2021 Volume 85 Issue 2 Pages 69-78
The Manila clam (Ruditapes philippinarum) catch plunged to 1/10 of the catch of FY 2013 in FY 2017 in Ise Bay and Mikawa Bay, Aichi Prefecture, Japan. One of the suggested causes is the reduction of phytoplankton content associated with reduced nutrition. Therefore, we examined the relationships between Manila clam growth and the aged changes of chlorophyll a, TN, and TP. We calculated the aged changes of clam soft body weight and gonad weight with a clam growth model using the concentration of chlorophyll a and water temperature. Consequently, from FY 2007 to FY 2017, clam soft body weight continued to decrease below the minimum weight required for survival, and gonad weight was lower from FY 2007 to FY 2017 than from FY 1998 to FY 2006. We hypothesized that the concentration of chlorophyll a, especially from June to November of each year, has a large influence on clam soft body weight and influences habitat. TP had a high contribution rate with chlorophyll a of TP and TN. When clam growth remained stable from FY 1998 to FY 2006, the average concentration of TP from June to November maintained at 56.5 μg･l−1. This value was equivalent to class 3 (50 μg･l−1<TP≤90 μg･l−1) of the appropriate concentration for clams in the quality standards for fishery waters. However, when the clam catch decreased because of continued poor growth from FY 2007 to FY 2017, the average concentration of TP from June to November had decreased to 45.2 μg･l−1. This average concentration was equivalent to class 2 (30 μg･l−1<TP≤50 μg･l−1). It is thought that class 3 TP concentration is required from June to November to maintain clam resources.
