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In this paper, we use the term “super-group” to describe groups of 20 or more individual humpback whales estimated to be within five body lengths of their nearest neighbour. Such observations are novel in that prior observations of feeding by humpback whales in the region (e.g. [42–44]) were of loose aggregations of small groups of whales (of up to 20 individuals). These records of the “super-groups” of humpback whales within the southern Benguela region arise from two sources, namely primary observations from dedicated research cruises, and incidental observations reported by the general public. Three dedicated research cruises were carried out in coastal waters between 32°20’S and 34°20’S off the south-western Cape region of South Africa (Fig 1) in late October–early November of 2011, 2014 and 2015 and focussed on identifying the distribution, relative abundance and movements of migratory large whales and other cetacean fauna in relation to biotic and abiotic factors (including upwelling areas) and the trophic ecology of large whales within this coastal area. The 2011 cruise was conducted from 10 November to 20 November, the 2014 cruise from 28 October to 8 November, while the 2015 cruise was conducted from 29 October to 7 November. The 2011 cruise was conducted aboard the South African Department of Environmental Affairs’ research vessel RV Algoa, while both of the 2014 and 2015 cruises were carried out aboard the South African Department of Agriculture Forestry and Fisheries research vessel the FRS Ellen Khuzwayo (hereafter both vessels are referred to as “mothership” below). During 2014 and 2015 concurrent oceanographic cruises (to those carried out on the FRS Ellen Khuzwayo) were carried out in the region aboard the RV Algoa. Due to limitations in the launching and recovery of rigid hull inflatable boats (RHIBs) from the FRS Ellen Khuzwayo in the prevailing south-westerly swell conditions during 2014 and 2015 this vessel generally anchored in sheltered conditions at night with the overnight position defining the area of search effort the following day. Searching for whales was generally initiated each morning from the bridge or monkey island of the mothership en route to the 100m isobath (some 5 n. miles offshore) and positions of whale groups were communicated by radio to one or two RHIBs following astern or parallel to the FRS Ellen Khuzwayo if conditions for RHIB operations allowed. Once approachable groups of whales were sighted, research personnel were transferred to the RHIBs for approach of whale groups, and RHIBs could be directed from whale group to whale group by the FRS Ellen Khuzwayo, until late afternoon when personnel were transferred back to the ship prior to seeking a sheltered anchorage or providing a lee, if swell conditions allowed for the recovery of the RHIBs. In 2011, the RHIBs were deployed from the RV Algoa at sea only once the first approachable whale group was sighted, and thereafter travelled astern or parallel to the mothership between whale groups. Where weather conditions precluded RHIB operations, whale groups were approached by the mothership. All approaches to whale groups and all research activities reported in this study were carried out under research permits (numbers RES2011/70, RES2014/61, RES2015/94) granted to Mammal Research Institute, University of Pretoria by the South African Department of Environmental Affairs and research permit (number RES2015/DEA) granted to the South African Department of Environmental Affairs by the Department of Agriculture, Forestry and Fisheries under provisions of the South African Marine Living Resources Act (Act 18 of 1998) and under ethics permits of the University of Pretoria, South Africa (Reference numbers—EC020-12 and EC061-15).
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0172002.g001 Locations of the Study Region and dedicated (open circles) and incidental observations (open squares) of feeding “super-groups” off the south west Cape coast of South Africa.The 200m isobath is shown. The inset expands the area shaded in grey.
Due to the tight spacing and repeated diving behaviour of whales within “super-groups” (at times less than 2–3 m apart) and the high risk of entangling whales within sampling gear, no plankton sampling could be carried out from the RHIBs in the near vicinity of these groups. Group sizes of dedicated sightings were estimated independently by experienced observers as upper, lower and best estimates during the close approaches of the group, either by RHIBs or by the mothership so that group size estimates were not carried out at distance and consequently not influenced by Beaufort Scale or Sea State. Best estimates of group size were not necessarily the mean of the overall upper and lower estimates as independent estimates were made by all of the personnel observing the group and the final estimated group size was based on consensus of these individual estimates. The estimates of the sizes of “super-groups” pertain only to those individuals within five body lengths of the nearest neighbour at the time of the largest aggregation during the encounter. Fluidity of the associations within groups and movement of identified individuals between groups within aggregations (including in and out of “super-groups”) was difficult to determine in the field, due to the repetitive deep dive behaviour of the individual whales, which made the determination of group sizes difficult. Where such difficulty precluded accurate group size estimation, the lowest estimate was selected. Incidental observations of “super-groups” were reported by the general public from aircraft during 2015 including during two sightseeing flights made by DH between Koeberg and St Helena Bay (including offshore of Dassen Island) on 19 October and 26 October 2015. These two flights were carried out at over 300m above sea level in a Cessna 185 aircraft. All of the reported incidental observations were verified from photographs. Group size estimates of the incidental sightings were estimated by the observers who recorded these sightings. Although other reports were received from boat-based and shore-based observers these have been excluded from the database as they were not accompanied by adequate photographic verification.
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