Fish Bulletin 167. Climatic Variation and Exploitation In The Pacific Mackerel Fishery

This bulletin provides an in-depth analysis of the California Current Pacific mackerel (Scomber japonicus) fishery. It includes descriptions of the fishery and the species population biology, a cohort analysis, density and environmental-dependent spawner-recruit models, and yield simulations. The cohort analysis (1928–1968), using an instantaneous natural mortality rate of M = 0.5, shows a fluctuating stock size with a maximum total biomass of 965 million pounds (438,000 MT) in 1933 and a minimum of 3.3 million pounds (1500 MT) in 1968. The number of recruits-per-spawner shows large fluctuations with considerable coherence between adjacent years. There was no marked downward trend in recruits-per-spawner over the 1928–1968 period. Density-dependent spawner-recruit models accounted for a maximum of 24 percent of the observed variation in recruitment. Multiple regression models, including both population and environmental variables, were fitted to the data available for two time periods; 1931–68 and 1946–68. The 1931–68 model accounted for 59 percent of the variation in recruitment; increased recruitment was associated with increased sea surface temperature, reduced sea level and reduced atmospheric pressure during the spawning season. The 1946–68 model accounted for 76 percent of the variation in recruitment; increased recruitment was associated with increased coastal upwelling and decreased offshore convergence during the spawning season. Maximum yield-per-recruit occurs with an age at recruitment of 1 or less, and with instantaneous fishing mortalities (F) in excess of 1.0. A dynamic pool model incorporating a Ricker spawner-recruit model predicts that extinction of the stock will occur with the above fishing strategy. Maximum sustained yield (MSY) with the steady state dynamic pool model is above 94 million pounds (41,000 MT). This MSY occurs with an age-at-recruitment of 4 and with an exploitation rate of 0.25. Simulations incorporating the density and environmental-dependent spawner-recruit functions predict that the above MSY cannot be attained when there is serial coherence in the annual recruitment fluctuations. Mean longterm annual yield with the above fishing strategy, under the environmental conditions occurring between 1931–1968, would have been only 56 million pounds (25,000 MT). With an age-at-recruitment of 1, maximum steady state yield (69 million pounds, 31,000 MT) occurs at an exploitation rate of 0.2. Maximum long-term yield with this fishing strategy, under the 1931–68 environmental conditions, would have been 45 million pounds (20,000 MT)