Current state and structure of native aggregations of pacifc oyster in the northern Amur Bay (Peter the Great Bay, Japan Sea)

Abstract

Spatial distribution and reproductive ability of the pacifc oysterCrassostrea gigasaggregations in the northern Amur Bay (Peter the Great Bay, Japan Sea) are investigated using the data of direct and remote accounts. Totally 772 oyster banks and reefs are detected with the summary area of 284.6 hectares that is signifcantly larger than earlier assessments. The 35 giant beds (> 1 ha) occupy 53.2 % of the summary area, but 79.8 % of mollusks gather in medium and large aggregations which occupy 45.5 % of the summary area. Majority of the aggregations (> 95 %) are located on shallows with the depth less than 5 m. The oyster matur-ing is completed when it reaches the size of 50 mm. The ratio of males to females is 1.0 : 1.5, on average, that indicates indirectly favorable habitat conditions forC. gigas. Males prevail among the small-sized individuals, but females prevail among the large-sized ones that corresponds to reproductive strategy of pacifc oyster and type of its reproduction. The males to females ratio changes from year to year. Portion of young mollusks is rather high (13.9–56.2 %), portion of the main reproductive groups changes from 40.6 to 88.9 % that indicates active processes of natural reproduction and the oyster reefs forming. Mollusks with high reproductive rate prevail among the main reproductive group with the size 70–130 mm (58.2 %). The largest number of such individuals is observed in the largest aggregations of oysters at Rechnoy Island (85.0 %) and at Skrebtsov Island (60.7 %). Spatial variations of the shells shape and ratio of size-physiological groups in the aggregations reflect patterns of the habitat conditions. Generally, the state of native oyster aggregations in the northern Amur Bay is stable with a tendency to their population increasing.