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Assessing commercial fishery bait in Dungeness crab (Cancer magister) feeding ecology:  EMBED Equation.DSMT4  and  EMBED Equation.DSMT4  stable isotope and gut content analysis

Toby Harbison*, Matthew Rogers, Sarah Henkel

*Corresponding author:

ABSTRACT: Dungeness crabs support Oregon’s largest fishery, which inputs millions of pounds of bait into the coastal ocean every year. This paper assesses evidence for seasonal consumption of commercial Dungeness crab (Cancer magister) fishery bait off the Oregon coast by that target species through δ13C and δ15N stable isotope and gut content analysis. Trophic ecologists commonly use δ13C and δ15N isotope ratios in consumer tissues to assess prey provenance and trophic level. Using Dungeness crab samples collected on Oregon’s inner to mid-continental shelf, variation in δ13C and δ15N values are assessed according to sex, size class, region, and season using nested analysis of variance (ANOVA), Euclidean vector statistics, and Bayesian standard ellipse areas (SEAc) in R. The isotopic signatures of sampled crabs show statistically significant variation by sex, region, and season; the differences in the isotopic niches of large male versus female crabs (carapace width ≥ 159mm) are especially pronounced. δ15N in female crabs peak in spring and decline to 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 vary seasonally in males with similar mean δ15N. The approximated trophic niches (using 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 west coast of the United States.