The Ruditapes philippinarum catch in Japan has drastically declined and its recovery is a major challenge. The spawning rates and egg output need to be estimated to discuss stock management strategies. However, it is difficult to observe these directly in a field survey. This study proposes a state-space model to estimate spawning rates in an R. philippinarum population using the number of individuals at each developmental stage based on traditional histological observations of the gonads. An example of its application to monthly observed data of an R. philippinarum population on the western Japanese coast is also presented. The spawning seasons were quantitatively detected in spring and fall, and the durations were estimated to be 2.8 months in spring and 3.0-3.5 months in fall, based on the spawning rates. The mean condition factor was lower in fall than in spring, but females with a high condition factor were also found in fall. The egg output of these individuals during the fall may be large. The spawning frequency per individual was estimated to be 6.5 times per year based on the model, indicating that R. philippinarum is an r-strategy species capable of multiple spawning. Questioning how the spawning rates vary with individual size/age and environmental conditions is essential for estimating egg output in R. philippinarum populations. The state-space model in this study can contribute to the quantification of spawning rates. The results will contribute to estimating the egg output and stock management strategies.

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In order to consider measures for improving sediment quality atˆsh farms in the southern part of Mie Prefecture, Japan, we surveyed the sediment quality (acid volatile sulˆde [AVS] content), farming intensity (ˆsh density, daily feeding rate,ˆsh cage area, and migration ofˆsh cages) and environmental characteristics (water depth and current velocity) in red sea bream Pagrus major farms. Regression analyses revealed that the daily feeding rate,ˆsh cage area, migration ofˆsh cages, water depth, and current velocity in‰uence spatial variation of AVS levels. AVS level was particularly in‰uenced by the current velocity, an increase in which was associated with a reduction in the sedimentation rate of organic matter on the seabed and increase in supply of dissolved oxygen.

Perna mussels have been introduced into Japanese waters since 1967. We investigated the occurrence and wintering of the Perna mussels in Ise and Mikawa Bays, central Japan. Most of specimens collected in the bays are identical with Perna viridis, but several specimens were difficult to identify to a specific level based on the past taxonomical studies. It is difficult to identify the specimens of the Perna mussels introduced into Japanese waters where commercial vessels have often come from oversea. Several specimens of P. viridis and Perna spp. were wintering at Nagoya port located in the innermost of Ise Bay and also at cooling water discharge of the Thermal Power Plant in Hekinan city located in Mikawa Bay.