Flow‐field observations of a tidally driven island wake used by marine mammals in the Bay of Fundy, Canada

Abstract

Correlations between fine-scale oceanographic features and aggregations of marine mammals are frequently reported, but the physical forces shaping these relationships are rarely explored. We conducted a series of oceanographic observations and remote sensing surveys of an oceanographic feature near Grand Manan Island known to attract marine mammals on flood tides. We tracked drift drogues from cliff-top with a theodolite and conducted boxtype surveys with an acoustic Doppler current profiler (ADCP) to assess flow patterns within the oceanographic feature. The feature was also visualized with Synthetic Aperture Radar (SAR) scenes. Drift drogues were advected towards a shear line originating near the northern tip of the island and entrained in one or more eddies downstream. ADCP surveys confirmed the presence of the shear line between rapid easterly flow and slower return flow. As the tide progressed, the shear line extended and manifested a single anti-cyclonic eddy at its distal end. As the flood tide progressed, northerly flow along the eastern shore of the island intensified and deflected the shear line northwards, shedding the eddy at slack high water. SAR images confirmed the presence of the shearline and eddy system, illustrating the evolution of a wake behind the island on flood tides. Profiles of flow direction and acoustic backscatter revealed secondary flows within the wake consistent with models and observations of other wakes. Oceanographic and remote sensing observations confirm that an island wake is generated by tidal flow past Grand Manan Island and provide an ecological context for the predictable aggregations of odontocete and mysticete cetaceans observed foraging within this region.

Fine-scale biophysical interactions drive prey availability at a migratory stopover site for Phalaropus spp. in the Bay of Fundy, Canada

“…We designed box transects to cover a variety of habitats over and beyond the ledges (Fig. 1), and repeatedly sampled the same locations through time to evaluate physical and biological parameters throughout the tidal cycle (as in Johnston & Read 2007). This allowed us to correlate zooplankton samples (detailed in the next section) with physical parameters relative to tidal cycle and bathymetric relief.…”

Biophysical characteristics of a morphologically-complex macrotidal tropical coastal system during a dry season

“…Regions inshore of reef and island matrices are isolated by varying degrees from adjoining offshore oceanic waters, potentially leading to localised physical and biogeochemical processes that can have a large influence on the local biological response (e.g., Furnas et al, 2005;Wyatt et al, 2012). Furthermore, the interaction of currents with island archipelagos and coastal headlands can create distinct frontal zones with converging and diverging flows, thereby influencing the distribution of sediment, phytoplankton and weakswimming nekton (e.g., Johnston and Read, 2007). The combined effect of these processes can lead to large variations in the function of planktonic ecosystems over small spatial areas.…”

The use of area–time closures as a tool to manage cetacean-watch tourism

“…Johnston and Read (2007) highlighted an ecological link between a predictable oceanographic feature in time and space that attracts cetaceans to the Bay of Fundy, Canada. In the summer and during flood tides, the Grand Manan Island wake attracts foraging fin whales (Balaenoptera physalus), minke whales (Balaenoptera acutorostrata) (Johnston et al, 2005a) and harbour porpoise (Phocoena phocoena) (Johnston et al, 2005b).…”

“…In the summer and during flood tides, the Grand Manan Island wake attracts foraging fin whales (Balaenoptera physalus), minke whales (Balaenoptera acutorostrata) (Johnston et al, 2005a) and harbour porpoise (Phocoena phocoena) (Johnston et al, 2005b). Oceanographic observations provided an understanding of the spatial and temporal variability in the physical forces controlling the island wake (Johnston & Read, 2007). Secondary flows in the wake aggregate prey to predictable locations where the cetaceans focus their foraging efforts.…”

“…These ecological links between the foraging habitats and foraging behaviour over space and time are therefore important factors when considering the spatial boundaries of a marine protected area in this region. Johnston and Read (2007) recommended that a proposed protected area at the Grand Manan Island encompass the island wake. The predictable nature of this oceanographic feature also allows human activities to be controlled within the area during the tidal flows that generate the island wake when mega fauna are foraging in the wake.…”