The USGS Great Lakes Science Center (GLSC) has a long history of significant contributions to the understanding of aquatic resources in the Great Lakes, through partnerships and interactions with state, tribal, and U.S. and Canadian federal agencies. The main focus of the Centerís research is on the long-term dynamics of native and non-native aquatic species and the sustainability of Great Lakes fisheries. Since the Center was established, the fish community structure of the Great Lakes has changed significantly. Each lake has seen the introduction of exotic species and the dramatic impact of over fishing. As a product of this long-term monitoring, data sets exist that describe the abundance of predators and their prey fish in each lake and are highly valued resources for understanding the long-term dynamics of the fish community in relation to biotic and abiotic influences, and for modeling Great Lakes ecosystem dynamics. Research is underway on each lake, conducted by the field stations and research vessels, as well as at headquarters in Ann Arbor.
The GLSC conducts annual bottom trawl surveys in all five Great Lakes to assess the status of both predator and prey species, although much of our work is focused on prey fish populations. The prey fish assemblage, including alewife, gizzard shad, emerald shiner, rainbow smelt, bloater, sculpins, and lake herring is a vital trophic link in the aquatic ecosystem; prey fish populations may be limited both by their food supply and predators. Within and among the Great Lakes, prey fish abundance and species composition vary greatly annually. Adequate prey fish populations are required to sustain walleye, lake trout, Pacific salmon, and other salmonid top predators in the Great Lakes. The top predator populations support economically valuable fisheries and thus maintaining vigorous predator populations are of great concern to resource management agencies. An accurate assessment of the prey fish populations is vital in determining the appropriate management decisions regarding these top predators. Managers across the Great Lakes basin request our help in providing current information on the status of prey fish populations and rely on us to provide the understanding of population dynamics needed for anticipating future direction of prey fish populations. As a federal agency, we can conduct fish surveys at wide spatial scales that cross political boundaries and are viewed as an unbiased and objective source of information for resource managers. We are the only agency with true deepwater capabilities on all Great Lakes and can sample habitats and aquatic communities that are beyond reach of individual States and Canadian resource agencies.
Fish community structure has changed substantially in the Great Lakes since the Center was established and research questions abound regarding understanding and predicting the dynamics of the prey fish communities. Species extinctions and establishment of invasive and non-native species (e.g., sea lamprey, alewife, rainbow smelt, ruffe, and gobies) have been observed in each lake. In addition, top predator fish species and abundance have changed dramatically through time with the loss of native lake trout and severe reduction of burbot populations throughout most of the basin, loss of Atlantic salmon in Lake Ontario, subsequent stocking of hatchery lake trout for restoration efforts, and the stocking of Pacific salmon to control alewife invasions. In general, Lake Erie, the warmest and shallowest lake, is the most productive and has the highest species diversity of prey fish. In contrast, Lake Superior is a deep, cold and relatively unproductive lake. Lake Ontario prey fish community is dominated by two non-native species, alewife and rainbow smelt. Lakes Michigan and Huron have lost a large complex of native ciscoes coincident with the alewife invasion. Both lakes are dominated by only a few species: alewife, rainbow smelt, bloater, sticklebacks, and sculpins.
Providing current information to fisheries managers is a highly visible and public aspect of our program, but of equal importance is research resulting from continual development of our long-term database, which now contains over 30 years of data for most of the lakes. Long-term data are being used to examine a diverse suite of ecological issues, including species extinctions and establishment of invasive species, effects of continued predator stocking on the prey fish community, identification of recruitment mechanisms that regulate prey fish abundance, and response of prey species to changes in food webs. Results of these studies have been published in peer-reviewed journals sine the 1960ís.
Fish community dynamics investigations are possible because of the multi-decade data series collected by the Center and are augmented by the ability to perform genetic analyses to identify population substructure and patterns of gene flow. A variety of techniques are used at the Center to investigate spatial and temporal genetic relationships within and among Great Lakes fish populations. Center researchers are using genetic methods to monitor the success of stocking programs designed to rehabilitate top predators like lake trout. Center scientists are also focusing on the use of non-lethal tissue extraction techniques and archival material to survey endangered fish species.
The lake trout, a federal trust species, was historically the top native predator fish in the Great Lakes and the mainstay of the early commercial fishery. A combination of over-fishing and predation by the exotic sea lamprey along with other contributing factors caused extinction of lake trout in all of the Great Lakes except Lake Superior where populations were greatly diminished and Lake Huron where an isolated, remnant population persisted. Managers have been deeply concerned about the lake trout and have instituted a variety of rehabilitation efforts. Because lake trout are a long-lived fish, effectiveness of management efforts are only determined through a long-term surveillance. Hence, the GLSC has conducted long-term gillnet assessment of native lake trout in Lake Superior and of hatchery-reared lake trout in all of the Great Lakes to evaluate the status of lake trout rehabilitation. As with our prey fish long-term data series, the lake trout database can provide a wealth of information in understanding lake trout population dynamics and movements. In addition, intensive research has been done on factors suspected of adversely influencing the establishment of self-reproducing, sustainable stocks of lake trout in the Great Lakes. Research has included evaluating the effect of various factors on survival of hatchery-reared trout, including rearing density, life stage at stocking, adult abundance on juvenile survival, season of stocking, genetic strain, and stocking method and location. Other research has included population modeling, observing geographic and bathythermal distribution and movements of adults and juveniles, verifying contaminant and energy uptake with laboratory and field diets and bioenergetic modeling, measuring growth and maturity, determining factors affecting fecundity, and understanding the role of thiamine deficiency in lake trout survival. Although many of the factors thought to influence survival of hatchery fish have been investigated, these studies have to be repeated periodically as fish communities change. We also demonstrated differential survival among genetic strains of lake trout in vulnerability to and survival from sea lamprey attack. As a consequence, hatcheries switched strain of lake trout used in rehabilitation program. Because of our observance of naturally produced larval lake trout on a mid-lake spawning reef in Lake Huron, that reef was declared a refuge.
