Ecology of the Razor Clam Solen gordonis and Fishery Impact on the Population In Sasebo Bay, Kyushu, Japan

In Sasebo Bay, northwestern Kyushu, Japan, the population of the razor clam Solen gordonis inhabits a coarse sandy substratum at 5- to 20-m water depths. A bundle of long metal rods with cone-shaped tips is used to vertically pierce and harvest clams from fishing boats. Knowledge of the ecological traits of this species and the fishery impact on the population is crucial for sustainable management. In April 2014 to March 2015, quadrat sampling by divers, experimental fishing surveys, and retrieval of clams detached from the fishing gear were conducted monthly. The grand mean individual density at the fishing ground was 555.7 inds/m3. New recruitment occurred in mid-November 2014, and the population excluding the recruits was divided into four cohorts by age according to shell-length-frequency distribution. Individual growth was described based on von Bertalanffy growth function, with L ∞, K, and t0 being 95.7 mm, 0.548/y, and -0.109 y, respectively. The estimated mean shell lengths at the 1+, 2+, 3+, and 4+ years of ages were 43.6, 65.5, 78.3, and 85.6 mm, respectively. The clams targeted for the fishery were mainly composed of the 2+ and 3+ years of age cohorts. Based on published information on the smallest mature sizes for other Solen species populations (46.1–58.2 mm in shell length), reproduction in the present population would begin at the 2+ years of age cohort. The macroscopic observation for gonadal development in the population also indicated the occurrence of the spawning event only in October-November in the year. The population is harvested from January to May, and thus no reproductive clams just before spawning are removed. Fishing efficiency was about 50%, caused by a greater loss in the smaller clams that were detached and damaged from the fishing gear (noncatch mortality). Fishing size selectivity was determined from the relationship between the diameters of cross-sectional area of clams and of cone of the fishing gear tip (9.5 mm). Downsizing the cone diameter to 8.0 mm and lowering the fishing effort by 15% are recommended to attain the present yield level toward local population persistence.