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Quantifying new top-down influences on the rapidly changing food web in the main basin of Lake Huron
James R. Bence1, J.X. He2, and Norine Dobiesz1
1 Quantitative Fisheries Center, Department of Fisheries and Wildlife, Michigan State University, 293 Farm Lane - Room 153, Giltner Hall, East Lansing, MI 48824 -1101
2 Alpena Fisheries Station, Michigan Department of Natural Resources, 160 E. Fletcher, Alpena, MI
Estimating total consumption by key predator species (Chinook salmon, lake trout, walleye, and lake whitefish) and how this consumption is allocated among prey for the main basin is challenging as this requires estimates of absolute abundance of the predators by age over time, as well as their diets, mortality rates, growth, and condition over time. To accomplish this, we developed or contributed to the development of new age structured assessment models for lake whitefish in previously unassessed areas of the main basin and walleye and to major modifications to assessment models for Chinook salmon, lake trout, and lake whitefish in other areas. We also worked to improve bioenergetics modeling approaches for the key predators, and developed new growth and condition models for all these predators, and used newly collected diet information to develop new integrated diets over time. We further used our growth and condition results to evaluate whether these could be used to indicate a regime shift. In addition we developed an approach for assessing round goby abundance based on our estimates of consumption by the predators of this species. Finally we used our results on abundance, diet, growth, and condition in updating and parameterizing a fish community simulation model for the main basin, and we interacted with a broader group of scientists to discuss implications of both simulation and other results as guidance for future research and management. Our results surprisingly suggest that the total fish consumption by the key predators we considered had not declined precipitously after the collapse of alewife in the system. Instead, the composition of fish consumption had changed, with lake whitefish becoming important as a predator and Chinook consumption becoming minor, and with the exotic benthic prey, round goby, becoming important. Our growth modelling results showed that the prey community changes had profound influences on individual fishes, and that growth can be a sensitive indicator, which provides indications of change before it is fully evident in abundance variables. Not surprisingly growth reflects changes in the prey community differently for the different predators, this in turn reflecting the extent to which they appear to be obligatorily reliant on fish that are at least partially pelagic (alewife and rainbow smelt) or can take advantage of truly benthic prey such as round gobies. Our simulation model and comparison of consumption and prey trends suggests that predation was one important factor involved in the collapse of alewife in Lake Huron. Forward simulations suggest that prey such as alewife and rainbow smelt may be trapped in a predator pit, with per capita mortality rates remaining very high on these species despite the overall levels of consumption on them having declined. The model incorporated positive feedbacks whereby lake trout and walleye recruitment is higher when alewife are scarce, whereas Chinook salmon recruitment declines under these conditions. A major limitation to both the simulation model and the study overall was the lack of quality information on the dynamics of round goby. We developed initial estimates of the biomass of round gobies, although these are accompanied by very large uncertainties. Those results suggest that round goby biomass is large relative to the levels of biomass for other species we assumed based on trawl surveys and that predators provide non-trivial levels of mortality on this species. However our results do not definitively answer the question of whether predation is currently sufficient to prevent a future increase in the biomass of this exotic species. Results were communicated to the scientific and management community, not just through publications but also through many presentations to a diversity of audiences and a culminating workshop that has suggested some future research and management directions.