Competitors and fouling organisms in the hanging culture of the pacific oyster,crassostrea gigas(thunberg)

Generally speaking the “enemies”; of oysters can be divided ecologically into three classes: 1) “parasites”; which live within the body of the oyster and cause harm, 2) “competitors (fouling organisms)”; which occupy the same ecological niche as the oyster and cause harm indirectly by, contesting the available food and living space, 3) “predators”; which feed on the oyster. The sort of harm which can affect oysters differs widely with cultivation technique and habitat conditions. For instance, with the raft hanging method, long line method and other intensive cultivation practices which involve submersion in the sea, the harm from competitors such as Mytilus edulis gcdloprovincialis, tunicates and barnacles is most noticeable. On the other hand, with natural reproduction and sowing methods or the rack hanging method, in which the cultivation generally takes place in the littoral zone or on sea bottom, the most prevalent harm is caused by such bottom dwelling predators as drills (murices, naticids), sea stars, and so on. Of course, damage caused by parasites can be seen equally on all of the above. Here we will deal mainly with competitors (fouling organisms) as these cause the most conspicuous damage in the raft hanging cultivation method, which is the one in general use in Japan. In recent years, as eutrophication of the coastal areas has proceeded, year by year we have seen an increase in the damage caused to marine cultivated animals to which fouling organisms have become attached. Also, as ocean‐going traffic with foreign countries has become more frequent, damage from the abnormal occurrence of imported species is surpassing even the vigorous increase of the American crayfish, Procambarus clarld, and tall goldenrod, Solidago altissima, on the land. In 1969–1970 an explosive outbreak of one of the polychaetous annelids, Hydroides elegans, in Hiroshima Bay caused the most extensive damage to date on cultivated oysters in which the losses reached ¥ 3 000 000 000. Then the damage in the intertidal seed collecting areas due to noxious fouling animals such as barnacles and tunicates grows worse every year and there are several instances where it has impeded an assured production of spat from cultivated oyster species.