Shell growth history of geoduck clam (Panopea abrupta) in Parry Passage, British Columbia, Canada: Temporal variation in annuli and the Pacific Decadal Oscillation

An assemblage of geoduck clam shells from the marine environment of coastal British Columbia was studied. Shells were cut and the widths of internal shell growth increments, annuli, were measured from the hinge plate. The largest shells showed more than one hundred annual increments. Shell growth exhibited juvenile maxima at ontogenetic ages 3–6 years. The growth maximum was followed by a distinct decline that continued until the death of each individual. Further, this ontogenetic growth trend was mathematically removed from the data in order to examine growth variations other than ageing. The longest growth records from the oldest shells were compared to monthly indices of the Pacific Decadal Oscillation (PDO). It was found that the PDO exerts the strongest influence on the shell growth during the very start of the growing season, in February and March. We also detected increased magnitude of growth variations towards the end of the 20th century. Similar trends were apparent in the PDO record. Moreover, the shell specimen displaying the strongest trend of increasing variance had the strongest PDO-linked regional growth signal. Our results support the view that PDO exerts a governing influence on the biological and ecological system along Northeast Pacific coastal areas. Incorporation of geoduck shell growth increment analyses into multi-disciplinary studies dealing with palaeoceanography and archaeology is suggested as a promising future approach.