The changes in the distribution pattern and the condition coefficient with the ascending pro gress of the juvenile Japanese sea bass were examined.In the Chikugo River estuary of Ariake Bay, surface towing by a larva net was performed from March to April. A total of 5314 postlarvae and juveniles (SL, were collected by 183 tows. The body depth ratio and condition coefficient increased in proportion to the growth up to 20 mm length.As the research proceeded, the mode of the distribution moved from higher to lower salinity.The concept of distance was introduced to the temporal changes in catch of the sea bass and salinity.In mid March, sea basses were mainly distributed in the sea water area. In mid April, half the number had already reached the fresh water area, and they had condition coefficient remarkably increased. Sudden change in catch when the tide is at turn, and more catch during rising than ebbing tide were ob served. Preliminary notes on the ascending mechanism of the sea basses were given.
The stock abundance of chub mackerel (Scomber japonicus) in the Pacific Ocean off Japan declined in the 1980s and remained at low levels through the 1990s. There were recruitment successes in 1992 and 1996. However, the cohorts born in these years were heavily fished before the age of maturity and chub mackerel has not begun to recover. To investigate the effects of conserving immature fish, we created four recovery policies: (i) policy 0, actual fishing mortality during the 1990s; (ii) policy 1, conserve strong year classes; (iii) policy 2, apply the average fishing mortality in the 1970s–1980s after 1992; and (iv) policy 3, a 55% reduction of the mortality adopted by policy 2. Policy 3 was considered to be the best in terms of final stock abundance and total catch from 1992 to 1999. We also calculate the future projection of stock and catch under these three policies as well as using average fishing mortality from 1993 to 1999. Using average fishing mortality from 1993 to 1999, the stock will not be recovered within the next 20 years. Even under the best policy, the risk that the final stock is not recovered to 3 million tons within the next 10 years is 40%.
SUMMARY. Stock biomass of the Pacific stock of Scomber japonicus decreased from ca. 4-6 million tons during the early and mid 1970's to less than one million tons in the 1990's. This decline was probably caused from over-fishing and decreased reproductive success corresponding to regime shifts in the late 1970's and 1980's. We decided to keep the spawning stock biomass (SSB) above 0.45 million tons, because 1) no recruitment failures were observed above this level and 2) this SSB can be attained by preserving dominant year classes until their maturation (age-3). Problems of the allowable biological catch determination method in Japan were discussed and a practical solution was presented.
Earlier opinions that Macroramphosus is monotypic are refuted, with two species apparently occurring in Japan (tentatively identified as M. gracilis and M. scolopax). In postsettlement young and adults, the former is characterized by a dark slender body (vs. red-orange and deep) and short second dorsal fin spine with a smooth posterior margin (vs. long spine with a serrated margin). Food habits also differ between the two species, which are either plankton or benthos feeders. Two types of Macroramphosus larvae and juveniles occurring at the surface were recognized, one having a straight ventral body profile of the body (identified here as M. gracilis) and the other having a notch in the anal region. The dark body of postsettlement M. gracilis is considered to be a retention of the character suited to the neustonic distribution of the larval and juvenile stages, the species remaining to ca. 40 mm in standard length (SL) in that habitat (vs. to ca. 12 mm SL in M. scolopax).
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