Carbon and oxygen stable isotope analysis of Pacific razor clam (Siliqua patula) shells from Cook Inlet, Alaska

Pacific razor clam abundance in the eastern Cook Inlet has declined significantly over the past several years. In 2015, all east-side Cook Inlet beaches were permanently closed to sport harvesting. The cause of this decline remains unknown, although habitat modification and shifting environmental conditions have been hypothesized as underlying factors. We analyzed 20 clams from 10 Cook Inlet beaches for their carbon and oxygen stable isotope composition. Each clam shell was sampled along a transect from the outer aspect towards the umbonal area, at approximately yearly intervals (targeting visible annuli). No relationship was found between δ¹⁸O and clam growth rate. However, we observed a general decline in Cook Inlet razor clam δ¹³C, beginning around 2014. This decrease in δ¹³C is coeval with above-average sea surface temperatures in the Gulf of Alaska, associated with "the blob," and could be indicative of a change in nutrient levels. Around this time (2013-2014) is also when razor clam abundance dropped to a record low, and spurred the first clamming beach closure on the Kenai Peninsula. Ocean conditions may have a strong effect on clam growth and survival, as temperature is known to influence the timing of spawning and maturation rates. Sea surface temperatures also impact primary productivity, creating a potential temporal and spatial predator-prey mismatch.