Suspension culture of clams at a port in a cold sea area

１．Introduction

　In a fishing port area surrounded by seawalls, greater calmness than in the open sea is ensured. Use of cultivation facilities including the effective application of fishing port stock taking advantage of this calmness is expected to contribute to the promotion of regional fisheries and revitalization of fishing communities, which are facing such challenges as depopulation, shortage of successors, declining birthrate and aging.
　To utilize the calm area in a fishing port, the Fisheries Engineering Research Team focused on clams, of which the domestic production has decreased significantly in recent years, and conducted a suspension culture test by suspending round aquaculture baskets (Photo 1) containing clams in a port. In Hokkaido, clams are mainly produced in the eastern sea area where tidal flats have developed, and account for approximately one-third of Japan’s total production in 2020. Meanwhile, there are not many natural tidal flats where clams can live on the Sea of Japan coast in Hokkaido, and fisheries production is generally sluggish. To improve this situation, the test was conducted at an aquaculture facility in Era Fishing Port on the Sea of Japan coast in southwestern Hokkaido.

Photo 1　 Round baskets for clam culture

２．Evaluation of the culture test

　Clams require a certain extent of water flow to supply the feed they consume by filtering suspended matter in the water. Fries grow up under sand or other bottom sediments. In Hokkaido, clams spawn only once a year, and it takes two years or longer before they can be shipped.
  In consideration of these factors, the suspension test used round baskets (40 cm in diameter, 10 cm in height), one type filled only with 100 fries of approximately 8 mm in shell length and the other, in which gravel or pumice was spread to a thickness of 3 cm, in addition to the fries. The baskets were suspended in the water in three ways (Fig. 1): (1) fixed to the land and sea bottom with ropes attached to the top and bottom of the basket, (2) fixed directly to a tube pipe installed on the quay, and (3) suspended from the land only with a rope. The shell length and survival rate of clams in the respective baskets were studied for two years or longer.
  The results indicated that the growth rate of clams was high in baskets fixed to a tube pipe, where waves caused less swaying. The growth rate was also high in baskets with gravel or another substrate, and was higher with gravel when the fries were small and with pumice after they had grown to be a certain size. The survival rate approximately one year after commencement of the study was approximately 85%, which was almost the same as that of a previous case at an adjoining fishing port using fries of approximately 12 mm in shell length. It was found that approximately 10% of the surviving clams grew large enough for shipment.
  Through these studies, the preferable handling conditions for growth of clams cultured in a fishing port were identified, and it was confirmed that suspended culture can contribute to the utilization of water in fishing ports. However, since growth in the second year was slower than in the first year, the team intends to continue identifying better conditions, including comparison with other sea areas.

Fig. 1　 Round basket installation methods

(Contact  :  Fisheries Engineering Research Team, CERI)