POLYDORA SP. INFESTATION AND HEALTH OF THE PACIFIC OYSTER CRASSOSTREA-GIGAS CULTURED IN BAJA-CALIFORNIA, NW MEXICO

The culture of the Oyster Crassostrea gigas Thunberg is the most impor tant aquaculture activity in the state of Baja California, NW Mexico. Its production is obtained from Bahia Falsa, at South of the state, re aching 2,500 metric tons per year and is sold in national and internat ional markers. However, there is a lack of information on commensals a nd parasites affecting cultured oysters. A survey of worm blister on t he inner valves of cultured oysters was performed from May 1996 to Apr il 1997. Two locations were sampled: the outer and inner areas of the bay, at three depths. Direct counts of worm blisters and quantificatio n of the area occupied by worm blisters on the inner valves were used as evaluation methods. The burrowing worm belonged to the genus Polydo ra (Polichaeta: Spionidae). Its prevalence was higher in oysters from the outer relative to the inner locations, and was also higher in oyst ers placed near the substrate relative to those near the surface of th e water. There was a trend of lower condition index and higher mortali ty in oysters placed near the muddy bottom. The placement of culture o ysters away from the bottom and exposed to air during low tides may re duce the prevalence of Polydora sp. and oyster mortality.