Larval recruitment of the manila clam Ruditapes philippinarum in Ariake Sound, southern Japan

Abstract

The mechanisms by which catch yields of the manila clam Ruditapes philippinarum have decreased drastically in Kumamoto Prefecture, southern Japan, and the larval recruitment by which benthic populations of the clam are maintained were studied. Samples at different growth stages (planktonic larvae, new settlers, and small to large bivalves of the species) were collected from February 1997 to December 1998 on two tidal flats (Kawaguchi and Nameishi, 20 km distance from each other) along the eastern coast of Ariake Sound. Densities of planktonic larvae, new settlers and small bivalves were generally higher at Kawaguchi than Nameishi. The higher density of large bivalves at Kawaguchi depends on a high larval density. Larval retention mechanisms coupled with water movement at Kawaguchi, such as tidal residual currents, may be related intimately with the higher larval density at Kawaguchi. Predation by shore birds at and after recruitment may be related to the lower density of large bivalves recorded at Kawaguchi in 1998, despite larval densities being higher there.

Variability of diet-tissue isotopic fractionation in estuarine macrobenthos

“…We supplied 0.06 ml (0.32 mg C) and 0.5 ml (1.35 mg C) d -1 of the diet for the bivalves and ghost shrimps, respectively. The growth rates (mean ± SD) of these animals were 0.032 ± 0.025 mg dry wt d Ishii et al 2001). However, the obesity index showed that the 2 bivalves were well fattened up at the termination of the feeding experiment in comparison with clams in the field (i.e.…”

Critical events in the Ariake Bay ecosystem: Clam population collapse, red tides, and hypoxic bottom water

“…These effects are predation by a carnivorous snail (Glossaulax didyma) (Kajiyama et al 1983), a ray (Aetobatus flagellum) and shore birds (wild duck) (Nakahara & Nasu 2002) and negative impact of biological disturbance of the sediment by a ghost shrimp (Nihonotrypaea japonica) on the survival of juvenile clams (Tamaki 2004). Ishii et al (2001), Ishii & Sekiguchi (2002) and Sekiguchi & Ishii (2003) emphasized the decrease in survival rate in the pelagic stage and the decrease in the number of recruits on the sediment of the tidal flats after the pelagic stage as one of the major factors that caused a marked decrease in the clam harvest on the tidal flats.…”

Evaluation of Dredged Sediment as a Silt and Clay Source for Artificial Tidal Flats

“…Macrobenthos are near the bottom of the food web, providing food for large crustaceans, fish and birds (Day et al, 1989;Evans et al, 1998), and also play a role in water purification by the tidal flats (Herman et al, 1999;Magni and Montani, 2006). Certain bivalves, such as clams, form fisheries (Ishii et al, 2001). To survey the emerging macrobenthos, we periodically sampled the artificial tidal flat with sediment core samplers: 25 cm × 25 cm × 25 cm for the real sea shore and 10 cm × 10 cm × 10 cm for the tidal flat simulator.…”

“…By the 1980s, however, about 40% had unfortunately disappeared (Kimura, 1994). In recent years, useful functions of tidal flats such as biological production, water quality clarification and recreation have been recognized more and more (Miyoshi et al, 1991;Kimura et al, 1992;Imamura, 1997;Evans et al, 1998;Ishii et al, 2001;Morrison et al, 2002;Tiner et al, 2003;Sakamaki et al, 2006;Magni and Montani, 2006). For example, Kimura et al, (1992) compared water quality at the seashore where an artificial tidal flat existed, and its offing region.…”