By Shawn Sitar, Michigan Department of Natural Resources
MICHIGAN – 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’ research vessel Lake Char discovered the deepest occurrence of lake char (or lake trout) spawning in the Great Lakes at a depth of over 400 feet off the north side of Isle Royale in Lake Superior.
Until this study, the spawning location and habitat of the deep-water lake trout was a mystery. Lake Superior is the last lake to retain the deep-water subspecies of lake trout, the siscowet, which is also the most abundant type found here due to the lake’s vast depth. This diehard animal can be found at depths starting around 130 feet out to the deepest location in the entire Great Lakes – 1,320 feet – which was documented first by this research team back in 2006.
However, where these fish spawn and lay their eggs was a mystery until this recent discovery.
Lake trout, the top native predator in the deeper waters of the Great Lakes, play a critical role in the stability of the ecosystem by helping to “manage” the prey species.
Lake Superior was the only Great Lake not to suffer extinction of lake trout and today likely has the most abundant native lake trout population in the world, serving as a learning ground to help recover the species in the other lakes.
As part of a Great Lakes Fishery Commission-funded study, an interagency team of researchers from the GLFC, the University of Wisconsin-Milwaukee, the Great Lakes Acoustic Telemetry Observing System, Michigan State University, the U.S. Geological Survey, the National Park Service and the DNR 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 spawned in the far-off reaches of Lake Superior at Isle Royale National Park.
First, the mission was to find a spawning population of siscowets.
Lake trout typically spawn in the fall on rocky lake bottoms; however, the research vessel Lake Char team had discovered a spring-spawning group of siscowets (the first and only documented population in the Great Lakes) at Isle Royale back in 2008.
Based on this information, the scientists returned to Isle Royale in 2018, where they captured both spring- and fall-spawning siscowets, fitting fish with acoustic telemetry – or tracking – tags, and simultaneously the team deployed an array of about 90 acoustic receivers. This array recorded the positions of the fish and was the deepest-ever deployment of acoustic technology in the Great Lakes, at depths over 325 feet. The idea was that the tagged spawners would reveal to the researchers where the tagged fish congregated to lay their eggs.
Deploying the array in deep water 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 because they had to be in the water over the winter, a surface buoy could not be used as an anchor. Since the anchors had to be abandoned, Lake Char Captain Chris Little came up with the simple solution of using rocks, as they are environmentally safe.
Fortunately, the National Park Service had the vessel Angelique – a former military landing craft that has the lift capacity to handle such a heavy payload – to assist with moving the rocks.
After tagging the fish and getting the tracking array set, the team had to patiently wait. The unattended receivers collected information to pinpoint the exact locations where the tagged fish were spawning. They collected data on the positions of the fish through the end of 2020, and once analyzed, the data pinpointed target locations for the next phase of the study.
A deep-water remotely operated vehicle, or ROV, was deployed from the Lake Char to seek evidence of spawning. The telemetry data revealed a few very pinpointed concentrations of spawning fish at around 330 feet for the spring siscowets and at about 90 feet for the fall spawners.
In June of 2021, the team deployed the ROV at 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, capturing sculpins (small, bottom-dwelling fish) and small lake trout with siscowets’ eggs in their stomachs, further verifying these deep-water spawning sites.
The mystery of where deep-water lake trout spawn was solved! Further research is necessary 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 highly adaptive cold-water fish that have diversified into numerous ecotypes to adapt to the key habitat variable of the Great Lakes – depth. Historically, deep-water ecotypes existed in all the Great Lakes, but they were driven to extinction in the late 1940s due to the combined effects of excess commercial fishing and the invasion of the parasitic sea lamprey.
The extinction of lake trout in the four lower Great Lakes led to the historic, disastrous changes in the food webs, requiring the intervention of stocking non-native salmon to control the invasive alewife that permeated the lower lakes. Subsequently, natural resource agencies began 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 borne fruit, with some natural reproduction of lake trout occurring first in Lake Huron and now in Lake Michigan. Lake Erie and Lake Ontario are further behind in the process.
The fishery management plans in all of these lakes call for stocking deep-water forms of lake trout in deep-water areas, and that’s where this project comes into play. The findings of this study will help fisheries managers know where best to stock and what is needed for success.
Learn more about DNR research that provides crucial information on how to manage Michigan’s fisheries at Michigan.gov/FishResearch.