A comparison of baleen whale density estimates derived from overlapping satellite imagery and a shipborne survey

Abstract

As whales recover from commercial exploitation, they are increasing in abundance in habitats that they have been absent from for decades. However, studying the recovery and habitat use patterns of whales, particularly in remote and inaccessible regions, frequently poses logistical and economic challenges. Here we trial a new approach for measuring whale density in a remote area, using Very-High-Resolution WorldView-3 satellite imagery. This approach has capacity to provide sightings data to complement and assist traditional sightings surveys. We compare at-sea whale density estimates to estimates derived from satellite imagery collected at a similar time, and use suction-cup archival logger data to make an adjustment for surface availability. We demonstrate that satellite imagery can provide useful data on whale occurrence and density. Densities, when unadjusted for surface availability are shown to be considerably lower than those estimated by the ship survey. However, adjusted for surface availability and weather conditions (0.13 whales per km 2 , CV = 0.38), they fall within an order of magnitude of those derived by traditional line-transect estimates (0.33 whales per km 2 , CV = 0.09). Satellite surveys represent an exciting development for high-resolution image-based cetacean observation at sea, particularly in inaccessible regions, presenting opportunities for ongoing and future research. Gathering data on cetacean distribution and densities has traditionally employed visual observers operating from various platforms, typically either ships, aircraft or land 1-5. Much of our understanding about baleen whale population recovery and ecology depends on these methods 6-8. In oceanic regions close to population centres, these methods are often used to monitor regional population densities 8-10. However, regular applications of these methods are often constrained in remote, inaccessible regions, where their use represents a significant logistical and financial commitment 11. Consequently, such surveys are infrequent, making monitoring of population trends more challenging. In the Southern Ocean, the only comprehensive surveys south of 60° S (i.e. the putative summer foraging area for a range of cetacean species) were those undertaken by the International Whaling Commission (IWC) during the International Decade of Cetacean Research and the Southern Ocean Whale Ecosystem Research (IDCR SOWER) surveys, between 1978/9 and 2003/4. These surveys circumnavigated the continent three times, and based on these data Southern Ocean baleen whale recovery trends have been estimated 6,12,13. However, smallscale, sometimes ad hoc studies are far more common. These are generally biased towards the most accessible regions of the Southern Ocean 14 , the Western Antarctic Peninsula 3,4,15-18 , with more limited studies also conducted in the Scotia Arc 19 , Weddell Sea 20 and limited areas of East Antarctica 21,22. The Southern Ocean represents

Dynamic Species Distribution Models in the Marine Realm: Predicting Year-Round Habitat Suitability of Baleen Whales in the Southern Ocean

“…However, the availability of high-quality, unbiased data at appropriate spatial and temporal resolution is challenging in many situations, especially over large spatial scales and in remote regions (Rondinini et al, 2006;Hortal et al, 2015;Menegotto and Rangel, 2018). Particularly, this is evident for whales due to their imperfect detectability and the high logistic, environmental, and financial constraints (Kaschner et al, 2006;Bamford et al, 2020). Failing to obtain unbiased data can misdirect the globally limited conservation resources and compromise assessments of climate change's potential impacts on these species (Rondinini et al, 2006;Doney et al, 2012;Jewell et al, 2012).…”

“…The seasonal existence of sea ice hampers human access to most of the SO during winter and causes a high temporal bias in species data toward austral summer (Scheidat et al, 2011;El-Gabbas et al, 2021a). Whale distribution data in the SO are spatially biased toward repetitive navigational routes (e.g., to and from fixed research stations), easy-access and ice-free regions, or areas of particular interest (e.g., the Antarctic Peninsula) (Bombosch et al, 2014;Bamford et al, 2020;El-Gabbas et al, 2021a). Supplementary Figure 1 shows spatiotemporal biases in available baleen whale sightings in the SO.…”

Cetacean Strandings From Space: Challenges and Opportunities of Very High Resolution Satellites for the Remote Monitoring of Cetacean Mass Strandings

“…VHR satellites orbiting the Earth collect regular images across large, minimally populated/unpopulated and inaccessible areas (Abileah, 2002;Fretwell et al, 2014Fretwell et al, , 2019LaRue et al, 2017;Cubaynes et al, 2019;Bamford et al, 2020) and have the potential to significantly improve the monitoring of mass strandings in remote regions, particularly the priority areas identified in section "Data gaps and future risk areas for monitoring strandings" (Fretwell et al, 2019). Since satellite companies also regularly take images across the world, they also hold archives that can be used retrospectively to study past mass stranding events (Fretwell et al, 2019; Figure 2).…”

“…Another way in which satellite monitoring of strandings could be augmented is by the parallel satellite-based monitoring of whales at sea (Fretwell et al, 2014;Cubaynes et al, 2019), for example to monitor the densities of whales using the local area (Bamford et al, 2020) also in relation to local environmental conditions. This could provide important context to understanding the local oceanic conditions and their impact on local whale densities at the time of stranding.…”

Use of satellite imagery to identify southern right whales (Eubalaena australis) on a Southwest Atlantic Ocean breeding ground

“…Furthermore, this method has proven to be an invaluable tool for research and conservation (LaRue et al, 2017; Leimgruber et al, 2005; Turner et al, 2003). Satellite images have already been used as a tool for detecting large whales, with surveys using commercially available images being conducted for humpback whales ( Megaptera novaeangliae ) in Australia (Cubaynes et al, 2019), southern right whales in Argentina (Fretwell et al, 2014; Cubaynes et al, 2019) and Antarctica (Bamford et al, 2020), gray whales ( Eschrichtius robustus ) in Mexico, and fin whales ( Balaenoptera physalus ) in the Mediterranean Sea (Digital Globe, 2017).…”