Characteristic changes in the population dynamics of asari (Manila) clam Ruditapes philippinarum in a period of stock decrease on the Banzu intertidal flat, Tokyo Bay

Abstract

In Tokyo Bay, harvesting of asari (Manila) clam Ruditapes philippinarum has decreased since the early 2000s. In the present study, based on the abundance of sampled clams in four sites from 1998 to 2015 in the Banzu intertidal area, the authors discussed changes in the processes of population dynamics of asari clam which are possibly related to harvest decrease in Tokyo Bay. The findings are summarized as following two points. Firstly, the recent decline in the harvest of asari clams in Tokyo Bay may be caused by the mortality of benthic juveniles. The spatial density of juvenile clams at recruitment showed a varied increase during the study period. In contrast, from the recruitment to adult stages, the mortality coefficient z and von Bertalanffy growth function k consistently increased and decreased, respectively. Moreover, decreasing trends in the spatial density of adult clams were apparent in the concurrent period. Thus, the decrease of adult clams was suggested to be due to poor survival in juvenile clams after recruitment. Secondly, the depletion of the autumn cohort and spatial relocation of the large juveniles were apparent after 2009. The autumn cohort significantly decreased in size-frequency composition, despite the increased abundance and size of juvenile clams at recruitment after 2009. Referring to recent reports, predation by fish may be one of the processes that depleted the abundance of the autumn cohort. Additionally, the density of year-class cohorts increased for 2–4 months from their first appearance to the completion of recruitment, which was likely due to the spatial immigration of large juveniles into the study area. The physical stress of sea water movement caused by increasing strong wind may have encouraged the spatial transport of the juveniles. The depletion of the autumn cohort and spatial immigration of the large juveniles were temporally overwrapped with the period of an outbreak and continuing infestation of the parasitic sea spider Nymphonella tapetis after 2007. This observation can hypothesize that poor burrowing activity due to the parasitic infestation allowed the clams to suffer predation or passive transport. However, above changes in population dynamics after the late 2000s cannot be solely connected to decline of asari harvest, because that decrease in the harvest of asari clams in Tokyo Bay began in the early 2000s; there remain factors other than this one that may have acted on the stock decline after the early 2000s in Tokyo Bay.