Sea otter interactions with mariculture oyster farms

Abstract

Sea otters (Enhydra lutris) are considered a keystone species and can be found around mariculture oyster farms. While oyster farms and sea otters have been coexisting in some locations, sustaining these farms, and the growing interest in expanding them, necessitates a comprehensive evaluation of potential interactions. Here, sea otter interactions with oyster farms were assessed through behavioral observations (i.e., activity and foraging dives) in oyster farms, adjacent non-farm areas (controls), and bays with no farming activity (references). 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). This study hypothesized that sea otters preferentially use oyster farms for foraging and resting activities compared to non-farm areas. Contrary to our hypothesis, sea otter activities showed no significant difference in these behaviors between oyster farms and controls. Similarly, foraging behavior, including success and prey diversity did not significantly differ among the areas. The dominant prey items in our study included clams (e.g., Saxidomus gigantea), crabs (e.g., Telmessus cheiragonus, Cancer productus), and mussels (e.g., Mytilus trossulus). Notably, there were no observations of farm oysters being consumed by sea otters. There were significant 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), and the reference area nearly doubling to 4.8 prey items per dive (SD = 7.3). 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.