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GLERL 2001 Milestone Reports
Milestone 2001 HomeGOAL: BUILD SUSTAINABLE FISHERIES
OBJECTIVE 1: Eliminate and Prevent Overfishing and Overcapitalization
PM: By 2005, reduce by 40% (34) the number of overfished fisheries (currently 86 of 269 fisheries are overfished).
Milestone-01-2-5: Report on initial studies to evaluate the impact of declining food resources on Great Lakes fishes.
Scientists: S. Brandt and T. Nalepa
Purpose: The purpose of this milestone was to determine whether recent declines in the amphipod Diporeia spp. could affect planktivorous fish in the Great Lakes. The objectives were to determine the importance of Diporeia in the diets of fish and the effect on prey selection and fish conditions with the disappearance of Diporeia in Lake Michigan. Planktivorous fish such as alewife, rainbow smelt, slimy and deepwater sculpin, and bloater support a $4 billion a year sportfishing industry in the Great Lakes for trout, salmon, and yellow perch. Additionally, state and tribal commercial fisheries target species such as bloater and lake whitefish for commercial harvest. Diet-induced changes in the growth or abundance of planktivorous fish may limit the carrying capacity of stocked salmonids and commercial fish in the Great Lakes.
Efforts: Forage fish were collected in southeastern Lake Michigan off Muskegon (MSK), Michigan in 1998 and 1999. Stations at St. Joseph (STJ) and Little Sable Point (LSP), Michigan were added in 2000. These sample stations represented locations where Diporeia were absent (STJ), in decline (MSK), and abundant (LSP). Diet and condition of the collected fish were determined in the laboratory. When Diporeia were available, they were an important part of the diet in at least one season for bloater, alewife, and lake whitefish and were the main part of the diet year around for slimy sculpin. Diporeia were not an important part of the diet of rainbow smelt. As Diporeia decline, other invertebrate prey such as Mysis relicta, microzooplankton, and Chironomids (the larval stage of midge flies) became more important in the diets of fish that normally feed on Diporeia. For example, at STJ, where Diporeia have disappeared, M. relicta dominated the diet of bloater; at sites further north where Diporeia persist (MSK, LSP), bloater diets mainly contain Diporeia (Figure 1).
Changes in the invertebrate community of Lake Michigan may also be affecting the growth and condition of some planktivorous fish. Body condition (measured as the caloric density of body tissue) of bloater was noticeably lower in 1998 and 1999 than in the 1970s. Whole-body energy density of alewife and rainbow smelt was similar or better in 1998 and 1999 than in the 1970s. These fish may be better adapted to utilizing alternative foods in the absence of Diporeia. Lake whitefish condition (weight adjusted for length) and growth has declined in Lake Michigan since 1993 with the appearance of zebra mussels. Changes in lake whitefish condition are attributed to the consumption of zebra mussels and other shelled prey, the absence of Diporeia in the diet, and density dependent growth reductions. Commercial harvest of lake whitefish are expected to decrease in the future.
Customers: State and tribal fishery managers in the Great Lakes need to know the health of planktivorous fish to make informed decisions on salmonid stocking and commercial fishing regulations and quotas. Our results have been provided to the multi-agency Yellow Perch Task Group for work on yellow perch recruitment.
Significance: The decline of Diporeia clearly has the potential to reduce the carrying capacity of planktivorous fishes in the Great Lakes. Changes in invertebrate food resources might alter competitive interactions between planktivorous fishes, which may affect salmonid production, yellow perch recovery, lake whitefish recruitment and growth, and success of natural reproduction of lake trout.
Success: The project met its goal of determining diet and condition of planktivorous fishes in Lake Michigan. Our results indicate the decline of Diporeia has the potential to affect the production of planktivorous fish in the lake, and may affect sport and commercial fisheries. The project results have been presented in 2 scientific publications (Pothoven et al. 2000a, Pothoven et al. 2000b), in several newspaper articles, and at over 10 scientific and public meetings.
Next Steps: Fish diet and condition will be followed for the next two years at St. Joseph, Muskegon, and Little Sable Point. Continued efforts will assess whether food web changes affect the abundance and distribution of fishes in Lake Michigan. Further, the ability of alternative food types to support planktivorous fish production will also be assessed.
Literature Cited:
Pothoven, S.A., Nalepa, T.F. and Brandt, S.B. 2000a. Age-0 and age-1 yellow perch diet in southeastern Lake Michigan. J. Great Lakes Res. 26:235-239.
Pothoven, S.A., Fahnenstiel, G.L., Vanderploeg, H.A. and Luttenton, M. 2000b. Population dynamics of Mysis relicta in southeastern Lake Michigan, 1995-1998. J. Great Lakes Res. 26:357-365.
Last updated: July 9, 2002 mbl