Abstract
Blue whalesBalaenoptera musculusat South Georgia were heavily exploited during 20thcentury industrial whaling, to the point of local near-extirpation. Although legal whaling for blue whales ceased in the 1960s, and there were indications of blue whale recovery across the wider Southern Ocean area, blue whales were seldom seen in South Georgia waters in subsequent years. We collated 30 yr of data comprising opportunistic sightings, systematic visual and acoustic surveys and photo-identification to assess the current distribution of blue whales in the waters surrounding South Georgia. Over 34000 km of systematic survey data between 1998 and 2018 resulted in only a single blue whale sighting, although opportunistic sightings were reported over that time period. However, since 2018 there have been increases in both sightings of blue whales and detections of their vocalisations. A survey in 2020 comprising visual line transect surveys and directional frequency analysis and recording (DIFAR) sonobuoy deployments resulted in 58 blue whale sightings from 2430 km of visual effort, including the photo-identification of 23 individual blue whales. Blue whale vocalisations were detected on all 31 sonobuoys deployed (114 h). In total, 41 blue whales were photo-identified from South Georgia between 2011 and 2020, none of which matched the 517 whales in the current Antarctic catalogue. These recent data suggest that blue whales have started to return to South Georgia waters, but continued visual and acoustic surveys are required to monitor any future changes in their distribution and abundance.
Highlights
Before the onset of industrial whaling in the Southern Ocean, there was perhaps nowhere in the world more densely populated with large whales than South Georgia (Richardson et al 2012)
When industrial whaling began in the early 20th century, South Georgia was the first place in the Southern Ocean to be targeted
We examined blue whale population identity around South Georgia, considering evidence of movement patterns provided by photo-identification and Discovery mark−recaptures and evaluated whether blue whales may be recovering around South Georgia
Summary
Before the onset of industrial whaling in the Southern Ocean, there was perhaps nowhere in the world more densely populated with large whales than South Georgia (Richardson et al 2012). South Georgia was part of a hemisphere-wide nearextirpation of Antarctic blue whales, so repopulation from a wider area after whaling ceased could not readily occur; the few decades of industrial whaling in the Southern Ocean had reduced Antarctic blue whales to 0.15% of their estimated pre-exploitation abundance (Branch et al 2004). Marine mammal population recoveries from exploitation that resulted in local extinction have been patchy and influenced by a number of factors (Clapham et al 2008) These include the abundance and demographics of a remnant and/or surrounding population, and the continued presence or introduction of other anthropogenic threats such as ongoing deliberate killing, fisheries bycatch, ship strikes, reduced food availability and pollution, or several of these factors combined (Corkeron et al 2018). We examined blue whale population identity around South Georgia, considering evidence of movement patterns provided by photo-identification and Discovery mark−recaptures (see Section 2.4) and evaluated whether blue whales may be recovering around South Georgia
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