Substrate Mapping at Offshore Reefs in the Illinois Waters of Lake Michigan

Stacy-Duffy, William L.; Czesny, Sergiusz J.

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https://hdl.handle.net/2142/124117 Copy

Description

Title

Substrate Mapping at Offshore Reefs in the Illinois Waters of Lake Michigan

Author(s)

• Stacy-Duffy, William L.
• Czesny, Sergiusz J.

Issue Date

2024-07-01

Keyword(s)

• Lake Trout
• Lake Michigan
• Habitat
• Spawning

Geographic Coverage

Illinois

Abstract

Lake trout historically were an apex predator in Lake Michigan and were the target of productive commercial fisheries in the late 19th and early 20th centuries. Over-fishing, combined with the invasion of the predatory sea lamprey in the 1930s, led the population to collapse by mid-century (Hansen 1999). Lake-wide stocking of hatchery-reared lake trout commenced in the 1960s and over the following decades led to a build-up in the adult stock, but research documented only a negligible amount of wild reproduction. Beginning in 2011, the Illinois Department of Natural Resources (IDNR) began observing an increase in the proportion of unclipped (putatively wild-born) mature lake trout in fall gillnetting surveys targeted at offshore spawning habitat. Because lake trout exhibit natal homing common to salmonid fishes, the increase in unmarked adults is assumed to be due to the occurrence of successful spawning on offshore habitats in Illinois waters. Offshore spawning habitats for lake trout are critical for the restoration of the species in Lake Michigan (Janssen et al., 2007). In the Laurentian Great Lakes, lake trout have often been observed congregating over glacial bedforms, such as drumlins, during the spawning season (Riley et al., 2014). It is generally believed that these bedforms (colloquially and hereafter referred to as “reefs”) contain assorted sediments that provide adequate interstitial space for safe egg deposition, while the surface roughness created by the bedforms forces interaction between water currents and benthic sediments. This interaction results in hypolentic (sub-lake bed) flow that provides fresh aeration for incubating eggs and embryos (Riley et al., 2019). The micro- habitat characteristics associated with successful egg deposition and incubation have been the subject of extensive research and discussion (e.g., Claramunt et al., 2005; Fitzsimons & Marsden, 2014; Farha et al., 2020). The identification and documentation of offshore reefs in Illinois waters of Lake Michigan has progressed sporadically over the past three decades. Unlike the drumlin fields that serve as well-documented lake trout spawning sites in northern Lake Huron, reefs in southwest Lake Michigan are typically composed of isolated exposed bedrock that provides sharp bathymetric relief in contrast to the surrounding gently sloped lakebed. The bedrock consists of dolomite from the Silurian Niagaran formation (Hough, 1958) exposed by Pleistocene glaciers that left behind glacial till scoured from older deposits (Collinson et al., 1979; Holm et al., 1987; Janssen et al., 2005). This glacial till, consisting of gravel, cobble, and boulders, provides interstitial space necessary for lake trout egg deposition at large bedrock reefs in southwest Lake Michigan. Early side-scan sonar surveys documented different substrate types on Julian’s and Wilmette Reefs (Holm et al., 1987; Edsall et al., 1996). As side-scan sonar technology progressed, Julian’s Reef was re-surveyed, along with Waukegan Reef, to provide high- resolution digital imagery of the lakebed and allow quantification of potential lake trout spawning habitat (Redman et al., 2017). The locations of major reefs in Illinois were originally documented by Collinson et al. (1979) and high-resolution bathymetry maps for several reefs were created by Redman et al. (2019) and Stacy-Duffy and Czesny (2020). Locating and describing potential lake trout spawning habitat is critical to monitoring the recovery of the species in Lake Michigan (Marsden, et al., 1995; Patterson et al., 2016). We sought to continue these efforts by conducting side-scan sonar surveys on five reef areas to quantify the extent of potential spawning habitat available on each reef.

Publisher

Illinois Natural History Survey

Series/Report Name or Number

INHS Technical Report 2024 (12)

Type of Resource

text

Language

eng

Sponsor(s)/Grant Number(s)

Federal Aid Project F-196

Copyright and License Information

This document is a product of the Illinois Natural History Survey, and has been selected and made available by the Illinois Natural History Survey and the University Library, University of Illinois at Urbana-Champaign. It is intended solely for noncommercial research and educational use, and proper attribution is requested.

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