Possible processes responsible for the decline in the stock of Manila clam based on long-term observations in Banzu tidal flat

Abstract

In this study, we conducted three surveys of the Manila clam, Ruditapes philippinarum, in the Banzu tidal flat, Tokyo Bay, from the late 1980s to early 2020s. The surveys included core sampling at six sites along the onshore–offshore transect line to assess density changes after settlement, fishing gear sampling at 64 sites to examine changes in clam distribution across the area, and sampling at 14 sites to monitor changes in condition index. We identified two seasonal cohorts of clams per year derived from spring and autumn spawning, which consisted of complementary and primary components, respectively, to create the next-generation clam stock. The decline in the autumn spawning cohort is a fundamental aspect of the decrease in clam stocks in Banzu. This decline was attributed to reduced survival after settlement, primarily affecting relatively large individuals with a shell length (SL) of approximately >20 mm. As the autumn cohort reaches an SL of >20 mm only after the summer following spawning, it is assumed that size-dependent factors affecting clams with SL > 20 mm after summer were responsible for the recorded decrease in clam stocks in Banzu. Thus, the decline in Manila clam stocks can likely be attributed to mortality from energy loss associated with spawning under oligotrophic and high-temperature conditions and increased predation pressure of black porgy populations with synergistically acting reduced burrowing ability of clams because of low water salinity.