Estimation of the sediment purification ability of sea cucumbers
At the start of the experiment
(before feeding on sediment)
During the experiment
(while feeding on sediment)
Consumption per sea cucumber
in laboratory testing
(click to enlarge)
The economic value of sea cucumbers - a major marine species in Hokkaido - has recently seen a significant increase in line with greater exports to China. Their unit price has risen rapidly since around 2002, and is now five times higher than it was 10 years ago. Indeed, sea cucumbers are now referred to as black diamonds because they are in such high demand. Sea cucumber harvests have also doubled in the last 10 years, leading to increased need for resource management. In some areas, work on the production and release of hatchery-reared eggs and larvae is currently under way. In response to the promotion of development of ports and harbors in harmony with the environment and fishing port development involving fisheries stock enhancement, the Fisheries Engineering Research Team focuses on work relating to sea cucumbers that feed on organic matter deposited on the sea bed, and has begun efforts to fatten sea cucumbers in a harbor environment in order to examine approaches for sediment remediation at berths in the harbor. However, little is known about the life of sea cucumbers and they have seldom been the subject of studies on sediment purification, especially in cold waters.
Against this background, the team attempted to estimate the sediment purification ability of sea cucumbers through laboratory testing using a specimen from Hokkaido. Its annual feed intake and excretion were measured, and the temperature of the water in the tank was adjusted to reproduce seasonal variations observed in actual marine environments. The second photo here shows the sea cucumber feeding on sediment. Based on the measurement data, the amounts of carbon and nitrogen (both causes of sediment pollution) contained in organic matter consumed by the specimen were determined. The results of sediment purification are shown in the figure here in terms of consumption per 100 g of sea cucumber. It can be seen that organic matter purification by the specimen varied greatly with the season and water temperature. The remediation values were approximately 50 mg carbon/day and approximately 6 mg nitrogen/day during the peak consumption seasons. In marine environments, organic matter settles on the sea bed, exerting a load on sediment. The results obtained here indicate that the purification ability of sea cucumbers can be quantified if the load in their actual habitats is determined.
In this context, a fishing port inhabited by sea cucumbers was surveyed. The carbon load was found to be 273 mg/(㎡·day), and the nitrogen load was 53 mg/(㎡·day). The results of a previous study suggested two sea cucumbers/㎡ as an adequate population for purification. When these figures are applied to the present case, it is estimated that the sediment load can be reduced by approximately 20% to 30% due to feed intake by sea cucumbers.
This approach enables the quantification of feed consumption and the sediment purification ability of sea cucumbers based on their feeding habits in cold waters.
(Contact: Fisheries Engineering Research Team, CERI)