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.

Localised anthropogenic wake generates a predictable foraging hotspot for top predators

“…The latter mechanism is currently the least explored in a natural setting despite its importance in high-flow environments, highlighting the relevance of our findings. While natural bathymetric features and associated patterns of shear lines and wake effects have been shown to attract top predators 8 , the man-made wake in this study promoted the most persistent and intense foraging aggregations of terns among all wake features investigated. While we did not assess prey vertical distribution, turbulent vertical velocity fluctuations within the wake were greater than 0.5 ms −1 (Supplementary Fig.…”

Localised anthropogenic wake generates a predictable foraging hotspot for top predators

“…The latter mechanism is currently the least explored in a natural setting despite its importance in high-flow environments, highlighting the relevance of our findings. While natural bathymetric features and associated patterns of shear lines and wake effects have been shown to attract top predators 8 , the man-made wake in this study promoted the most persistent and intense foraging aggregations of terns among all wake features investigated. While we did not assess prey vertical distribution, turbulent vertical velocity fluctuations within the wake were greater than 0.5 ms −1 (Supplementary Fig.…”

Seasonal distribution of an opportunistic apex predator (Tursiops truncatus) in marine coastal habitats of the Western Mediterranean Sea

“…Distribution of the expected number of sightings over the whole area during summer (red) and winter (blue). peculiar, fixed features of the study area such as islands, headlands and seamounts may have played a role in the constant distributional pattern over seasons here observed, as they are known to permanently generate eddies, fronts and water masses circulation, enhancing the amount of available nutrients and aggregating different species able to attract apex predators (Johnston and Read, 2007;Bailey and Thompson, 2010;Dinis et al, 2016). Bathymetry is considered a proxy for prey availability indirectly linked to the common bottlenose dolphin distribution and habitat selection as well (Marini et al, 2015;Gnone et al, 2022).…”

Tere Tohorā, Karanga Tāngata: Weaving Māori Knowledge With Conventional Science to Characterise a Biodiversity Hotspot for Marine Megafauna in an Area Facing Multiple Anthropogenic Impacts

“…This may occur via aggregation (e.g., entrainment) or via increases in productivity via upwelling events or oceanographic features such as fronts, eddies or island wakes (Owen 1981 ; Johnston et al. 2005 ; Johnston and Read 2007 ), that are known to occur along this coast (Stevens et al. 2021 ).…”

“…The high use of the area by wide-ranging species (e.g., mautai/false killer whales, ūpokohue/pilot whales, tohorā/ Bryde's whale) suggests there are persistent ecological features that may enhance prey availability and draw these species to the study area (Johnston et al 2005;Hazen et al 2011;Brough et al 2023). This may occur via aggregation (e.g., entrainment) or via increases in productivity via upwelling events or oceanographic features such as fronts, eddies or island wakes (Owen 1981;Johnston et al 2005;Johnston and Read 2007), that are known to occur along this coast (Stevens et al 2021). The ability for Māori knowledge to provide a holistic understanding of 'why' this area may be important for the marine megafauna further showcases the value of considering dual knowledge systems for baseline biodiversity studies, a finding that has been shown in similar studies elsewhere (Stern and Humphries 2022;Gryba et al 2025).…”

Localised anthropogenic wake generates a predictable foraging hotspot for top predators

“…The latter mechanism is currently the least explored in a natural setting despite its importance in high-flow environments, highlighting the relevance of our findings. While natural bathymetric features and associated patterns of shear lines and wake effects have been shown to attract top predators 8 , the man-made wake in this study promoted the most persistent and intense foraging aggregations of terns among all wake features investigated. While we did not assess prey vertical distribution, turbulent vertical velocity fluctuations within the wake were greater than 0.5 ms −1 (Supplementary Fig.…”

Seasonal distribution of an opportunistic apex predator (Tursiops truncatus) in marine coastal habitats of the Western Mediterranean Sea

“…Distribution of the expected number of sightings over the whole area during summer (red) and winter (blue). peculiar, fixed features of the study area such as islands, headlands and seamounts may have played a role in the constant distributional pattern over seasons here observed, as they are known to permanently generate eddies, fronts and water masses circulation, enhancing the amount of available nutrients and aggregating different species able to attract apex predators (Johnston and Read, 2007;Bailey and Thompson, 2010;Dinis et al, 2016). Bathymetry is considered a proxy for prey availability indirectly linked to the common bottlenose dolphin distribution and habitat selection as well (Marini et al, 2015;Gnone et al, 2022).…”

