Optimal Harvesting Policy for Chub Mackerel (Scomber japonicus) in Japan under a Fluctuating Environment

Temporal fluctuation in the reproduction rates of chub mackerel (Scomber japonicus) was estimated for the period 1971–86, when stock abundance of chub mackerel varied from a high to low level. To estimate the effect of various harvesting policies on stock conservation and the long-term catch, six harvesting policies were considered: (1) actual catch data during 1975–88, (2) 1/3 constant rate of exploitation, (3) low maximal catch amount, (4) 1/5 constant rate of exploitation, (5) prohibition of fishing when the stock is rare, and (6) constant-escapement policy. Using the data of reproduction rates during 1975–85, the final stock abundance at 1988 and the total catch amount during 1975–88 under each harvesting policy are calculated. We concluded that harvesting policies (4), (5), and (6) were effective for the stock conservation and that policies (2), (5), and (6) were effective for increasing the total catch.

Managing Nonrandomly Varying Fisheries

Theoretical studies indicate that constant effort or constant escapement harvest strategies are viable long-term management policies for fisheries influenced by random environmental variability. But the marine environment and fishery recruitment do not characteristically vary randomly: their time series tend to be autocorrelated. I examined the response of a model of an age-structured fish population to these harvest strategies, if recruitment is influenced by periodic variability. Variance in fish yield and the incidence of low stock size increased with both increasing amplitude and period length of environmental forcing. Sensitivity to environmental fluctuations increased sharply at periodicities between one and several times the generation length. Low-risk fisheries, which are subject to little nonrandom recruitment variability, may be satisfactorily managed by either management strategy, although constant effort harvesting provided greater stability to industry with little risk to the fish population. Neither strategy maintained stability in high-risk fisheries with high-amplitude, low-frequency recruitment variability, but a constant escapement policy minimized risk of stock collapse.

Alternative Harvest Strategies for Pacific Herring (Clupea harengus pallasi)

A simulated Pacific herring (Clupea harengus pallasi) population is used to evaluate alternative management strategies of constant escapement versus constant harvest rate for a roe herring fishery. The biological parameters of the model are derived from data on the Strait of Georgia herring stock. The management strategies are evaluated using three criteria: average catch, catch variance, and risk. The constant escapement strategy provides highest average catches, but at the expense of increased catch variance. The harvest rate strategy is favored for its reduced variance in catch and only a slight decrease in mean catch relative to the fixed escapement strategy. The analysis is extended to include the effects of persistent recruitment patterns. Stock–recruitment analysis suggests that recruitment deviations are autocorrelated. Correlated deviations may cause bias in regression estimates of stock–recruitment parameters (overestimation of stock productivity) and increase in variation of spawning stock biomass. The latter effect favors the constant escapement strategy, which fully uses persistent positive recruitment fluctuations. Mean catch is depressed for the harvest rate strategy, since the spawning biomass is less often located in the productive region of the stock–recruitment relationship. The model is used to evaluate the current management strategy for Strait of Georgia herring. The strategy of maintaining a minimum spawning biomass reserve combines the safety of the constant escapement strategy and the catch variance reducing features of the harvest rate strategy.

Mean Value and Variability of Fish Catches in Fluctuating Environments

The mean value of the catch and its variability due to environmental fluctuations are analyzed for a very general stock-recruitment model. Particular attention is devoted to the comparison of two standard fishing strategies (constant effort and constant escapement) in terms of mean catch, variance in catches, and maximum deviation of catch. It is demonstrated analytically that constant escapement policies should always give higher mean catch, but should give higher catch variance and more extreme catches only under certain conditions of environmental variability.