Control rule performance for intermixing lake whitefish populations in the 1836 Treaty waters of the Great Lakes: A simulation-based evaluation

Abstract

We conducted a simulation-based evaluation exploring the effects of population intermixing on the current 65% total annual mortality control rule used to manage lake whitefish (Coregonus clupeaformis) fisheries in the 1836 Treaty waters of the Laurentian Great Lakes. The simulations incorporated intermixing among four populations with characteristics similar to those of lake whitefish in northern lakes Huron and Michigan. Dynamics of each population were simulated for 100years with each stock exploited by a single fishery. An age-structured assessment of each stock was conducted every third year, with the abundance, mortality, and recruitment estimates used with the current control rule to set future harvest limits. Overall aggregate yield of the modeled system was generally not affected by intermixing, but did depend on assumed productivity levels. Mean annual yields for individual fisheries were sensitive to mixing levels, with yields from fisheries coinciding with low productivity spawning populations often similar to those coinciding with high productivity populations when intermixing occurred. Variability in yield tended to decrease slightly as mixing rates increased. Intermixing did not have a large influence on measures of spawning population sustainability, but detection of declining population sizes in low productivity populations may be difficult when intermixing occurs because of fishery yields remaining high due to immigration of fish from other spawning areas. This raises concern about the current control rule given that low productivity populations had spawning population sizes less than 20% of the unfished level in most years.