Illinois Natural History Survey - University of Illinois

INHS Reports January-February 1996

Identification of Genetic Stocks in Midwest Game Fish

Since the turn of the century, transfer of fish from one lake to another has been a common practice among fisheries managers. An underlying assumption has been that all individuals within a species are similar and able to survive and reproduce in diverse habitats. Recent evidence indicates, however, that populations within a species can be genetically quite different from one another. It has also been assumed that artificial propagation and subsequent introduction of fish into a lake is effective in increasing the number of harvestable fish in that lake, but despite years of stocking efforts, many natural populations remain static or are declining.

Population geneticists have shown that species often are composed of genetically distinct stocks. Each stock represents a group that has survived and evolved in isolation from other stocks. Unfortunately, many management programs have failed to acknowledge potential stock differences. As a result, individuals from one stock have often been transported and introduced into waters containing a different stock. Such stock mixing poses a significant genetic risk to native populations; when transplanted individuals survive and interbreed with the resident stock, the resulting offspring may be poorly adapted.

For management programs to incorporate conservation genetics principles, information on existing stock boundaries is critical. Unfortunately, little work has been done to define stock boundaries in the upper Midwest. One reason for this has been an inability to detect significant levels of genetic variation within a regional context. Newly developed molecular genetic techniques, however, now allow us to detect those levels of genetic variation.

Illinois Natural History Survey scientists and graduate students are working with biologists from the Wisconsin and Minnesota Departments of Natural Resources to study the population genetic structure of a number of fish species in the upper Midwest. Fifteen species with diverse life history traits and management priorities are being targeted. These include heavily managed game species, such as muskellunge and walleye, as well as nongame species, such as the johnny darter. The data generated will be used to determine if watershed boundaries correlate with the boundaries of genetic stocks. If patterns of stock structure are consistent among the diverse species studied, managers may be able to implement effective stock management for all species of concern and predict the most likely stock boundaries for species for which genetic data are unavailable.

Hypothetical sampling sites in the Mississippi, Great Lakes, and Hudson Bay drainage basins.

The study area is of interest because of the proximity of three large drainage basins: the Mississippi River, Hudson Bay, and the Great Lakes. Where possible, three populations of each species were sampled from each of the major tributaries within each of these drainage basins. This sampling hierarchy allows scientists to determine the relatedness of populations within and among tributaries and, therefore, the limit of detectable genetic differences using the newest available techniques.

Genetic data obtained thus far (with proteins, mitochondrial DNA, and nuclear DNA) confirm the presence of distinct genetic stocks within the upper Midwest. In addition, results obtained for each of the diverse species studied are in general agreement. Stock boundaries generally conform to existing watersheds, although in some areas genetic stocks appear to correlate with older, postglacial watersheds. For example, populations in the upper Fox and upper Rock Rivers (Mississippi River basin) are genetically distinct from downstream populations located closer to the mainstem Mississippi River. Instead, they are genetically similar to populations from tributaries of Lake Michigan (Great Lakes basin). For management purposes, such results indicate that existing watersheds may form a starting point for the reduction of genetic risks associated with stock transfer, but additional information provided by this study will allow more precise identification of stock boundaries.

When the study is complete, recommendations for alterations to existing management practices (e.g., establishment of stock-based management units and geographical limits to transportation and introduction of fish) will be discussed with management biologists from all states involved. We hope that the information will be used to improve the efficacy of stocking where it is deemed necessary, and to protect the genetic integrity of our fisheries resources for long-term productivity.

Robert D. Fields and David P. Philipp, Center for Aquatic Ecology

Charlie Warwick, editor