The Deepest Egg Hunt in the Great Lakes

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MARQUETTE, Mich. (WLUC) — Fisheries researchers go to great lengths, or sometimes deep waters, to seek answers to their questions.

Recently, an interdisciplinary team of scientists aboard the Michigan Department of Natural Resources (DNR) research vessel Lake Char discovered the deepest presence of lake trout (or lake trout) spawning in the Great Lakes at a depth 400 feet on the north side. of Isle Royale in Lake Superior.

Until this study, the deepwater lake trout spawning location and habitat remained a mystery. Lake Superior is the last lake to retain the deepwater lake trout subspecies, siscowet, which is also the most abundant type found here due to the lake’s great depth. This diehard animal can be found at depths starting at around 130 feet down to the deepest spot in the entire Great Lakes – 1,320 feet – which was 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 native top predator of the deep waters of the Great Lakes, play a vital role in ecosystem stability by helping to “manage” prey species.

Lake Superior was the only Great Lake not to experience lake trout extinction and today is home to possibly the most abundant native population of lake trout in the world, serving as a learning ground to help re-establish the species in the other lakes.

In a study funded by the Great Lakes Fishery Commission, an interagency team of researchers from the GLFC, University of Wisconsin-Milwaukee, Great Lakes Acoustic Telemetry Observing System, State University of Michigan, the U.S. Geological Survey, the National Park Service, and MNR’s Marquette Fisheries Research Station embarked on a labor-intensive project in 2018. The team deployed advanced acoustic telemetry and underwater drone technologies to track, locate, identify and verify where siscowets have spawned in remote areas of Lake Superior at Isle Royale National Park.

First, the mission was to find a breeding population of siscowets.

Lake trout generally spawn in the fall on the rocky bottoms of lakes; however, the team from the research vessel Lake Char had discovered a group of spring-spawning siscowets (the first and only documented population in the Great Lakes) at Isle Royale in 2008.

Based on this information, scientists returned to Isle Royale in 2018, where they captured siscowets breeding in the spring and fall, fitting the fish with acoustic telemetry – or tracking – tags and simultaneously the team deployed a network of about 90 acoustic receivers. This array recorded fish positions and was the deepest deployment of acoustic technology ever in the Great Lakes, at depths greater than 325 feet. The idea was that the tagged spawners would reveal to researchers where the tagged fish congregate to lay their eggs.

Deploying the deep-sea array was no easy task, requiring the team to collect and transport over 28,000 pounds of boulders to Isle Royale to anchor the deep underwater receivers. Receivers are signaled to release and float to the surface for recovery, but as they had to be in the water during the winter, a surface buoy could not be used as an anchor. Since the anchors had to be abandoned, Char Lake Captain Chris Little came up with the simple solution of using rocks, as they are environmentally safe.

Fortunately, the National Park Service had the ship Angelique – a former military landing craft that has the lifting capacity to support such a heavy payload – to help move the boulders.

After tagging the fish and setting the tracking device, the team had to wait patiently. Unattended receivers gathered information to identify the exact locations where tagged fish were spawning. They collected data on fish positions through the end of 2020, and when analyzed, the data identified target locations for the next phase of the study.

A remotely operated deep-sea vehicle, or ROV, was deployed from Char Lake to search for evidence of spawning. Telemetry data revealed a few very accurate concentrations of spawning fish at around 330 feet for spring siscowets and around 90 feet for fall spawners.

In June 2021, the team deployed the ROV to the target sites and, for the first time in history, found the deepest lake trout eggs ever recorded in all of the Great Lakes. The team also deployed nets, catching sculpins (small groundfish) and small lake trout with siscowet eggs in their stomachs, further checking these deep-water spawning grounds.

The mystery of the deep water lake trout spawning location has been solved! Further research is needed to understand the adaptations and factors necessary for young lake trout to survive in such a harsh environment, which will help those in the lower Great Lakes seeking to fully restore the abyssal areas of their ecosystems.

Lake trout are a highly adaptive cold-water fish that have diversified into many ecotypes to accommodate the key Great Lakes habitat variable: depth. Historically, deep-sea ecotypes existed in all of the Great Lakes, but were driven to extinction by the late 1940s due to the combined effects of excessive commercial fishing and the invasion of parasitic sea lamprey. .

The extinction of lake trout in the four lower Great Lakes led to historic and disastrous changes in food webs, necessitating the intervention of stocking non-native salmon to control the invasive alewife that permeated the lower lakes. Subsequently, natural resource agencies launched an international collaborative effort to restore lake trout by regulating commercial fishing, managing sea lampreys, and stocking hatchery fish.

In recent years, these efforts have finally paid off, with some natural lake trout reproduction occurring first in Lake Huron and now in Lake Michigan. Lake Erie and Lake Ontario are further behind in the process.

Fisheries management plans for all of these lakes call for the stocking of deep-sea lake trout in deep-water areas, and that’s where this project comes in. Findings from this study will help fisheries managers know where best to store and what is needed to be successful.

Learn about DNR research that provides crucial information on how to manage Michigan’s fisheries at Michigan.gov/FishResearch.

Check out previous Showcasing the DNR stories in our archive at michigan.gov/DNRStories.

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