Confusion reigns? A review of marine megafauna interactions with tidal-stream environments

Energetic tidal-stream environments are characterised by frequent, variable yet broadly predictable currents containing ephemeral flow structures that change across multiple spatiotemporal scales. Marine mammals and seabirds (marine megafauna) often frequent such sites but increasingly these locations are targeted for renewable energy extraction; little is known however about how marine megafauna use these habitats and any potential impacts. This review aims to summarise existing knowledge concerning usage by marine megafauna and considers their wider ecological significance. The review describes the physical processes occurring within tidal-stream environments that generate the oceanographic structures of potential ecological relevance such as jets, boils, eddies and fronts. Important physical features of these environments include lateral transport, turbulence-driven 3-dimensional flow structure at various spatial scales, and upwelling. Foraging opportunities appear to be the main attractor to marine megafauna, likely driven by enhanced prey abundance, vulnerability and/or diversity. Many megafauna associate with particular tidal phases, current strengths and flow structures, likely in response to tidally-forced prey distribution and behaviours. Occupancy patterns, distributions and foraging behaviours are discussed. Local site fidelity by ‘tidal-stream experts’ suggest non-uniform conservation risks within larger metapopulations. The review discusses data gathering techniques and associated challenges, the significance of scaling and information gaps.