Graduate Thesis Or Dissertation

Assessing the Impact of Fishery Bait on Spatiotemporal Variation in the Feeding Ecology of Dungeness Crab (Cancer magister) Using Stable Isotopes, Gut Content Analysis & Fishermen Interviews

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  • The commercial Dungeness crab (Cancer magister) fishery is Oregon’s largest trap-based fishery. Each year, crab vessels discard millions of pounds of bait, including squid, razor clams, and sardines, into the near-coastal ocean. Although the fishery season typically runs from December 1st through August 14th, most landings occur within the first six weeks of the open season. Regional and seasonal variation in Dungeness crabs’ δ13C and δ15N stable isotope (SI) composition and gut contents across multiple sex and size classes were assessed to investigate the potential consumption of discarded bait by the crabs. Stable isotope mixing models were employed to approximate possible spatiotemporal variability in bait contribution to Dungeness crab diets. Additionally, fishermen interviews were conducted to determine the bait species utilized in the fishery as well as the regional and seasonal distribution of bait inputs into the marine system. The δ13C and δ15N signature of crabs collected during four seasons (December 2019, March-April 2020, June-July 2020, and October 2020) across three regional study areas showed highly significant regional and seasonal variation. Tissues of female crabs sampled in the springtime across multiple sites contained increased δ15N ratios compared to all other seasons, which could be indicative of trophic enrichment through bait consumption. Fishermen interviews indicated that the types and amounts of bait used vary over the course of the commercial fishery season, but not substantially between regions. Overall, the results demonstrated the complex and dynamic nature of coastal food webs and the importance of considering multiple habitats over time when evaluating trophic relationships.
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