Impact of low temperature during the winter on the mortality in the post-settlement period of the juvenile of short-neck clam, Ruditapes philippinarum, on the tidal flats in Hichirippu Lagoon, Hokkaido, Japan

Abstract

We conducted field surveys to monitor the population dynamics of short-neck clam, Ruditapes philippinarum, in Hichirippu Lagoon, in eastern Hokkaido, Japan, focussing on the negative impact of extremely low temperatures during the winter on juveniles in the post-settlement period. The clam population had a short annual breeding season between September and November. The newly settled juveniles experienced the severe winter as the daily mean temperature of the surface sediment descended below 0°C soon after settlement. The mortality of small juveniles (less than 3 mm in mean shell length) in the post-settlement period between October 24, 2006, and August 1, 2007, was estimated at 1.34% d⁻¹, which was approximately 5.6 times higher than after the post-settlement periods (0.24% d⁻¹ and 0.17% d⁻¹). We compared the mortality of the juvenile in and after the post-settlement period with the results of the population study conducted on Kikuchi River Tidal Flat, Kumamoto, Kyushu (Tsukuda 2008), where the daily mean temperature was approximately 6°C higher in the post-settlement period of the juveniles. On Kikuchi River Tidal Flat, the mortality in and after the post-settlement period of the juvenile was estimated at 1.62% d⁻¹, and 0.49% d⁻¹ and 0.44% d⁻¹, respectively. These facts indicate that the extremely low winter temperatures in this study area (eastern Hokkaido) does not bring about large-scale mortality, and this is true not only for juveniles after the post-settlement period but also small juveniles in the post-settlement period.