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U.S. Geological Survey - Great Lakes Science Center

Predator Assessment

Predator fish have long supported valuable fisheries in the Great Lakes, but overfishing and introduction of invasive species, such as the parasitic sea lamprey, have caused declines in multiple predator species, such as lake trout. Consequently, a major goal of Great Lakes fishery management is restoration of predator fish. Since the 1970s, the USGS-GLSC has been providing valuable observational data on predator fish population dynamics in all five Great Lakes to managers. Predator population monitoring by the USGS-GLSC has been instrumental in restoring and identifying factors limiting the success of native predator fish. Current research focuses on understanding burbot ecology, yellow perch stock assessment, and walleye gillnet selectivity. The long-term dataset on predator fish populations makes the USGS-GLSC a world leader in freshwater predator fish stock assessment.


Walleye represent the most important fishery in the Great Lakes in combined terms of harvest and economic value. A cornerstone of management for this fishery is a spatially explicit understanding of movements, habitat use, and habitat availability as these relate to seasonal and age-dependent changes in fishing vulnerability. LEBS scientists are partnering with Ohio and Ontario to develop this information for walleye using cutting edge electronic tags and acoustic telemetry (see In addition, LEBS scientists are developing novel data comparing inter-agency gill net sampling gears for improved inferences on walleye distribution and demographics in stock assessments.

Principal Investigator: Richard Kraus

Lake Michigan supports popular Chinook salmon fisheries that have been sustained by stocking since the late 1960s. Natural recruitment of Chinook salmon in Lake Michigan has increased in the last few decades and currently contributes over 50% of Chinook salmon recruits. LEBS scientists are evaluating life history characteristics between naturally recruited and stocked Chinook salmon in Lake Michigan to help discern potential changes resulting from naturalization and implications for fisheries.

Principal Investigator: Mark Rogers

A balanced cold water fish community in Lake Erie’s eastern basin is a Lake Erie Committee objective. Rehabilitation of lake trout and healthy populations of burbot, the native top predators in this community, are a primary component of reaching this management goal. LEBS scientists cooperate with partner agencies to assess and monitor these target coldwater species via standardized field and biological sampling and report relative abundances and population dynamics metrics of importance to the Lake Erie Committee. The long-term cold water assessment data facilitates research that explores limitations to cold water community restoration and evaluates strategies to overcome restoration challenges.

Principal Investigator: Mark Rogers

Objectives & Justification
Since 1938 when the first documented nesting occurred in Lake Ontario, the population of double-crested cormorants (Phalacrocorax auritus) has risen to 10,000 breeding pairs (Weseloh and Ewins 1994). The most likely reasons for the dramatic increase in population levels include reductions in environmental contaminant levels that adversely affected reproduction, protection under the Migratory Bird Treaty Act of 1972, and an abundant food supply in the lake, mainly alewife (Alosa pseudoharengus), a non-indigenous forage fish (Weseloh et al. 1983, Price and Weseloh 1986).

Double-crested cormorants are piscivorous, consuming about 0.5 kg of fish per day (Orta 1992). Proliferation of cormorant breeding populations in Lake Ontario has led to questions about their potential effects on fish populations. Not only are effects on gamefish such as smallmouth bass (Micropterus dolomieu), yellow perch (Perca flavescens), and stocked salmonines a concern, but also the loss of certain forage fishes. Alewife are of special interest since they are the primary forage species of salmonines, and their populations have been declining in Lake Ontario (Jones et al. 1993).

At stake for the sport fishing community in New York State alone is an annual catch of nearly 200,000 salmonids (Eckert 1994). The 1988 economic value of all sport fishing in New York waters of Lake Ontario was $87 million (Connelly et al. 1988). The total commercial harvest for the same location in 1994 was 81,000 lbs (in order of importance: yellow perch, Perca flavescens; brown bullhead, Ameiurus nebulosus; and white perch, Morone americana) valued at $83,000 (Cluett 1994). A satisfactory resolution to the problem requires accurate information about the cormorant’s life history, food habits ecosystem role, and impact on other species of special concern.

The objectives of this study are to (1) determine the annual diet composition of double-crested cormorants in eastern Lake Ontario and the St. Lawrence River, (2) estimate the number of fish consumed annually by cormorants in eastern Lake Ontario and the St. Lawrence River, and (3) evaluate the effectiveness of different management actions in reducing fish consumption.

Principal Investigator: James H. Johnson

While cormorant numbers have declined or remained steady across most of the upper Great Lakes, abundance in Saginaw Bay has continued to grow, doubling in recent years.  Cormorants inhabit two locations in Saginaw Bay; Little Charity Island, and the Spoils Island near the mouth of the Saginaw River.  Stakeholders have been increasingly calling for management action against cormorants in Saginaw Bay, concerned that cormorants are consuming large amounts of valued sportfish which live in the bay.  We are conducting a diet study of cormorants in Saginaw Bay to be compared and contrasted to the regular fish community survey work in the area.  We will describe double-crested cormorant diets over their entire breeding season (from first arrival in the spring to departure in the fall).  Work in other areas of the Great Lakes has demonstrated that cormorant diets will evolve over the season as the fish community changes over time with warming and spawning activity.  Results of this study will be incorporated into management decisions regarding cormorant control in Saginaw Bay and stakeholder communication.

Principal Investigator: Ed Roseman

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