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GLERL 2007 Milestone Reports
< GLERL 2007 Milestone main page
GOAL: Ecosystem
Scientist: Steven Pothoven (GLERL)
NOAA Performance Objective: Access, model and forecast ecosystem resources for management decisions.
Ecosystem Research Program Performance Measure: Identify Causes and Consequences of Changes in Ecosystem Condition
OAR Performance Measure: Annual number of coastal, marine, and Great Lakes ecosystem sites adequately characterized for management.
NOAA Program: Ecosystem Research
Milestone: Effects of dreissenid invasion on alewife and lake whitefish consumption in Lakes Michigan and Huron.
Purpose: The energy flow and availability of lower food web resources for fish production in aquatic systems including the Laurentian Great Lakes have been altered by dreissenid mussels. For example, declines of Diporeia spp., formerly the dominant benthic macroinvertebrate in offshore waters of the Great Lakes, have been attributed to the spread of dreissenid mussels, but the exact mechanism is uncertain. Management strategies for both recreational and commercial fisheries in the Great Lakes need to be evaluated relative to ongoing changes in the food-web. In Lake Michigan, adult alewife Alosa psuedoharengus growth, condition, and energy density declined following the dreissenid invasion and diet composition was altered. Historically Diporeia accounted for about 30% of adult alewife diet, and in turn, alewives accounted for about 70% of the diet of stocked salmonines in Lake Michigan. Growth and condition of lake whitefish Coregonus clupeaformis, a benthivore, also declined following dreissenid invasions in Lakes Michigan and Huron even though dreissenids were incorporated as a major food source. Lake whitefish are the most important commercial fish species in Lakes Michigan and Huron, accounting for 65% and 85% of the commercial harvest in each respective lake, for a total value of $15.7 million (USA dollars). The purpose of this study was to determine whether food consumption of the average alewife and lake whitefish changed following dreissenid invasions in Lake Michigan and Huron.
Efforts and Results (to date): Data describing the pre- and post-dreissenid invasion diets, growth, and energy content for alewife and lake whitefish were gathered. Data sources included published manuscripts, United States Geological Survey Great Lakes Science Center, NOAA Great Lakes Environmental Research Laboratory, Michigan Department of Natural Resources, Ontario Ministry of Natural Resources, Grand Traverse Band of Ottawa and Chippewa Indians and Chippewa Ottawa Resource Authority. The data gathered was used to as inputs for the Wisconsin bioenergetics model. Bioenergetics modeling was used to describe annual consumption before and after the dreissenid invasion and the subsequent decline of Diporeia for the average alewife in Lake Michigan and lake whitefish in Lake Michigan and Huron.
Annual consumption of zooplankton by the average alewife in Lake Michigan declined 37% and consumption of benthic macroinvertebrates (Diporeia spp., Mysis relicta, Chironomidae) declined 19% between pre- (1983-1994) and post- (1995-2005) invasion periods. Reduced consumption by alewives corresponded with reduced alewife growth. Consumption of benthic macroinvertebrates (Diporeia spp., M. relicta, Chironomidae, Mollusca) by the average lake whitefish declined 54%, 16% and 17 % in northern, central and southern Lake Michigan respectively during the post-invasion period relative to the pre-invasion period, despite large increases in consumption of molluscs in each area of the lake. Reduced consumption of benthic macroinvertebrates during the post-invasion period was mainly associated with a reduction in lake whitefish growth in central and southern Lake Michigan and to eating fish in northern Lake Michigan. In contrast, annual consumption of benthic macroinvertebrates by the average lake whitefish did not change between pre- and post-invasion periods in any area of Lake Huron even though the average consumption of molluscs was over 5 times higher and weight at age was on average 43% lower during the post-invasion period than during the pre-invasion period.
Some ecologists have proposed that the Diporeia decline in the Laurentian Great Lakes may cause alewives to substantially increase their rate of zooplanktivory. However, our bioenergetics modeling results have shown that the rate of zooplankton consumption by an average alewife has actually decreased with the invasion of dreissenid mussels. Decreases in alewife consumption, growth, condition, and energy density all have been linked to declines in Diporeia. An alewife in Lake Michigan appears unable to eat enough zooplankton or Mysis to offset the loss of Diporeia in their diets, so continued low growth, condition, and energy density is expected. We expect that as long as Diporeia abundance remains relatively low, fishery managers should continue to assume a relatively low energy density of alewives when determining salmonine stocking rates.
Heretofore published descriptions and models of the role of dreissenid mussels in Great Lakes food webs have not included a strong trophic link between lake whitefish and dreissenid mussels. Our bioenergetics modeling has revealed a strong trophic link between lake whitefish and dreissenid mussels in Lakes Michigan and Huron, with an average adult lake whitefish in Lakes Michigan and Huron consuming about 2.5 kg and 3 kg, respectively, of dreissenid mussels each year. Therefore, the concerns by fishery managers that dreissenid mussels represent a "dead end" in the food web have been allayed by our analysis.
Although lake whitefish are able to eat dreissenids, there are consequences for their growth even if fish maintain pre-invasion consumption rates. In Lake Huron and southern Lake Michigan, where enough molluscs were eaten so total annual consumption did not change during the post-invasion period, growth of lake whitefish still declined dramatically. It is unlikely that dreissenids are currently a limited food item in the environment, but lake whitefish appear unable to eat enough mussels to maintain pre-invasion growth rates. Therefore, regardless of lake whitefish abundance, the commercial catch of lake whitefish in areas where Diporeia are disappearing is likely to continue to mainly consist of slower-growing fish in relatively poor condition.
Customer(s): Fishery resource managers and policy makers at the state, tribal, and federal level in the Great Lakes region who are required to make decisions regarding commercial harvest quotas and regulations, recreational fish stocking levels, and management goals. Public parties interested in the effects of food-web changes on fisheries including state and tribal commercial fishermen, recreational fishermen, and sport charter boat operators.
Cause Factors (if milestone not met):
Milestone was met.
Revised Completion Date (if milestone not met):
Milestone was met.
Last updated: 2007-03-27 mbl