Predator-prey relationship between crabs and short necked clam, Ruditapes Philippinarum
2005
Kimura, H.(Yamaguchi-ken. Fisheries Research Center, Nagato (Japan))
According as the catch of short necked clam Ruditapes philippinarum has decreased since the late 1980s, harvest yield of the planted clams in the tidal flats has also decreased. Crabs are known to prey on clams, so author evaluated the predation pressure of crabs on the planted clams and on natural resources of the clam. A lot of broken red shells were found in the nursery fields in where clams stained red with paint for the mark were planted without protect net on the estuarine tidal flat of the Monzen river in Iwakuni city. There existed a considerable number of crabs such as Charybdis japonica and Portunus pelagicus around the nursery fields, and they prayed on the clam leaving broken shells in a rearing tank. The crab of 149.2g (average weight of the crabs captured around the nursery fields) was evaluated to eat 13.8g flesh of the clam in a day. If the biomass of the clam is high as it used to be observed in 1982 and 1992 on the same tidal flats in Iwakuni city, growth of the biomass can be larger than the crab's predation pressure. The author concludes that the predation pressure of the crabs is the largest factor which reduces the population of planted clam in nursery fields but is not the main factor which has reduced the natural resources of the clam. The predation pressure of crabs on natural resources of the clam has increased relatively as the result of the marked clam resources diminution caused by unexplained factor.
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