Aquatic invertebrates provide an important food source for most prey fish as well as for juvenile top fish predators. Understanding diets is essential in identifying the competition among native and non-native fish species. Invertebrate communities have also suffered from the invasion of non-native fish and invertebrate species because invasives have altered food webs and may sequester energy. Currently, zooplankton and benthic invertebrate populations and fish diets are analyzed in studies complementing ongoing long-term fish surveys. In addition, baseline information is used to document the impact of new invasive invertebrate species. As a result, Center researchers are developing bioenergetic models of recently-introduced mussels.
One problem encountered by most ecologists studying invertebrates is lack of current taxonomic expertise. Center researchers have created an interactive web key that describes over 100 species of free-living and parasitic copepods. Information regarding species identification, distribution, ecology, and literature references is centralized on a web site providing a reference tool for managers and researchers in the Great Lakes basin. Ongoing efforts also involve preparation of keys for rotifers and aquatic insects distribution around the Great Lakes.
Prey Fish Ecology
Our annual bottom trawl estimates have documented wide fluctuation in the Great Lakes prey fish communities. This important long-term data set can be used for a variety of long term ecological studies. Future research efforts will be directed at understanding the mechanisms underlying these changes to better address the managersí information needs. Specifically, the prey fish studies will be enhanced to provide more accurate abundance estimates and to allow better predictions of recruitment, growth, and survival. To improve surveys, Center researchers and peers will be scrutinizing sampling designs to improve the accuracy of biomass and abundance estimates and to allow statistical comparisons of data through time within and between the lakes. This will undoubtedly require some gear standardization and trawl comparison surveys that link modern gears with ones used historically. We are also incorporating remote sensing technology, particularly hydroacoustics, to complement bottom trawl information and provide more information about pelagic fish. Hydroacoustic surveys were initiated in Lake Michigan in the 1990s, and plans are currently underway to incorporate hydroacoustic surveys in each of the Great Lakes.
To better understand the factors influencing the prey fish community, we will incorporate value-added sampling of lower trophic levels to the overall survey design. We have begun documenting changes in the invertebrate community that affect the food base of prey fish by taking concurrent benthic samples with bottom trawls, and by sampling zooplankton communities concurrently with hydroacoustic surveys. Inclusion of benthic invertebrate samples can document the rapidly changing benthic invertebrate community in the Great Lakes with subsequent repercussions throughout the food web. Stable isotope analyses can complement direct diet analyses to document energy flow through the food web. In Lake Ontario, we are starting to track energy density of prey fish to detect consequences of food web disruption and impact on upper trophic levels. Habitat features such as depth and temperature are routinely obtained, but other factors such as nutrient content and substrate type can be incorporated in the survey designs. Larval fish investigations can complement typical surveys and enhance the understanding of prey fish recruitment.
Colonization of the Great Lakes by zebra and quagga mussels has shunted production to the benthos while at the same time reducing our ability to sample benthic fishes with our traditional bottom trawls. Moreover, round gobies, an exotic benthic fish, are increasing in abundance and poised to become a major part of the prey fish community. We need to develop tools to sample benthic fishes in those broad areas of the Great Lakes infested with
Lake Trout and Other Predator Fish
Center researchers are still investigating the important factors affecting the status of lake trout, a keystone species, in the Great Lakes. In Lake Superior in which stocking of supplemental lake trout are not required, investigators are interested in better understanding the deepwater forms of lake trout and their impact on the forage fish community. In the other lakes, a variety of factors are under investigation. Surveys of lake trout will continue to aid in development of management plans and will evaluate the effects and consequences of new management strategies. The influence of stocking methods on survivorship is being reevaluating in light of recent ecosystem changes. Factors affecting egg quality are being studied and energetic conditions of lake trout throughout the Great Lakes basin are being compared. The importance of a refuge from fishing pressure in aiding recruitment of lake trout is still an unanswered question in the Great Lakes. Studies involve documenting refuge use and movement patterns in and out of the refuge. The importance of early mortality syndrome on lake trout survival is being addressed in an initiative lead by the Center. The thermal regime used by different lake trout strains and their sea lamprey predators is producing surprising results that will lead to better understanding of lake trout survival. Center researchers are also studying the impacts of non-native fish and invertebrates on lake trout early life history. In addition, researchers are working on the population dynamics of other top predators such as burbot.