The 69-year-old research vessel (R/V) Chinook was fast approaching retirement. After years of generating valuable research data on the Great Lakes fishery for the Michigan Department of Natural Resources (MDNR), it was nearing the end of its useful life. Enter the R/V Tanner, a state-of-the-art 57-foot aluminum-hulled vessel with all the modern equipment its crew could hope for. This new member of the fleet, named after former MDNR Fisheries Division chief and director Dr. Howard A. Tanner, was funded in part by a grant from the Great Lakes Fishery Trust (GLFT). Although the Chinook will be missed, the Tanner is a welcome replacement that will enhance data collection and management decisions for years to come.
In the warmer waters of spring, yellow perch lay their eggs in long, gelatinous strands, usually among the thick vegetation found in the shallow waters of rivers and along the shores of lakes. A single female can lay more than 20,000 eggs; however, the vast majority of fish that come from these eggs will die very early, primarily due to predation and starvation.
To cultivate self-sustaining stocks of fish, fisheries managers need to understand how best to help young fish survive. For example, among all the different habitats in and around Lake Michigan, are there particular ones where larval and juvenile yellow perch—as well as other species, like round gobies and alewives—can find the food and water conditions they need to thrive? And how do these fish use different habitats as they grow?
A team of researchers hypothesized that river plumes—which have unique thermal, light, nutrient, and biological properties—would provide an excellent place for larval yellow perch, round gobies, and alewives to grow. They also wondered how the three species used river mouths, river plumes, and the nearshore waters of southeastern Lake Michigan. Did the young fish function the same regardless of their environment, or were there important differences among habitats?
To test their hypotheses, researchers (1) collected water samples from river plumes and adjacent nonplume areas in southeastern Lake Michigan to compare their physical, chemical, and biotic conditions; (2) estimated the movement of larval fish from tributaries into Lake Michigan; (3) compared the densities, diets, and growth rates of the three species in different areas; and (4) evaluated the extent to which later-stage fish used tributaries and river plumes as early life habitats.
The interdisciplinary research team—which included, among others, a fish ecologist, a physical scientist, and a water isotope specialist—used a number of different tools and methods (including otolith isotopic analysis) to better understand fish movement, as well as habitat contributions and linkages.
The Results Are In…
The researchers found that, while river plumes in southeastern Lake Michigan present a somewhat different environment than open lake waters (they are a little bit warmer and more turbid), they are very small and not the hotspots of production that researchers anticipated. Data do suggest, however, that river mouths have greater potential for production, particularly for alewives.
Perhaps the most important finding, though, is that there was a clear distinction between river mouth and nearshore populations of yellow perch and round gobies, but not alewives. An analysis of diets, fatty acids, and stable isotopes, as well as isotopic analyses of otoliths, show that alewives move back and forth between the habitats (the river and the open lake water) with some regularity, while yellow perch and round gobies stay mostly in the same habitat for life.
What Does It All Mean?
Although river plumes were not as productive as researchers originally surmised, river mouths show some promise as production hotspots and, therefore, warrant further study. They may be particularly good environments for larval alewives. Given recent concerns about the considerable year-to-year variations in the alewife population, and their important role as prey for other valuable fish, river mouths may hold some management answers.
More important, however, is that fish managers now have a clearer understanding of the yellow perch, round goby, and alewife populations. This information could be of great value when making decisions about regulations for managing these species and helping more larval and juvenile fish survive.