[A-CG40] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG40-08] Estimation of carrying capacity of Ruditapes philippinarum using net bags on tidal flats in Midorikawa River, Ariake Bay, Japan

*Nana Yamashita¹, Tomohiro Komorita¹ (1.Prefectural University of Kumamoto)

Suspension-feeding bivalves, one of most important fishery resources, often form dense populations on the sandy tidal flats, remarkably high secondary production among benthic animal communities, and play an important role in the material cycle through filter feeding and excretion. Such dense patches of bivalve populations should compete food resources due to strong intra- and/or inter-specific competition. Therefore, carrying capacity of the bivalves should be estimated for sustainable bivalve fishery.
In this study, we hypothesized that "individual and populations growth rates of the clam vary with density”. This hypothesis led to the prediction that " an carrying capacity of the clam is lower the maximum biomass in the previous field observations”. Based on this hypothesis, we control the density of clams within net bags at several levels on tidal flats facing to Ariake Bay, Japan, investigate clam growth rate and secondary production, and analyze the relationship of growth rate and initial density of the clam. The objective of this study was to estimate the appropriate density of clams through the field experiments.
In this study, field experiments were conducted on tidal flats at the river mouth in Midorikawa River facing the Ariake Bay. The clams were put into net bags , with five levels of biomass ranging from the maximum biomass (ca. 17 kgWW m^-2 ) to a normal biomass (ca. 1.2 kgWW m^-2 ) on March, 2022. The growth rate and secondary production of the clam were measured on June and October, 2022. Secondary production of the clam was calculated by the differences between the biomass of clams in June and March divided by the number of days elapsed.As a saturated type curve, the Michaelis-Menten equation was chosen in this study to analyse the relationship between initial density and the secondary production. This analysis was performed using the nls function in the statistical software R (version 3.6.1).
The results show that the biomass in bags was 1.2±0.49 kgWW m^-2 per bag in the lowest density and 18.8±7.51 kgWW m^-2 per bag in the highest density. In June, the value increased to 2.99±0.97 kgWW m^-2 per bag in the lowest density and 22.9±4.68 kgWW m^-2 per bag in the highest density.The mean secondary production was 0.43 ± 0.25 gC m^-2 d^-1 in the lowest density and 1.04 ± 1.20 gC m-2 d-1 in the highest density, indicating that the mean secondary production tended to increase with increasing initial density, but the increase in secondary production was smaller at the highest density.
Although with a large standard deviation, the mean value of secondary production and the initial biomass in the bag, using the Michaelis-Menten equation, the following equation was obtained.
y=1.27*x/4127+x
y is the secondary production and x is the initial biomass. Based on this relationship, the intersection of the straight line y=0.00015355x, whose slope through the origin is Vmax/2Km, and the horizontal line y=1.27, the maximum secondary production, yielded 8.3 kgWW m^-2. Other studies indicated that clams may grow better than wild clams because they are less to be moved by waves, thus consuming less energy and accumulate food. Therefore, it is considered most efficient to manage the density of clams in the actual field to be less than 8.3 kgWW m^-2.