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.