Seasonal patterns of coastal waterbird habitat use differ between floating and submerged oyster farms

Abstract

Oyster aquaculture is expanding rapidly in coastal ecosystems, introducing novel structures and human activity into habitats used by waterbird communities. However, few studies have evaluated how gear types and aquaculture operations influence waterbird presence, abundance, and behavior across seasons. We examined species- and guild-level responses of 31 coastal waterbird species to floating and submerged oyster gear and aquaculture-related disturbance across 47 estuarine sites in Rhode Island, USA, from 2020–2023. Floating gear arrays created elevated roosting platforms that were used by gulls (Larus spp.), terns (Sterna spp.), and cormorants (Nannopterum spp.) during post-breeding staging (4–15× higher odds of occurrence) and supported winter foraging for some diving species, while reducing Mareca americana (American Wigeon) abundance by ∼ 10-fold. In contrast, submerged gear reduced presence or abundance of geese (Branta and Anser spp.) and dabbling ducks (primarily Anas spp.), likely reflecting shading or obstruction of submerged vegetation and benthic foraging areas. Shorebirds and wading birds generally exhibited largely neutral responses, with notable exceptions: Ardea alba (Great Egret) and A. herodias (Great Blue Heron) were more likely to occur near floating gear but less likely near submerged gear, while Tringa semipalmata (Willet) showed reduced presence near submerged gear. Aquaculture-related disturbance had mixed effects: some waterfowl and cormorants were attracted to active farm operations, whereas Anas rubripes (American Black Duck) and T. semipalmata showed significant avoidance. Overall, the effects of oyster aquaculture on waterbirds are context dependent and mediated by gear type, siting, season, and species ecology. Floating gear often provides incidental habitat benefits, whereas submerged gear commonly constrains use by species dependent on shallow vegetation and benthic prey. Management strategies that avoid vegetated shallows, elevate or cluster gear, and align farm operations with sensitive periods can reduce conflicts, while maintaining meaningful habitat value for waterbirds.