Geology topics

Benjamin Scott Leonhardt

Publications and source records attributed to Benjamin Scott Leonhardt.

5 recordsLinked to original sources

USGS2025

Burbot population dynamics in northern Lake Michigan, 2001–2023, and predation on newly stocked lake trout by burbot

Although the worldwide status of burbot ( Lota lota ) across its Holarctic distribution has recently been assessed, adverse effects of dreissenid mussels on burbot abundance in waters invaded by mussels have not been investigated. We analyzed data from an annual spring gillnet survey to characterize burbot population dynamics in northern Lake Michigan during 2001–2023. Adult burbot abundance in northern Lake Michigan trended neither significantly upward nor downward during 2001–2023. Thus, expansion of the quagga mussel ( Dreissena rostriformis bugensis ) population into deeper waters of Lake Michigan during 2003–2008 had no detectable negative effects on burbot abundance. Burbot continued to feed on lake trout ( Salvelinus namaycush ) stocked in the Northern Refuge of Lake Michigan through 2023, which supported a hypothesis that the burbot population potentially impeded lake trout restoration in the refuge.

Michigan↗

USGS2025

Drowned river mouth lakes are winter foraging habitats for the expanding Lake Michigan cisco Coregonus artedi population

Characterizing fish movements is required for understanding habitat use, energy flow, and trophic structure and can inform fisheries management. Drowned river mouth (DRM) lakes are productive inland habitats in the Laurentian Great Lakes basin used by migratory fishes. Despite recognition of their ecological connections to the Great Lakes, the value of DRM lakes as seasonal habitats is not well understood for many fishes. One such species, cisco Coregonus artedi , has recently expanded in Lake Michigan from near extirpation to higher relative abundances in the northeastern portion of the lake. Cisco are recreationally harvested in some DRM lakes during winter, but little is known about cisco movement patterns and ecology. In winter 2022 and 2023, we collected cisco from three DRM lakes along the eastern shores of Lake Michigan (Lake Charlevoix, Portage Lake, Muskegon Lake) to characterize genetics, morphometrics, and diets. We also implanted telemetry tags in 20 cisco collected in Lake Charlevoix to examine movement patterns and determine DRM lake residency (i.e., seasonal vs. year-round). We found no consistent genetic or morphometric differentiation across DRM lakes, suggesting that recolonization began from a single stock. Fish were the only diet item found in cisco guts collected during winter months. Movement patterns from Lake Charlevoix indicated strong spawning site fidelity to Grand Traverse Bay as well as non-spawning site fidelity. However, given the presence of cisco in southern DRM lakes and some site-specific differences in morphometrics, managers could benefit from further research to determine whether spawning occurs in southern Lake Michigan.

Journal of Great Lakes Research↗

USGS2024

Spatial distribution and diet of Lake Michigan juvenile lake trout

Most studies of Lake Michigan lake trout ( Salvelinus namaycush ) have focused on adults, with scant attention to juveniles (<400 mm). We explored the spatial distribution and diet of juvenile lake trout using U.S. Geological Survey September bottom trawl data (2015–2022) and stomach content information opportunistically collected since 2012 by various agencies using multiple gear types. Most juvenile lake trout in the September bottom trawl survey were caught at 37–64 m bottom depths. Length frequency data from the bottom trawl survey identified three size classes likely associated with wild juvenile lake trout age: < 85 mm (∼age-0), 85–170 mm (∼age-1) and > 170 mm (∼age-2+). Largest catches of wild lake trout < 170 mm occurred along a northeastern transect (near Frankfort, Michigan), whereas most > 170 mm were collected along southern transects. Mysis diluviana was the dominant prey for juvenile lake trout < 170 mm, and > 250 mm were primarily piscivorous, while 170–250 mm appeared to be a transitional period of switching from Mysis to fish. Species composition of prey fishes consumed by lake trout varied spatially and we found evidence of seasonal and annual diet variation within Grand Traverse Bay. Diporeia , once an important component of juvenile lake trout diet, appears to no longer be consumed by juvenile lake trout in Lake Michigan to any measurable degree. Continued research on the ecology of juvenile lake trout may provide insight into the effects of a changing ecosystem on juvenile lake trout diet and growth, thereby contributing to the effort to rehabilitate the Lake Michigan lake trout population.

