Interannual to decadal variability of summer sea surface temperature in the Sea of Okhotsk recorded in the shell growth history of Stimpson's hard clams (Mercenaria stimpsoni)

Sclerochronological and shell stable oxygen isotopic analyses were conducted on live-caught specimens of Stimpson's hard clams, Mercenaria stimpsoni, from the southern Sea of Okhotsk, off northern Hokkaido, Japan. In this region, the main growing season of this species during early ontogeny (below the age of 12 years) lasts from mid-spring to mid-fall at sea surface temperatures (SST) between approximately 10 and 22 °C. Growth cessation begins between late fall and early spring at SST, below approximately 6 °C; however, shell growth was largely limited to the summer season later in life. Counting of annual increments indicated that this species had a relatively long life span of up to 100 years. Annual shell growth rates were high during early ontogeny and declined abruptly afterwards. Mean standardized shell growth indices (SGIs) of long-lived specimens were positively correlated to the mean summer SSTs near the sampling site and in the coastal waters off northern Hokkaido. The SGI chronology of the longest-lived specimen (99 years old) exhibited periodicities of approximately 10 and 5 years during the calendar years 1920-2011, possibly reflecting the quasi-decadal variability of summer SST in the southern Sea of Okhotsk. These findings indicate that M. stimpsoni could serve as an archive to reconstruct past marine climate changes in the Sea of Okhotsk.