Estimates of the Age and Depth of Formation of the Subtidal Oyster Reefs in Saroma Lagoon, Hokkaido

Abstract

Oyster reefs, composed of dead oyster (Crassostrea gigas) shells, abound on the subtidal mud bottom of Saroma Lagoon, Hokkaido. According to previous descriptions, these reefs might have been formed in the intertidal zone. However, they have also been observed in the subtidal zone in Saroma Lagoon. Thus, the following five hypotheses were proposed and tested; (1) the reefs sank down into the subtidal zone due to diastrophism after reef formation, (2) the sea level rose after reef formation, (3) the reefs formed in the subtidal zone naturally, (4) dead shells accumulated subtidally and were piled up by strong water movements, and (5) the reefs were piled up artificially. The distribution and contours of the reefs were examined with a sonic depth finder and through visual observations by scuba diving. The reefs, mainly located at depths of 8-10m, had a mean diameter of 16 m and a mean height of 1.5m. The right and left valves of many of the oysters were intact, indicating that these shells had grown in situ. Radiocarbon dating of a dead oyster shell revealed an age of 180 years or less. The sea and land levels did not change significantly during the period of reef formation. These results, combined with the history of the oyster fishery within the lagoon, strongly suggest that the reefs formed recently on the subtidal bottom. We suggest that the method of estimating sea level by excavation of fossil oyster reefs should be cross-checked with other methods.