Great Lakes Spawning Whitefish and Invasive Mussels (SWIM) Project
Project Updates
2025 Field Season
In the summer months of 2025, small scale dreissenid mussel control treatments were completed in Good Harbor Bay in Lake Michigan. Two types of benthic mats, canvas and polyvinyl, each 20 m x 20 m, were deployed for 30 days. Three 6 m x 18 m vinyl mats were constructed to infuse carbon dioxide infusion under a benthic mat. These mats were deployed for 4-5 days. Quagga mussel mortality was determined from caged, sentinel mussels and from quadrat surveys at 1-, 30-, and 60-day (CO2 mats only).
Environmental (nutrients, dissolved oxygen, and water quality) and lower food-web (invasive mussel densities and size structure, benthic algal and invertebrate communities) conditions relevant to fish spawning behavior and embryo survival were assessed. At the Thunder Bay site, measurements were conducted for year two of pre-treatment assessment. At the Good Harbor site, assessments were conducted before, immediately after, and 30 – 60 days after mussel control treatments to assess treatment effectiveness.
Movements of adult lake whitefish during the spawning season were tracked on Good Harbor Bay Reef and the Thunder Bay Reef Complex using acoustic telemetry. Preliminary data suggests lake whitefish are using and likely spawning in focal areas of both reef systems. Further analysis of fine-scale behavior is ongoing. Additional acoustic tagging of adult lake whitefish occurred at the new Lake Michigan focal site in Big Bay de Noc (N=60) and to supplement the sample size of tagged fish in Thunder Bay (N=26). Movements of these individuals will be tracked over the next 3 spawning seasons.
Both Good Harbor and Thunder Bay reef sites were intensively sampled for lake whitefish egg deposition from fall through early December 2025. At Good Harbor Reef (Red Shoals), egg densities remained extremely low—fewer than 5 eggs per square meter for the second consecutive project year. This continues a trend of persistently low egg densities observed since monitoring began in 2018. At Thunder Bay, egg densities averaged over 60 eggs per square meter but were still substantially lower than levels recorded a decade ago when the reefs were first constructed and lake whitefish populations were more abundant. Larval emergence traps were deployed in spring 2025, but no larvae were captured. Sampling will be repeated this spring to assess lake whitefish larval emergence at Thunder Bay.
Given the sustained low egg densities at Good Harbor, additional exploratory sampling was conducted in Big Bay de Noc (northern Green Bay, Lake Michigan) during fall 2025. Results indicated average egg densities exceeding 350 eggs per square meter—more than 70 times higher than those observed at Good Harbor. This higher egg deposition corresponds to greater relative adult spawning stock abundance. Larval emergence assessment is also planned for spring 2026 in Big Bay de Noc.
2024 Field Season
Field evaluations were completed in a lake in Sleeping Bear Dunes National Lakeshore to determine the efficacy of different benthic mats to produce mussel mortality in 10, 20 and 30 days of treatment. The study also tested the effectiveness of infusing carbon dioxide under a mat for just 5 days. Pellet formulations of existing molluscicides Zequanox® and niclosamide were evaluated in mesocosm studies to measure product release rate and lethality to invasive mussels.
Environmental (nutrients, dissolved oxygen, and water quality) and lower food-web (invasive mussel densities and size structure, benthic algal and invertebrate communities) conditions relevant to fish spawning behavior and embryo survival were assessed.
New high-resolution bathymetric and habitat maps were collected and processed for both sites. Outputs include bathymetry and acoustic backscatter intensity at three resolutions (0.25-, 0.50-, and 1.0-m), and bathymetric derivatives such as rugostity, geoforms, slope, aspect, topographic roughness index, and topographic position index. Autonomous underwater vehicle images were gathered over Good Harbor Reef, yielding estimates of live mussel coverage, substrate type, and benthic fish abundances. Methods development and testing is ongoing to quantify mussel shell hash (i.e., dead mussel shells) and the quality of interstitial spaces between rocks where fish eggs can lodge and develop. Substrate maps are also under refinement for both sites.
Movements of acoustic-tagged Lake Whitefish (n = 40) on the Good Harbor Bay reef in 2023 suggest that whitefish spawning activity is restricted to a “hot-spot” associated with a ridge-like feature on the northwest side of the reef, where experimental invasive mussel control will likely occur. During the 2024 spawning season, acoustic-tagged lake whitefish were also detected on or near the Good Harbor Reef hot spot, as well as the Thunder Bay reef complex; analyses of fine-scale movement behavior is forthcoming.
Both sites were intensively sampled for lake whitefish egg deposition during October through December. At Good Harbor reef, egg densities were less than 5 per square meter despite increasing and concentrating effort in the presumed hot spot of lake whitefish spawning activity. At Thunder Bay, egg densities were more than 50 per square meter but markedly lower than was measured ten years ago when the reefs were first constructed and when lake whitefish populations were at higher densities.