Application of a fish finder system to spatial distribution mapping for the razor clam Solen gordonis: Case study from coastal waters, northwestern Kyushu, Japan

Effective spatial management planning for sedentary fishery resources requires accurate spatial distribution mapping of a focal stock including both fished and associated unfished populations. This study focuses on (sub-) adult razor clams (Men gordonis), with shell lengths >= 25 mm, in Sasebo Bay and Omura Bay, northwestern Kyushu, Japan. They are commercially harvested, but little is known about their spatial distribution pattern. The objective was to estimate the spatial distribution and configuration of clam beds existing within an area of ca. 12 km(2). For this, clam and sediment sampling by scuba-equipped divers, acoustic bathymetric surveys using a recreational-grade fish finder (single-beam acoustic) system, and presence-absence modelling using generalized linear models were performed. For the collected clams, catch per unit effort (CPUE; number of clams collected/ h) was calculated and used as a proxy for clam individual density. The results showed that the presence-absence variation in the spatial distribution of (sub-)adult clams was associated with the bottom sediment type. (Sub-) adult clams occurred frequently in the sediments with higher gravel (particles with diameters a 2 mm, mainly shell fragments) and lower mud (diameters < 0.063 mm) contents than the sediments of the absence sites. Acoustic signals of the fish finder system were backscattered more intensely from gravel-sand beds (i.e., typical of the presence sites) than from muddy sand beds (i.e., absence sites). Using the relationships among the presence-absence variation of (sub-)adult clams, sediment type, and backscatter strength, spatial distribution of clams was predicted with high accuracy. The presence probability of (sub-)adults (= x; continuous values from 0 to 1) predicted from the results was significantly regressed on CPUE ( = y) [i.e., y = exp ( -0.7984 + 4.9350x) - 0.051. Using this relationship, a map for the relative abundance of (sub-)adult clams was obtained. This map depicted several clam beds including both exploited and unexploited ones. The latter beds virtually conform to no-take zones in the razor clam fishery.