Lake Michigan↗

USGS2024

Documentation of a probable spawning run of cisco Coregonus artedi in the Spanish River, Ontario, Canada

Coregonines were historically diverse and abundant in the Laurentian Great Lakes but declined throughout the 19th and 20th centuries due to multiple factors, including overfishing, habitat degradation, and interactions with invasive species. Great Lakes fishery managers are now working to restore coregonines across the basin. An important aspect of restoration efforts is the identification of key habitats, such as spawning habitats. Our understanding of the extent and diversity of coregonine spawning habitats in the Great Lakes is limited. Historical accounts suggest some cisco Coregonus artedi migrated to tributaries to spawn, but contemporary runs have not been reported. Moreover, cisco display considerable phenotypic variation, with different “forms” often occupying specific habitats or regions. We provide evidence for a previously undocumented spawning run of cisco in the Spanish River, a tributary to the North Channel of Lake Huron. We captured 31 mature and ripe fish (25 males, 6 females) approximately 52 km upstream of the river mouth on 15–16 November 2022. The fish ranged in age from 3-9 yr and total length from 287-394 mm. Of these 31 individuals, 27 were morphologically similar to C. artedi manitoulinus , a subspecies of cisco found in the North Channel; however, four individuals more closely resembled cisco captured in other areas of Lake Huron. These findings represent the best evidence of tributary spawning by cisco since the 1880s, and they have important implications for coregonine conservation and restoration efforts. Ongoing work aims to augment our understanding of the ecological and evolutionary importance of this population.

Ontario↗

USGS2023

Lake Trout population dynamics in the Northern Refuge of Lake Michigan: Importance of stocking rate

Objective The Northern Refuge of Lake Michigan was established under the auspices of the Great Lakes Fishery Commission in 1985 as an additional step toward restoring a self‐sustaining Lake Trout Salvelinus namaycush population to the lake. The overall goal of our study was to assess progress toward Lake Trout rehabilitation in the Northern Refuge through 2021. Methods We conducted annual (1992–2021) gill‐net surveys in the fall to assess the adult population and beam trawl surveys in the spring to assess naturally reproduced age‐0 Lake Trout. Result Spawner abundance averaged 45 fish • 305 m of gill net −1 • day −1 during 1992–1999, just 4 fish • 305 m of gill net −1 • day −1 during 2000–2014, and then 67 fish • 305 m of gill net −1 • day −1 during 2018–2021. Sea Lamprey Petromyzon marinus wounding rate decreased by nearly 90% between the 2000–2014 and 2015–2021 periods. The average annual mortality rate of adult Lake Trout decreased from 43% during the 1990s and 2000s to 22% during 2015–2021. All of these population dynamics were partly driven by changes in Lake Trout stocking rates, which declined by roughly 50% during 1994–1996 but then increased roughly twofold during 2006–2010 and remained at this higher level during 2010–2018. In 2021, early signs of natural recruitment of Lake Trout became evident. Beam trawling during June 2021 yielded a wild age‐0 Lake Trout, the first ever caught since the surveys began in 1992. In addition, 15 (4% of total catch) of the Lake Trout caught in the fall 2021 gill‐net survey were unclipped (wild) fish. Estimated ages of these wild fish indicated that natural recruitment had begun sometime around 2015. Conclusion With Alewife Alosa pseudoharengus abundance in Lake Michigan at record low levels, Sea Lamprey wounding rate greatly reduced, and Lake Trout spawner abundance at relatively high levels, the outlook for Lake Trout rehabilitation in the Northern Refuge holds promise.

Northern Refuge of Lake Michigan↗