery.

Lake trout, the main native predator in the deeper waters of the Great Lakes, plays a critical role in ecosystem stability by contributing to it “administer” the prey types.

A SISCOWET LAKE TROUT released in September 2018 from the research vessel Lake Char with a surgically implanted acoustic transmitter near Hawk Island in Isle Royale National Park. (Andrew Muir, Great Lakes Fishery Commission)

Lake Superior was the only Great Lake not affected by the lake trout extinction and today has probably the most abundant native lake trout population in the world, serving as a learning area to help restore the species in the other lakes.

As part of a study funded by the Great Lakes Fishery Commission, an inter-agency team of researchers from the GLFC, the University of Wisconsin-Milwaukee, the Great Lakes Acoustic Telemetry Observing System, Michigan State University, the US Geological Survey, the National Park Service and The DNR Marquette Fisheries Research Station began a labor intensive project in 2018.

The team used advanced acoustic telemetry and underwater drone technologies to track, locate, identify and verify where Siskowaten spawned in the far-flung foothills of Lake Superior in Isle Royale National Park.

First, the mission was to find a spawning population of Siskowets.

Lake trout typically spawn on rocky lake floors in the fall; However, as early as 2008, the team on the research vessel Lake Char had spotted a group of spring spawning Siscowets on the Isle Royale – the first and only documented population in the Great Lakes.

THE RESEARCH VESSEL Lake Char at dock on Amygdaloid Island in Isle Royale National Park in September 2018. (Andrew Muir, Great Lakes Fishery Commission)

Based on this information, the scientists returned to Isle Royale in 2018, where they caught spawning Siskowa in both spring and autumn, fitting fish with acoustic telemetry or tracking tags and simultaneously deploying an array of about 90 acoustic receivers. This array recorded the positions of the fish and was the deepest deployment of acoustic technology in the Great Lakes, reaching depths of over 325 feet. The idea was that the tagged spawning animals would show researchers where the tagged fish would congregate to lay their eggs.

Deploying the array in deep water was no easy task, as the team had to collect over 28,000 pounds of boulders and transport them to Isle Royale to anchor the deep underwater receivers. The receivers are signaled to release themselves and swim to the surface for recovery, but as they had to remain in the water over the winter, a surface buoy could not be used as an anchor. With the anchors having to be abandoned, Lake Char Captain Chris Little found the simple solution of using rocks as they are environmentally safe.

Luckily, the National Park Service had the ship Angelique — a former military landing craft that has the lifting capacity to handle such a heavy payload — to help move the rocks.

After tagging the fish and receiving the tracking array set, the team had to wait patiently. The unsupervised receivers gathered information to pinpoint the exact locations where the tagged fish are spawning. They collected data on the fish’s locations through the end of 2020, and after analysis, the data indicated target locations for the next phase of the study.

A remotely operated deep water vehicle (ROV) was deployed from Lake Char to search for evidence of spawning. The telemetry data showed some very pinpoint concentrations of spawning fish at about 330 feet for the spring pennants and at about 90 feet for the fall spawners.

In June 2021, the team deployed the ROV to target locations and, for the first time in history, found the deepest lake trout eggs ever recorded in any Great Lakes. The team also deployed nets and caught small bottom-dwelling fish called bullheads and small lake trout with Siskowets eggs in their stomachs to further verify these deep-water spawning sites.

The mystery of where deepwater lake trout spawn has been solved.

More research is needed to understand the adaptations and factors required for juvenile lake trout to survive in such a harsh environment, which will aid those in the lower Great Lakes trying to fully restore the abyssal reaches of their ecosystems.

Lake trout are highly adaptable cold-water fish that have diversified into numerous ecotypes to adapt to the Great Lakes’ most important habitat variable – depth. Historically, deep-sea ecotypes have existed in all of the Great Lakes, but they were driven to extinction in the late 1940s due to the combined effects of over-commercial fishing and invasion by the parasitic sea lamprey.

The extinction of lake trout in the four lower Great Lakes led to the historic, catastrophic changes in food webs that required the intervention of non-native salmon to control the invasive aleweib that invaded the lower lakes. Subsequently, natural resource authorities began an international collaboration to restore lake trout by regulating commercial fisheries, managing sea lamprey, and employing hatchery fish.

In recent years, these efforts have finally borne fruit, with some natural reproductions of lake trout occurring first in Lake Huron and now in Lake Michigan. Lake Erie and Lake Ontario are further behind in this process.

The fisheries management plans in all of these lakes call for stocking of deep sea trout in deep sea areas and this is where this project comes in. The results of this study will help fisheries managers know where best to camp and what is required for success.

Visit Michigan.gov/FishResearch to learn more about DNR research, which provides important information on the management of Michigan’s fisheries.



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