Date

Summer 2025

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Coastal development and tourism have increased human-wildlife interactions, raising concerns for southern sea otters, Enhydra lutris nereis, an ecologically important and charismatic species that often rests in nearshore habitats near active harbors and tourist centers. This study examined the effects of human activity and group dynamics on rafting behaviors across three sea otter subpopulations with varying exposure to human stimuli. I hypothesized that sea otters would exhibit more resting and parental behaviors in low human activity areas and larger rafts, and that increased human activity within an area would lead to larger raft sizes, supporting the many-eyes hypothesis. Fieldwork involved bi-weekly scan surveys and focal follows of otter rafts across seven sites in Elkhorn Slough, Garrapata, and Morro Bay for one year (2020-2021 ). The scans recorded human stimuli within 30 m of any otter raft, a known disturbance threshold. Assumed site-specific chronic and acute human activity levels were confirmed using ANOV As. General Additive Mixed Models provided a flexible framework for assessing human impacts on rafting sea otter behavior and examining how these impacts were influenced by group size. Findings revealed regional differences in sea otter behavior and reactivity, influenced by stimulus type and the availability of protected areas. In Elkhorn Slough, where water-based human activity is high but nearby sheltered areas exist, otters selectively used protected sites. They also demonstrated increased tolerance to short-term increases in human activity when resting in sites with consistently high human presence. In contrast, Morro Bay otters, chronically exposed to high levels of both land- and water-based human activity with no nearby refuges, displayed tolerance up to a threshold, beyond which sensitive individuals left the raft. Across locations, larger raft sizes were correlated with increased resting, supporting the many-eyes hypothesis. These results highlight the importance of group size effects when examining the impacts of disturbance and the need for region-specific management strategies to mitigate human impacts. Further, our findings can be used to inform sea otter reintroduction efforts in human-impacted coastal zones, reinforcing the need for sea otters to have access to sheltered areas with low human activity.

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