South Georgia blue whales five decades after the end of whaling

Abstract

Blue whales Balaenoptera musculus at South Georgia were heavily exploited during 20th century 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.

Developing UAV Monitoring of South Georgia and the South Sandwich Islands’ Iconic Land-Based Marine Predators

“…(Table 1) given the abundance, diversity and widespread distribution of marine predators at South Georgia (Trathan et al, 1996) we have now initiated UAV aerial monitoring at key locations to supplement the existing monitoring, which was limited to colonies proximal to scientific stations. Here, we highlight three case studies to demonstrate the efficacy of UAV surveys, which we now consider to be essential, given the recent eradication of introduced mammalian species (Bazilchuk, 2013;Martin and Richardson, 2019), ongoing glacial retreat (Cook et al, 2010), and changing foraging opportunities at sea given the recovery of cetaceans (Zerbini et al, 2019;Calderan et al, 2020). Changes in the local ecosystem are now certain, but ascribing cause remains difficult.…”

Cetaceans Sightings During Research Cruises in Three Remote Atlantic British Overseas Territories

“…This may be due to the different focus of these surveys between coastal versus offshore waters (e.g., Hedley et al, 2001). The frequencies of species sightings have changed over time around South Georgia (Richardson et al, 2012;Calderan et al, 2020) with humpback whales becoming increasingly common in recent years (Jackson et al, 2020). Previously a very rare sighting, blue whales have been seen more frequently since 2018 (Richardson et al, 2012;Calderan et al, 2020).…”

“…The frequencies of species sightings have changed over time around South Georgia (Richardson et al, 2012;Calderan et al, 2020) with humpback whales becoming increasingly common in recent years (Jackson et al, 2020). Previously a very rare sighting, blue whales have been seen more frequently since 2018 (Richardson et al, 2012;Calderan et al, 2020). Both 2019 surveys, the DY98 survey in January/February (Baines et al, 2019) and the DY99 survey in February/March, recorded humpback whales as the most frequent sighting, not only across South Georgia, but also across the South Sandwich Islands as well.…”

“…Dedicated surveys of cetaceans in the waters surrounding these three British Overseas Territories have been relatively limited, although they have increased in recent years, particularly around South Georgia (see Calderan et al, 2020). Thus these opportunistic data, during marine ecosystem research cruises, provide valuable information on the distribution of cetaceans in these territories and can help inform about the trajectories of whale populations, in particular, where future studies should focus.…”

“…Despite their different climate zones, all three territories are characterized by deep oceans with steep-sided volcanic seamounts and guyots rising from the deep ocean floor (Makler and de Matos Mello, 2007;Geissler et al, 2020); only the island of South Georgia has a significant area of shelf shallower than 700m deep distinguishing it from the rest of SGSSI (Hogg et al, 2016) and the Atlantic territories. The seamounts and shelf breaks provide upwelling areas of productivity, supporting a rich food chain that includes many cetacean species (MacLeod and Bennett, 2007;Rossi-Santos et al, 2007;Best et al, 2009;Calderan et al, 2020;Jackson et al, 2020). An understanding of which areas within the exclusive economic zones are key regions for biodiversity, the "biodiversity hotspots" (Hogg et al, 2016;Requena et al, 2020), is required to inform management decisions.…”