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Sharks surf the slope: current updrafts reduce energy expenditure for aggregating marine predators

Citation

Papastamatiou, Yannis (2021), Sharks surf the slope: current updrafts reduce energy expenditure for aggregating marine predators, Dryad, Dataset, https://doi.org/10.5061/dryad.00000003q

Abstract

An animal’s energy landscape considers the power requirements associated with residing or moving through habitats. Within marine environments, these landscapes can be dynamic, as water currents will influence animal power requirements and can change rapidly over diel and tidal cycles.

In channels and along slopes with strong currents, updraft zones may reduce energy expenditure of negatively buoyant fishes that are also obligate swimmers. Despite marine predators often residing within high-current area, no study has investigated the potential role of the energetic landscape in driving such habitat selectivity.

Over 500 grey reef sharks (Carcharhinus amblyrhinchos) reside in the southern channel of Fakarava Atoll, French Polynesia. We used diver observations, acoustic telemetry, and biologging to show that sharks use regions of predicted updrafts and switch their core area of space use based on tidal state (incoming vs outgoing).

During incoming tides, sharks form tight groups and display shuttling behavior (moving to the front of the group and letting the current move them to the back) to maintain themselves in these potential updraft zones. During outgoing tides, group dispersion increases, swimming depths decrease, and shuttling behaviors cease. These changes are likely due to shifts in the nature and location of the updraft zones, as well as turbulence during outgoing tides. Using a biomechanical model, we estimate that routine metabolic rates for sharks may be reduced by 10-15 % when in updraft zones.

Grey reef sharks save energy by using predicted updraft zones in channels and ‘surfing the slope’. Analogous to birds using wind-driven updraft zones, negatively buoyant marine animals may use current-induced updraft zones to reduce energy expenditure. Updrafts should be incorporated into dynamic energy landscapes and may partially explain the distribution, behavior, and potentially abundance of marine predators.

Methods

Sharks were tagged with acoustic transmitters which are detected by fixed listening stations. Tags include sensors that also measure swimming depth and animal activity. Data has been separated out based on tidal state.