Harvest levels that conserve spawning biomass can provide larger and more stable and sustainable yields in intermixed fisheries

Abstract

We conducted a simulation-based evaluation of constant fishing mortality control rule performance for four intermixing fish populations patterned after lake whitefish (Coregonus clupeaformis) fisheries in the Great Lakes. Various scenarios consisting of different assumed productivity and mixing levels for the spawning populations were examined to explore how harvest level performance changed under conditions that lake whitefish, as well as other fish populations, may encounter. The simulation framework included an operating model that represented the true dynamics of the populations, the harvests from fisheries exploiting the intermixed populations, and a full age-structured assessment of each exploited stock. Four harvest levels were evaluated: a 65% total annual mortality harvest level that presently is used to manage lake whitefish fisheries in northern lakes Michigan and Huron and three more conservative levels (35, 45, and 55% total annual mortality harvest levels). Under many investigated scenarios, the current 65% harvest level was found to have a high risk of overexploiting lower productivity spawning populations, which reduced aggregate yield for the modeled system. The 55 and 45% harvest levels resulted in the largest aggregate yields for many of the examined scenarios, although the 45% harvest level was better at reducing risks to sustainability of individual spawning populations. Inter-annual variation in yield generally declined as harvest levels became more conservative. Our analysis suggests that harvest levels that conserve spawning biomass can provide multiple benefits for intermixed fisheries including greater yields, reduced inter-annual variability in yields, and lower risk of depleting low productivity spawning populations. We encourage the implementation of precautionary harvest rates for intermixed fisheries to help protect less productive populations.