Increased impact of predators and coastal warming on Ruditapes philippinarum populations in Japan

Abstract

Fishery production of the asari clam Ruditapes philippinarum has decreased significantly from the western to the central coasts of Japan, excluding the northern coasts. It has been observed in the Nakatsu tidal flat, a representative population-collapsed habitat in western Japan, that survival rates are low, even if recruitment of juvenile clams has occurred. Predation is thought to be the main cause of mortality, and predation prevention has been confirmed to reduce mortality and enable clam production in many areas of western Japan. However, other factors also contribute to high mortality in clam populations, in particular the combined stress of high temperature and Perkinsus infection. Camera monitoring has shown that the main predators were black seabream Acanthopagrus schlegelii in summer and ducks in winter. Research has shown that foraging by black seabream damages nori seaweed (Neopyropia yezoensis) farms, suggesting that the foraging pressure from this fish species substantially affects coastal ecosystems. The rise in seawater temperature in winter may have prolonged the active period of black seabream, a temperate to subtropical species, enhancing its population growth and habitat expansion. Increased anthropogenic nutrient loads and hypoxia in colder seas during the period from the 1960s to the 1980s may have led to clam population growth with lower predator abundance. It is still unclear whether the re-eutrophication of recent warm seas will increase clam populations. Although increased food availability enhances clam growth and reproduction, reducing mortality by alleviating combined stressors; this effect will likely be masked if predation pressure and combined stressors strongly affect clam populations.