Using Time-Varying Asymptotic Length and Body Condition of Top Piscivores to Indicate Ecosystem Regime Shift in the Main Basin of Lake Huron: A Bayesian Hierarchical Modeling Approach

We evaluated the ecosystem regime shift in the main basin of Lake Huron that was indicated by the 2003 collapse of alewives, and dramatic declines in Chinook salmon abundance thereafter.  We found that the period of 1995-2002 should be considered as the early phase of the final regime shift.  We developed two Bayesian hierarchical models to describe time-varying growth based on the von Bertalanffy growth function and the length-mass relationship.  We used asymptotic length as an index of growth potential, and predicted body mass at a given length as an index of body condition.  Modeling fits to length and body mass at age of lake trout, Chinook salmon, and walleye were excellent.  Based on posterior distributions, we evaluated the shifts in among-year geometric means of the growth potential and body condition.  For a given top piscivore, one of the two indices responded to the regime shift much earlier than the 2003 collapse of alewives, the other corresponded to the 2003 changes, and which index provided the early signal differed among the three top piscivores.  We discussed management strategies based on our findings.