Autumn Meeting of Japan Society of Fisheries Sciences, Abstract (last updated on May 22, 2002)

Is the tuna critically endangered? Intrinsic inconsistency between Red List criteria

H. Matsuda, T. Yahara and Y. Uozumi

The southern bluefin tuna is being considered to be listed as a critically endangered (CR) species in the 1996 IUCN (the World Conservation Union) Red List. The tuna is still exploited by commercial fishing. Because of overfishing, the number of mature individuals has been reduced to 1/5 of the stock level of 30 years ago. This reduction falls into CR by IUCN criterion A1(b,d). In accordance with IUCN criterion E, however, the extinction probability of a CR within the next 10 years or 3 generations is at least 50%. Because the tuna is still abundant, the extinction probability of the tuna within the next 3 generations is currently negligible. Despite that the levels for different criteria within categories were set against a common standard, there is an intrinsic inconsistency between these criteria. There is no biological reason that the tuna should be regarded as CR.

Effect of age- and season- limits for maximum sustainable fisheries

Yasuto Takenaka and Hiroyuki Matsuda

We consider the optimal harvesting policy under sustainable fisheries including the age of first capture and seasonal harvesting. To obtain this policy, we use spawning biomass per recruit (SPR) and yield per recruit (YPR) simultaneously. We use data on a chub mackerel population in the Pacific Ocean off eastern Japan. We consider three cases of seasonal fishing, fishing throughout the year, half year after the spawning season and variable length of fishing season. We conclude that (1) the maximum sustainable is unlikely pulse fishing. (2) The optimal yield under the fixed fishing season policies is close to that under variable harvesting season. (3) In any case, the fish should be preserved until the end of the first spawning. (4) Under variable fishing season, the optimal fishing season includes fishing season of the two major types of actual fisheries, the dip-net and the purse-seine, on the chub mackerel in Japan.