Effects of oyster aquaculture on elasmobranch abundance and habitat use in Humboldt Bay, CA

Author

Rose HarmanFollow

Graduation Date

Fall 2023

Document Type

Thesis

Program

Master of Science degree with a major in Biology

Committee Chair Name

Paul Bourdeau

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Jose Marin Jarrin

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Eric Bjorkstedt

Third Committee Member Affiliation

HSU Faculty or Staff

Fourth Committee Member Name

Eve Robinson

Fourth Committee Member Affiliation

Community Member or Outside Professional

Keywords

Oyster aquaculture, Elasmobranchs, BRUVS, Humboldt Bay, Habitat use, Habitat structure, Habitat modification, Environmental stressors

Subject Categories

Biology

Abstract

Habitat modification from aquaculture can have large effects on natural communities, with the habitat complexity provided by aquaculture structure positively influencing benthic invertebrates and small fish abundance. However, the effects of aquaculture on larger predatory fish like elasmobranchs (i.e., sharks and rays), which use nearshore habitat to forage and provide top-down control of these ecosystems, is largely unknown. Over two years, I deployed baited remote underwater video systems (BRUVS) to examine the effects of oyster aquaculture and environmental variables such as habitat (mudflat or eelgrass), salinity (ppt), turbidity (m), temperature (°C), pH (mV), dissolved oxygen (mg · L^-1), and tidal height (m) on the presence and abundance of elasmobranchs and their main benthic prey (cancrid crabs) in Humboldt Bay. Humboldt Bay, in Northern California, is a large estuarine embayment that yields ~70% of California’s oysters and provides critical habitat for elasmobranchs and crabs. I found significant positive effects of aquaculture and warmer water on bat ray presence in both mudflat and eelgrass habitats. When bat rays were present, they were more abundant on mudflats during periods of high turbidity. In contrast, I found no effect of aquaculture on leopard shark presence, which was better predicted by increasing turbidity. Surprisingly, I also found no effect of aquaculture on abundance of cancrid crabs, however, aquaculture had interactive effects with crab abundance on bat ray presence, which was positively associated with crab abundance where aquaculture structure was present, but not when it was absent. Further, aquaculture also had interactive effects with bat ray presence on crab behavior, with crabs taking longer to visit BRUVS when bat rays were present in general, but more so when aquaculture was absent. Taken together, these results suggest that bat rays (1) may be attracted to aquaculture for increased feeding opportunities in a habitat that provides refuge from larger predators; and (2) that their presence affects crab activity. Based on these findings, I hypothesize that the increased presence of bat rays in and around aquaculture could intensify their top-down effects on crabs, through both consumptive and non-consumptive pathways, thus decreasing the abundance of crabs in aquaculture-modified habitats. Future field experiments should directly test the hypothesis that aquaculture presence increases bat ray consumptive and non-consumptive effects on crab prey in aquaculture-modified coastal habitats.

Citation Style

APA

Recommended Citation

Harman, Rose, "Effects of oyster aquaculture on elasmobranch abundance and habitat use in Humboldt Bay, CA" (2023). Cal Poly Humboldt theses and projects. 695.
https://digitalcommons.humboldt.edu/etd/695