Abstract
Pen shells, Atrina spp., were important marine product in the northern Ariake Sea until recent years, but most of the pen shell populations have collapsed over the past two decades. In the present study, recruitment, growth, and mortality were compared among three pen shell populations, at Site A (intertidal zone), Site B (upper subtidal zone, 3 m in depth), and Site C (subtidal zone, 7 m in depth) in the northeast fishery zone of the Ariake Sea from 2000 to 2006. The maximum mortality rate (maximum proportion of dead shells on each sampling date) was low at Site A (12.8%) and Site B (7.8%). On the other hand, at Site C, most pen shells have died, with undamaged shells mainly from spring to summer and, in most years, a maximum mortality rate of over 80% present. At Site C, shell density decreased greatly during the study period, but it was much higher than at Site A or B. Growth was slower and the soft body weight was lower at Site C than at Sites A and B, but the seasonal pattern of relative gonad weight was not very different among the three sites. These results suggested that there has been a deterioration of habitat conditions at Site C and that excessive reproductive investment under bad conditions might have led to the massive die-offs there. Although the direct cause of these die-offs is unclear, it is certain that degradation of sea-bottom conditions as a result of, e.g., hypoxia, increase in sulfide concentration, and decrease in grain size of the sediment, affects the survival of pen shells directly.
