Negative Impact of Kelp Crab on Released Juvenile Sea Cucumber Apostichopus japonicus

The Japanese sea cucumber Apostichopus japonicus (Selenka, 1867), in particular, harvested from Hokkaido, the northern island of Japan is regarded as the highest quality due to characteristics like large lined-up warts and thicker flesh. Despite the vigorous release of hatchery-produced juvenile have been carried out around the coast of Hokkaido, the stock size have not been recovered. In this study, the hatchery-produced juveniles of A. japonica were released at artificial reefs to monitor their distribution for two years at a fishing port located in southwest Hokkaido, Japan. Co-occurring crabs (Anomura and Brachyura) as possible predatory organisms were also monitored during the survey.

Densities of juvenile A. japonica gradually decreased from the initially released two thousand to 130, 256 and 252 individuals within two years at three different sites, respectively. The body size of A. japonica decreased in summer, indicating a dormant state known as aestivation, but approximately doubled after two years. Fifteen species of crabs (4 hermit crabs and 11 true crabs) were observed, in particular, two hermit crabs Paguristes ortmanni, Pagurus proximus and one kelp crab Pugettia sp. were the dominant species. Sea cucumber ossicles were microscopically observed from stomach contents of these three species. There was a significant, negative correlation between the density of juvenile A. japonica and co-occurring crabs (p < 0.05). Predation of juvenile A. japonica by hermit crab Paguristes ortmanni and kelp crab Pugettia sp. were also confirmed in laboratory predation experiments. These results indicate possible predation of juvenile A. japonica by these crabs in a natural environment.

In this study, the results demonstrate an important insight that predation by these crabs may influence significantly on an abundance of juvenile A. japonica, negatively affecting sea cucumber fisheries in Hokkaido.