The Effects of Unaccounted Discards and Misspecified Natural Mortality on Harvest Policies Based on Estimates of Spawners per Recruit

Abstract

The amount of exploitation in a fishery is typically controlled by regulatory harvest policies, of which the best for maximizing total yield is F _MSY, the fishing rate that provides the maximum sustainable yield. However, estimation of F _MSY can be difficult for many fisheries, and for this reason proxies such as the spawning-biomass-per-recruit rate (often denoted F _%) have been adopted for some fisheries. Around 1992, the Pacific Fishery Management Council adopted an F _35% harvest policy for U.S. West Coast groundfish. Recently, this policy has been questioned, along with the overall productivity of West Coast groundfish. Estimates of appropriate harvest rates and productivity are computed from stock assessment models, which often rely on poorly estimated values for natural mortality and at-sea discard levels. The effects of unaccounted discards and misspecified natural mortality on estimates of appropriate harvest rates were investigated using a basic catch-age model. A deterministic, generalized groundfish population was simulated using an average fishing mortality time series computed for 11 major West Coast groundfish species. The model results indicated that unaccounted, non-size-selective discards had no effect on subsequent harvest rate estimates, while unaccounted, size-selective discards of fish ranging up to 50% by weight resulted in a 30–40% higher F _% in some cases. The largest changes in harvest policy resulted from lower spawner−recruit steepness parameters, younger ages of selection, and positively biased natural mortality values.