PREDATOR-PREY DYNAMICS IN THE MAIN BASIN OF LAKE HURON
Alewife abundance declined rapidly to very low levels during 2002-2004 in the main basin of Lake Huron. This led to the collapse of a sport fishery for Chinook salmon, but also to large increases in wild recruitment of many native species, including lake trout, walleye, and emerald shiners. Understanding the causes for the initial alewife decline and their continued suppression is essential for proactively managing the system (e.g., to manage for a native species dominated system with low alewife abundance) and for learning lessons from Lake Huron that could be applied to other systems such as Lakes Michigan and Ontario. The causes for the major shift in the Lake Huron main basin remain controversial. To a large extent this controversy revolves around the absolute biomass of prey fish relative to the magnitude of predation. Innovative application of modern stock assessment methods can resolve this issue. The objectives for this proposal include : (1) Develop estimates of the absolute abundance and dynamics of the major prey species that contributed substantially to piscivore diets up through the collapse period (i.e, alewife and rainbow smelt). (2) Determine the predator-prey linkage that determines prey mortality as related to predator and prey abundance. (3) Evaluate how variations in prey fish recruitment, mortality, and individual growth contributed to the prey dynamics and particularly the alewife collapse. (4) Evaluate the extent to which recruitment, mortality, and individual growth of alewife and rainbow smelt were related to piscivory, primary productivity, temperature, or other environmental factors. (5) Predict conditions under which alewife could reenter the Lake Huron main basin as a major ecological contributor. Our approach will start by combining and updating existing predator assessment models, growth estimates, and bioenergetics calculations to obtain updated estimates of piscivore consumption of prey fish. This involves using existing age-structured assessment models for the main predators, updating these models for growth and bioenergetics, and review of information on predators for which there is no existing assessment, and if appropriate development of such assessments. We will then develop an age-structured prey fish assessment model that jointly uses estimates of prey fish consumption and data on prey dynamics (e.g., trawl surveys, hydroacoustic surveys, biological information) . This would be done using an assessment approach similar to statistical catch-at-age but with predation taking the role that fishing usually does. We will relate the resulting estimates of prey recruitment and mortality to possible causes, including predation and temperature. This will be done as an integrated part of the prey fish assessment with alternative hypotheses (models) evaluated by data-based statistical model selection. We will use these assessment results as the basis of a simulation model that will be used to evaluate under what conditions the Lake Huron system might shift back to an alewife dominated community.