2025 Volume 89 Issue 1 Pages 28-40
The levels of nutrients (TN and TP) and chlorophyll-a (Chl-a) required for the success of the Manila (Asari) clam fisheries in Mikawa Bay were discussed based on the results of fishing ground surveys and long-term water quality monitoring data. The correlation coefficient between the change in abundance and water quality during fall and winter was positive and was particularly significant with Chl-a in summer. Long-term water quality data showed extremely low CPUA (clam catch per unit fishing area) below threshold values (TN: 385–417 μg∙ L-1, TP: 40–42 μg∙ L-1, Chl-a: 10.7–12.2 μg∙L-1, all annual means). Based on the logistic regression analysis, the concentration thresholds at which the risk of resource collapse increases were calculated to be 364 μg∙L-1 for TN, 39 μg∙L-1 for TP, and 9.3 μg∙L-1 for Chl-a, while the concentrations for the achievement of the clam fishery level were 457 μg∙ L-1 for TN, 54 μg∙L-1 for TP and 14.4 μg∙L-1 for Chl-a. These water quality concentrations were higher than the reference concentrations of environmental standard type II applied to the bay area including the clam fishing grounds in Mikawa Bay, which suggests that appropriate nutrient management is necessary for clam stocks to recover.
