Assessing drivers of habitat use by dolphins at dynamic oceanographic features to inform conservation management

Estuaries host important populations of marine megafauna. These environments provide shelter from the open ocean, alongside a myriad of consistent and predictable bathy-hydographic features linked to interactions between topography and tidal flows, where foraging opportunities may be enhanced. In tandem, anthropogenic activity (e.g. marine renewable energy installation and shipping) is oftentimes concentrated around estuaries, highlighting a need for informed management. One approach is to identify and conserve habitats used for essential purposes, such as feeding, using protected areas. In estuaries, this requires an in-depth understanding of how bathy-hydrographic features function, including knowledge on persistence and predictability alongside associated patterns in animal attendance, something that is currently missing, particularly over longer temporal scales. Here, using a resident population of bottlenose dolphins (Tursiops truncatus) in the Shannon Estuary (W. Ireland) as a model case study, we combine boat-based sightings data and oceanographic surveying to (1) assess dolphin distributions across the estuary and links between high-use habitats and local bathy-hydrographic conditions, (2) characterise the fine-scale hydrography of one of these high-use areas, and (3) determine the persistence and predictability of patterns in habitat use across the estuary over decadal scales. High-use habitats by dolphins coincided with bathymetrically complex locations throughout the estuary, and associations were tidally dependent. Oceanographic surveying at one of these sites indicated the presence of strong downwelling during flood tides when dolphin presence peaked, which may aid dolphin foraging. High-use habitats were generally used consistently through time, with populations surveyed a decade later displaying similar patterns in habitat use, although limitations were noted. Together our results highlight the importance of bathy-hydrographic features to a marine predator in estuarine environments and support their prioritisation for protection. We propose that adaptive management be used to safeguard the tidally driven use of the Shannon Estuary by dolphins, whilst wider ecosystem-based measures are implemented to protect the functioning of these important habitats. Growing evidence suggests that within coastal regions, bathy-hydrographic features are important to a range of marine megafauna species. We demonstrate that linking detailed data on bathymetry and dynamic oceanography to animal movement patterns provides a fine-scale understanding of long-term site use by mobile predators, which can facilitate effective decision making and marine planning. Data-sets of bottlenose dolphin presence across the Shannon Estuary, encoded as 0/1 along an interpolated survey track at a resolution of 2 minutes are included here for (1) 1997-2008 and (2) 2015 and 2018. Spatio-temporally matched data on bathymetric depth and slope, and tidal conditions (phase in the tidal cycle, tidal height, phase in the spring neap cycle) are also included. The data matches methods described in the referenced manuscript and associated supplementary materials. The reuse of this data requires permission from Simon Ingram ([e-mail]) and/or Emer Rogan ([e-mail]).