Condition and growth of Lake Whitefish are declining in Lakes Huron, Michigan, and Ontario. While studies have linked the decline to changes in food availability and hence diet in the latter two lakes, there have been no studies of whitefish feeding patterns in Lake Huron. This project examines the diet of Lake Whitefish relative to season, depth, and food availability. Growth, condition, and diet of Lake Whitefish in Lake Huron will be compared to that found in the other Great Lakes.
The Lake Whitefish is the most important commercial fish species in Lake Huron. The harvest of Lake Whitefish in this lake is approximately four million pounds per year, more than the total sport and commercial harvest of all other species combined. Tribal and state commercial fishermen have recently reported decreases in condition of Lake Whitefish in Lake Huron, eliciting concern about the stability of this important fishery. The decline in condition of Lake Whitefish has economic implications as harvest and marketability of Lake Whitefish decrease. Lake whitefish in poor condition could also experience poor reproductive success and recruitment to the fishery could decline. Additionally, some reports indicate that Lake Whitefish may be moving to deeper water where food such as Diporeia is more abundant. If Lake Whitefish are moving to deeper water, they may not be legally or practically accessible to commercial fisheries.
In Lake Michigan, similar declines in condition of Lake Whitefish were attributed to a combination of factors, including consumption of low-quality food items such as Zebra mussels Dreissena polymorpha and Sphaeriidae and declines of the high-energy food item Diporeia (Pothoven et al. 2001). Following declines of Diporeia off Muskegon, MI between 1998 and 1999-2000, the contribution of Diporeia to the diet of Lake Whitefish fell from 61% to 18% by weight (Pothoven et al. 2001). Changes in the thermal regime experienced by Lake Whitefish caused by feeding in deeper water could also affect growth. Understanding diet patterns and bioenergetics of Lake Whitefish was recently identified as a top-priority research item for funding by the Lake Huron Technical Committee, under the auspices of the Great Lakes Fishery Commission.
This project was initiated in 2002 to examine the relationship between feeding patterns of Lake Whitefish and the benthic macroinvertebrate community in Lake Huron. Whitefish and macroinvertebrates were collected in Lake Huron at various depths off Alpena (to 80 m) and off Tawas (to 50 m) in 2002-2004. All whitefish were analyzed for stomach contents and caloric density, and benthic organisms collected in 2002-2003 were counted and sorted by major taxonomic group. Stomach contents and energy density of additional whitefish collected throughout the lake were also analyzed. These fish were provided by other agencies: Michigan Department of Natural Resources, US Fish and Wildlife Service, and the Ontario Ministry of Natural Resources
Age-0 and medium (< 350 mm excluding age-0) Lake Whitefish at the Alpena site ate mainly large-bodied zooplankton along with ostracods, Mysis, and chironomids. At the Tawas site, age-0 and medium Lake Whitefish ate mainly zooplankton. Large Lake Whitefish (> 350 mm) ate mainly Quagga mussels at both sites (Figure 1). Prey such as zooplankton, ostracods, chironomids, and Quagga mussels are lower in caloric content than Diporeia. Thus, the fish would need to expend more energy foraging than prior to the Diporeia decline. In addition, chironomids are not abundant in the sampling region, suggesting the fish selected for this diet item but with high energetic costs.
Age-0 Lake Whitefish that ate mainly zooplankton had more food (by weight) and energy in stomachs than fish eating shelled prey or other macroinvertebrates. On the other hand, the weight of food in stomachs did not differ across prey groups for medium fish, but energy in stomachs was lowest for fish that ate shelled prey. For large Lake Whitefish, there was no difference in food weight or energy in stomachs for different prey groups
The size of benthic prey (that is, Diporeia spp., Chironomidae, and Dreissena spp.) eaten increased with fish size and influenced the energetic value of prey for medium and large Lake Whitefish. The type of prey eaten affected the food and energy intake differently for each size class of Lake Whitefish. For example, large Lake Whitefish obtained 8 times more energy from the average Quagga mussel eaten compared to medium Lake Whitefish. Therefore, it appears that Quagga mussels are a relatively unprofitable food source for medium Lake Whitefish and the loss of Diporeia spp. could have a disproportionate impact on medium fish.
The energy density of Lake Whitefish from northern Lake Huron increased with fish weight up to 800 g and then stabilized. The energy density of Lake Whitefish in northern Lake Huron was significantly lower than in southern Lake Michigan (Pothoven and Nalepa, in press) (Figure 2). The differences in energy content between lakes could be related to a combination of diet, prey availability and fish density.
Geographic differences in Lake Whitefish diets did not necessarily reflect corresponding differences in prey abundances as derived from the benthos samples. For example, although the highest contribution of Diporeia to small Lake Whitefish diets was in the northwest portion of the lake where Diporeia was most abundant, there was almost no Diporeia in large Lake Whitefish diets in the same region. Similarly, D. bugensis comprised most of the food eaten by large Lake Whitefish at nearly all sites although there were wide geographic differences in mussel abundance. Also, the highest contribution of D. bugensis to small Lake Whitefish diets came from the eastern side of the lake where this prey was least abundant. These discrepancies suggest that prey selection may occur at much smaller scales than we were able to examine with this study.
We will complete and submit a manuscript documenting the diet of Lake Whitefish in different areas of Lake Huron (i. e., different management zones). Diets will be examined relative to the distribution and abundance of benthic macroinvertebrate populations, particularly the amphipod Diporeia. Data for this manuscript were collected in 2002-2003.
Mohr, L. C., and Nalepa, T. F. 2005. Proceedings of a Workshop on the Dynamics of Lake Whitefish (Coregonus clupeaformis) and the Amphipod Diporeia spp. in the Great Lakes. Technical Report 66. Great Lakes Fishery Commission, Ann Arbor, MI, 310 pp.
Nalepa, T. F., Mohr, L. C., Henderson, B. A., Madenjian, C. P., and Schneeberger, P. J. 2005. Lake Whitefish and Diporeia spp. in the Great Lakes: An Overview. pp. 3-20. In Proceedings of a Workshop on the Dynamics of Lake Whitefish (Coregonus clupeaformis) and the amphipod Diporeia spp. in the Great Lakes. Edited by L. C. Mohr, and T.F. Nalepa. Great Lakes Fishery Commission, Technical Report 66, Ann Arbor, MI.
Pothoven. S. A. T. F. Nalepa, C. P. Madenjian, R. R. Rediske, P. J. Schneeberger, and J. X. He. Energy density of Lake Whitefish Coregonus clupeaformis in Lakes Huron and Michigan. (in press)
Pothoven, S.A. and Nalepa, T. F. Feeding ecology of Lake Whitefish in Lake Huron. Journal of Great Lakes Research. (in press)