Three cage tests for the growth and survival of Japanese littleneck clam juveniles were conduct ed on the tidal flat of Kuzuma along the coast of Kisarazu district in Chiba Prefecture, Tokyo Bay, and the sharpness index defined as width/length of shell was examined. The area having good condi tions for juvenile growth was not consistent with that for a high survival rate, and juveniles with a high survival rate did not always grow faster than those with a low survival rate.Juveniles on the offshore side grew faster than those on the shoreside, but the juvenile survival rate in the intermediate zone of the flat was higher than those on the offshore and shore sides. The juveniles having a high sharpness index tended to show a high survival rate. At the same age, the juveniles with a high sharpness index grew faster than or equal to those with a low sharpness index. But as the sharpness index approached the maximum value on the tidal flat, the shell growth of the juveniles was less than those of younger clams and the sharpness index was lower.
Bule tides,upwelling of anoxic waters,have been one of the most serious environmental problems issues in Tokyo Bay. The major sources of the blue tides are considered to be the anoxic waters containing hydrogen sulfide in dredged trenches,navigation channels and the flat bottom waters at the head of the bay. The objective of the present study is to reveal each contribution to blue tide,which is still unclear,based on the estimation of time variation in the amounts of hydrogen sulfide in these waters comparing to occurrences of blue tide events.We revealed the large contribution of the flat bottom anoxic waters compared to the waters in the largest dredged trench in case of large scale blue tides.
In Tokyo Bay, the harvestable quantity of asari (Manila) clams Ruditapes philippinarum has been decreasing since the late 1990s. We conducted a field investigation on clam density in the Banzu culture area from April 1988 to December 2014 and collected records spanning January 1986 to September 2017 from relevant fisheries cooperative associations to clarify the relationship between the temporal variation in stock abundance and the production activities of fishermen. The yearly variation in clam abundance over the study period was marked by larger decreases in the numbers of larger clams. A large quantity of juvenile clams, beyond the biological productivity of the culture area, may have been introduced as seed stock in the late 1980s despite the high level of harvestable stock. The declines in harvested quantity began in the late 1990s and may have been caused by decreases in harvestable stock despite the continuous addition of seed stock clams. The harvested quantity is likely to be significantly dependent upon the wild clam population, even within the culture area, as the harvestable quantity was not correlated with the quantity of seed stock introduced during the study period. These declines in harvested quantity may have resulted from a decreasing number of operating harvesters due to the low level of harvestable stock and consequently reduced profitability. Two findings were emphasized. A certain management style, based on predictions of the contributions of wild and introduced clams to future stock biomass, is essential for economically-feasible culturing. In areas with less harvestable stock, actions should be taken to maintain the incomes of harvesters while avoiding overexploitation, even if the total harvest quantity decreases.
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