Tere Tohorā, Karanga Tāngata: Weaving Māori Knowledge With Conventional Science to Characterise a Biodiversity Hotspot for Marine Megafauna in an Area Facing Multiple Anthropogenic Impacts

“…This may occur via aggregation (e.g., entrainment) or via increases in productivity via upwelling events or oceanographic features such as fronts, eddies or island wakes (Owen 1981 ; Johnston et al. 2005 ; Johnston and Read 2007 ), that are known to occur along this coast (Stevens et al. 2021 ).…”

“…The high use of the area by wide-ranging species (e.g., mautai/false killer whales, ūpokohue/pilot whales, tohorā/ Bryde's whale) suggests there are persistent ecological features that may enhance prey availability and draw these species to the study area (Johnston et al 2005;Hazen et al 2011;Brough et al 2023). This may occur via aggregation (e.g., entrainment) or via increases in productivity via upwelling events or oceanographic features such as fronts, eddies or island wakes (Owen 1981;Johnston et al 2005;Johnston and Read 2007), that are known to occur along this coast (Stevens et al 2021). The ability for Māori knowledge to provide a holistic understanding of 'why' this area may be important for the marine megafauna further showcases the value of considering dual knowledge systems for baseline biodiversity studies, a finding that has been shown in similar studies elsewhere (Stern and Humphries 2022;Gryba et al 2025).…”

Localised anthropogenic wake generates a predictable foraging hotspot for top predators

“…The latter mechanism is currently the least explored in a natural setting despite its importance in high-flow environments, highlighting the relevance of our findings. While natural bathymetric features and associated patterns of shear lines and wake effects have been shown to attract top predators 8 , the man-made wake in this study promoted the most persistent and intense foraging aggregations of terns among all wake features investigated. While we did not assess prey vertical distribution, turbulent vertical velocity fluctuations within the wake were greater than 0.5 ms −1 (Supplementary Fig.…”

Seasonal distribution of an opportunistic apex predator (Tursiops truncatus) in marine coastal habitats of the Western Mediterranean Sea

“…Distribution of the expected number of sightings over the whole area during summer (red) and winter (blue). peculiar, fixed features of the study area such as islands, headlands and seamounts may have played a role in the constant distributional pattern over seasons here observed, as they are known to permanently generate eddies, fronts and water masses circulation, enhancing the amount of available nutrients and aggregating different species able to attract apex predators (Johnston and Read, 2007;Bailey and Thompson, 2010;Dinis et al, 2016). Bathymetry is considered a proxy for prey availability indirectly linked to the common bottlenose dolphin distribution and habitat selection as well (Marini et al, 2015;Gnone et al, 2022).…”

Tere Tohorā, Karanga Tāngata: Weaving Māori Knowledge With Conventional Science to Characterise a Biodiversity Hotspot for Marine Megafauna in an Area Facing Multiple Anthropogenic Impacts

“…This may occur via aggregation (e.g., entrainment) or via increases in productivity via upwelling events or oceanographic features such as fronts, eddies or island wakes (Owen 1981 ; Johnston et al. 2005 ; Johnston and Read 2007 ), that are known to occur along this coast (Stevens et al. 2021 ).…”

“…The high use of the area by wide-ranging species (e.g., mautai/false killer whales, ūpokohue/pilot whales, tohorā/ Bryde's whale) suggests there are persistent ecological features that may enhance prey availability and draw these species to the study area (Johnston et al 2005;Hazen et al 2011;Brough et al 2023). This may occur via aggregation (e.g., entrainment) or via increases in productivity via upwelling events or oceanographic features such as fronts, eddies or island wakes (Owen 1981;Johnston et al 2005;Johnston and Read 2007), that are known to occur along this coast (Stevens et al 2021). The ability for Māori knowledge to provide a holistic understanding of 'why' this area may be important for the marine megafauna further showcases the value of considering dual knowledge systems for baseline biodiversity studies, a finding that has been shown in similar studies elsewhere (Stern and Humphries 2022;Gryba et al 2025).…”