Recovery of fish stocks in the Seto Inland Sea

Abstract

The total amount of fisheries’ yield in the Seto Inland Sea in 1999 was 571,000 MT, consisting of 256,000 MT by fishing and 315,000 MT from aquaculture. About 40,000 people engaged in the fishing and aquaculture industries, earned 129 billion yen (1100 million US$) by fishing and 89 billion yen (770 million $US) by aquaculture. The averaged annual catch for the Seto Inland Sea by fishing was 13 MT/km2. Division into time periods in terms of eutrophication levels can be made: before 1960 when red sea bream were abundant with ecological divergence (before eutrophication), from 1960 to 1990 when the biomass of anchovy was large (during eutrophication), and after 1990 when the jellyfishes were abundant (excessive eutrophication or high N:P ratio). The fish production will decrease in the sea of jellyfishes. Actually, the amount of catch was 462,000 MT in 1982 which decreased 265,000 MT in 1993, corresponding to 43% in twelve years, then keeping the same level. A big reduction was seen in the catches of the spotlined sardine, anchovy, Spanish mackerel, tiger puffer, short-necked clam, sea cucumber and others. The tiger puffer and Spanish mackerel were abundant as predators in the sea of anchovy. The biomass of anchovy was at its maximum in 1986 and decreased to less than one third in 1996. The stocks of tiger puffer and Spanish mackerel greatly decreased because of the higher fishing pressure compared to the anchovy stock. The fishing power of individual fisheries targeting on the tiger puffer and Spanish mackerel increased substantially when fishing vessel and fishing gear improved, resulting in an excessive fishing effort. A large quantity of small immature fishes is usually caught in the Seto Inland Sea, resulting in growth and/or recruitment overfishing for many species. Hence, it is necessary to promote management of the fisheries so as not to reduce the fish stocks, and to allow the Seto Inland Sea to return from being a sea of jellyfishes to a sea of anchovy, with decreased eutrophication levels.