Seasonal and interannual variation in the density of visible Apostichopus japonicus (Japanese sea cucumber) in relation to sea water temperature

Apostichopus japonicus (Japanese sea cucumber) is of ecological and economic importance in the coastal waters of Japan. To assess the apparent abundance of A. japonicus, density data of visible A. japonicus are required; however, the activity of this species on the sea floor changes with variations in sea water temperature. For example, at high sea water temperatures, A. japonicus takes shelter in the gaps of rocks or oyster shells. This change in its visibility on the sea floor makes it difficult to determine the apparent abundance of this species. In the present study, we aimed to clarify the optimal survey timing to assess the apparent abundance of A. japonicus based on the relationship between the density of visible A. japonicus and sea water temperature. Between 2010 and 2013, we evaluated how the density (ind. 100 m⁻²) of visible A. japonicus changes with sea water temperature in an area preferred by these organisms, Maizuru Bay, Kyoto, Japan, every 2 weeks. Seasonal changes in the density were consistent with seasonal variations in sea water temperature. Density increased when sea water temperature decreased and was at a maximum at minimum sea water temperatures. In addition, the density tended to change significantly over a short period. In some cases, the density after the lowest sea water temperature in March 2011 declined by half from 65 to 27 in just 2 weeks. Owing to considerable variations in the density of visible A. japonicus, we believe it is important to establish the appropriate survey timing to quantify the apparent abundance of this species, with our data suggesting that surveys should be performed during the season with the lowest sea water temperatures. Furthermore, at the lowest sea water temperatures in 2010, 2011, 2012, and 2013, the maximum densities were 99.0 (9.1 °C), 65.0 (9.7 °C), 41.3 (9.8 °C), and 8.7 (8.9 °C), respectively. Over 4 years, the density decreased to approximately 10% of that in the year with the peak density. Considering this decrease, it is suggested that A. japonicus in Maizuru Bay is overfished. Therefore, it is possible that the density of visible A. japonicus at the lowest sea water temperatures provides useful information for assessing the apparent abundance of this species.