A Method to Consistently Approach the Target Total Fishing Fraction of Pacific Sardine and Other Internationally Exploited Fish Stocks

Abstract

AbstractThe “northern” stock of Pacific Sardine Sardinops sagax is fished by Mexico, the USA, and Canada. Without an international management agreement, the U.S. Pacific Fishery Management Council prorates its target total fishing fraction (F) in its harvest control rule (HCR) by 0.87. This is the proportion of the stock that it deemed in 1998 to be present, on average, during summer–fall 1964–1992 in the U.S. Exclusive Economic Zone. However, the portion of the stock in U.S. waters is variable, depending on the environment and stock biomass and demographics. Furthermore, irrespective of the stock distribution, the combined foreign and U.S. landings may exceed those determined by an F of 0.15, potentially reducing the stock biomass and yield. This occurred each year during 2001–2014, and the F (mean = 0.22; SD = 0.06) trended upward. To more consistently approximate the target F, a method was proposed to set harvest quotas with accounting for predicted foreign landings. We refined the method by adding a prediction error term and showed that the U.S. HCR, solved with predicted foreign landings and stock biomass from each annual assessment in 2001–2014, better stabilized F about the target F relative to the historical values (original method: mean = 0.16, SD = 0.02; new method: mean = 0.16, SD = 0.05). We also compared the historical F (mean = 0.18, SD = 0.07) and optimized F (original method: mean = 0.16, SD = 0.06; new method: mean = 0.17, SD = 0.08) calculated using updated biomass estimates from the 2013 assessment. Results showed that irrespective of the assessment and its assumptions, quotas that were optimized with respect to F better approached the target F‐value. Although the new method reduces bias due to trend in the foreign landings, its performance may be less precise than the original method if—as recently occurred—the assessments are significantly revised and the stock migration is abruptly abbreviated.