Assessing commercial fishery bait in Dungeness crab Cancer magister feeding ecology: δ13C and δ15N stable isotope and gut content analysis

Abstract

Dungeness crabs Cancer magister support the largest fishery in Oregon (USA), which inputs millions of pounds of bait into the coastal ocean every year. We assessed evidence for seasonal consumption of commercial Dungeness crab fishery bait off the Oregon coast by that target species through δ13C and δ15N stable isotope and gut content analysis. Using Dungeness crab samples collected on the Oregon inner to mid-continental shelf, variation in δ13C and δ15N values were assessed according to sex, size class, region, and season using nested ANOVA, Euclidean vector statistics, and Bayesian standard ellipse areas corrected for sample size (SEAc) in R. The isotopic signatures of sampled crabs showed statistically significant variation by sex, region, and season; the differences in the isotopic niches of large male versus female crabs (carapace width ≥159 mm) were especially pronounced. δ15N in female crabs peaked in spring and declined toward fall with similar mean δ13C, which could be indicative of higher trophic level bait consumption in the winter when the fishery is the most active, while δ13C varied seasonally in males with similar mean δ15N. The approximated trophic niches (based on SEAc) of both male and female crabs were greatest in fall, suggesting broader foraging in the absence of bait inputs. This work provides a step towards understanding the ecological role of fishery activities on one of the most economically valuable species on the US west coast.