Mortality of the Sakhalin Surf Clam in Sendai Bay in the Early Benthic Stage

Abstract

Mortality rate after settlement was estimated for 1983-1986 year classes of the young Sakhalin surf clam Spisula sachalinensis in Sendai Bay. A common pattern is found in their distribution, suggesting a regular, efficient mechanism to gather young clams. The pattern remains unchanged with time, showing an insignificant movement of young clams at least macroscopically.
Each year class declines exponentially and mortality rate fluctuates widely among year classes. The mean mortality rate ranges from 0.4 to 0.8 per month. Abundance of juveniles settling in the fishing ground also varies and no significant correlation is found between it and the subsequent mortality. The relative strength of each year class depends on the rate of reduction during about 6 months after it is produced, and would be controlled primarily by the fluctuation in mortality after settling rather than the heavy reduction during the pelagic life.