The Ozaukee County Planning and Parks Department is currently implementing the use of side-scan sonar and field assessments to classify habitat and substrate for use in a comprehensive habitat suitability model for Lake Sturgeon in the Milwaukee River in Milwaukee County and Ozaukee County. Lake Sturgeon (Acipenser fulvescens) is one of the Great Lakes basin’s oldest and largest indigenous species. Historically, this species had populations that numbered in the millions basin-wide, but were reduced to remnant populations by 1920 from overharvest, habitat loss and degradation, and water quality impairments. Furthermore, the construction of dams and other flood control measures along the Milwaukee River in the 1900s prevented natural sturgeon regeneration. Lake Sturgeon are potamodromous, periodic spawners, migrating from lake feeding grounds upriver to spawn in the spring. Slow growth, late sexual maturation, and natal spawning requirements complicate rehabilitation efforts.
Since 2006, Riveredge Nature Center (RNC), in partnership with the Wisconsin Department of Natural Resources (WDNR), has implemented the "Return the Sturgeon" reintroduction project in alignment with Wisconsin's statewide Lake Sturgeon Management Plan. This project includes the annual rearing and release of thousands of fingerling Lake Sturgeon into the Milwaukee River and Lake Michigan Basin. As of 2020, WDNR and RNC have released approximately 17,686 juvenile sturgeon, and adult Lake Sturgeon are beginning to return to the Milwaukee River to naturally spawn as they reach sexual maturity. In conjunction with these efforts, the Ozaukee County Planning and Parks Department (Department) and its partners have removed or remediated several fish passage impediments in the Milwaukee River Watershed, reconnecting 31 river miles between the Milwaukee River’s mouth at Lake Michigan to the Bridge Street Dam in the Village of Grafton. However, it is unknown if remnant, accessible habitat will support a naturally reproducing population, as no comprehensive Lake Sturgeon instream habitat suitability information exists for the Milwaukee River.
Successful, proactive sturgeon management requires knowledge of existing (or lack of) high quality spawning and nursery habitat. Traditional, comprehensive instream habitat surveys of large areas can be time intensive and costly as well as employing discrete sampling techniques requires data extrapolation. Alternatively, side-scan sonar is an innovative, low cost, effective method to identify substrate and capture continuous habitat data, particularly for species highly dependent on substrate requirements such as Lake Sturgeon. Side-scan sonar can capture substrate information over large areas where traditional, comprehensive habitat survey methods are not feasible due to time, financial, or other constraints. The Department is collecting sonar images from Milwaukee River confluence with the Milwaukee Harbor to the Bridge Street Dam in the Village of Grafton, and from Cedar Creek confluence with the Milwaukee River to the Nail Factory dam.
The final report generated by analysis of the sonar images will be used by the Department and partners to inform decisions involving the Milwaukee River Lake Sturgeon reintroduction project, identify potential habitat restoration projects and to determine priorities for future protection and restoration projects. The resultant substrate and bathymetric mapping of the Milwaukee River and Cedar Creek will have multiple, long term applicability to other studies and uses (e.g. aquatic habitat, sediment evaluation, etc) across the Great Lakes natural resources community.