2017 年 71 巻 2 号 p. 64-69
Lake Nakaumi is a brackish water body in the eastern part of Shimane Prefecture, Japan. The lake, which receives fresh water from Lake Shinji and several rivers, connects with the Japan Sea through a single narrow channel. Recently, aquaculture trials for the Manila clam, Ruditapes philippinarum, have been conducted in the lake. However, little is known regarding the ecological and physiological characteristics of the clam in this environment. In the present study, the reproductive cycle was investigated in clams sampled at a subtidal site from April 2008 to March 2011, using histological analysis and the change in condition index(CI). The water temperature, salinity and chlorophyll a + pheo-pigment content were monitored in lake water near the collection site of the clams. The CI of the clams increased from March to May and gradually decreased afterwards to September/October. The spawning period was determined to be August–October in 2008, Jun–August and October–November in 2009, and Jun–October in 2010. Thus, the time of initiation and annual number of the peak spawning periods varied yearly. While the water temperature(<5–>30℃)and chlorophyll a + pheo-pigment content(7–15 μg/l)were within the range for these parameters in the major clam habitats in the southern part of Japan, salinity(10–25 psu)was considerably lower than that in other areas. We presume that low and variable salinity conditions caused unstable spawning activity(delayed initiation and fluctuating annual peak of spawning period)of the clams in Lake Nakaumi.
