Sea otter interactions with mariculture oyster farms

Abstract

While oyster farms and sea otters (Enhydra lutris) coexist in some locations, sustaining these farms and growing interest in their expansion necessitate a comprehensive evaluation of potential interactions. We assessed sea otter interactions with oyster farms through behavioral observations of otters in oyster farms, adjacent non-farm areas (controls), or bays with no farming activity (references) in southcentral Alaska in 2022–2023. Behavioral observations, conducted through scan surveys, captured sea otter activities (e.g., resting, grooming, swimming, and foraging). Targeted foraging observations tracked foraging success and prey (species and count). We hypothesized that sea otters preferentially used oyster farms for foraging and resting activities compared to non-farm areas. Contrary to our hypothesis, sea otter activities showed no significant difference in foraging or resting behaviors between oyster farms and controls. Similarly, foraging behavior, including success and prey diversity, did not differ among areas. The dominant prey items in our study included clams (e.g., butter clam [Saxidomus gigantea]), crabs (e.g., helmet crab [Telmessus cheiragonus], red rock crab [Cancer productus]), and mussels (e.g., Pacific blue mussel [Mytilus trossulus]). Notably, there were no observations of farmed oysters being consumed by sea otters. There was strong evidence of differences in the average number of prey consumed per sea otter per dive between the control and reference areas, with the control area averaging 2.6 prey items per dive (SD = 3.6) compared to 4.8 prey items per dive (SD = 7.3) in the reference area. These differences may be attributed to variations in prey biomass and environmental conditions. Our observations indicate that there are no discernible differences in overall sea otter activity or foraging behavior in the presence of oyster farms.