Status
Global
Habitat
Ecosystem and cultural services
Bryde’s Whales are presumably able to satisfy their nutritional and reproductive needs within their warm, temperate distribution, freeing them from the need to make extensive latitudinal migrations (Bannister 2002). Although pelagic populations (e.g., the SE Atlantic population) undertake limited migrations towards the equator in winter and higher latitudes in summer, coastal populations do not migrate as such and their movements are primarily alongshore, most likely governed by the distribution of their prey. Year round occurrence has been reported from the coastal areas of south-western Japan, south eastern Brazil and South Africa. Bryde’s Whales feed at a constant and high rate throughout the year (Best 1967) and feeding events commonly involve multi-species aggregations (Best 2007, Penry et al. 2011). The South African inshore population is dependent on year round prey availability and feed predominantly on small pelagic fish (e.g., anchovy and pilchard).
The South African inshore population is resident over the Agulhas Bank (Best 2007) but shows a seasonal shift in distribution, with the majority of sightings on the southeast coast of South Africa, between Cape Agulhas and East London in the southern hemisphere summer and autumn (Best 2007, Penry et al. 2011). Off South Africa, aggregation size of Bryde’s Whales was positively correlated with the occurrence of feeding behaviour. During winter there are increased sightings of Bryde’s Whales further north along the East Coast, frequently in groups with Common Dolphins (Delphinus capensis) and Cape Gannets (Morus capensis) (Best et al. 1984, Best 2001, O’Donoghue et al. 2010). This appears to coincide with the annual north-eastward migration of sardines into KwaZulu-Natal waters (Fréon et al. 2010, Penry et al. 2011). In general, Bryde’s Whales do not display reproductive seasonality. However this is more apparent in offshore/pelagic populations. Penry et al. (2011) found no seasonality in the occurrence of calves off South Africa. The inshore form is polyoestrous and has a high frequency of ovulation, possibly due to the year round abundance of food (Best 2007). The period for lactation is not known.
The offshore form lives on the edge of the continental shelf and migrates seasonally between the equator in the southern hemisphere winter and about 34°S in summer (Best 1996). Breeding can occur year-round but is more seasonal than the inshore form, peaking in autumn/early winter. The form is also less piscivorous than the inshore form, with euphausiids being regularly consumed and the fish eaten being more mesopelagic in nature: there also seems to be a marked seasonality in prey type, with euphausiids being relatively unimportant in autumn and winter but predominating in summer (Best 2001). Because of its year-round presence in temperate and tropical waters this population is subjected to an unusually high level of attack by what are believed to be cookie-cutter Sharks (Isistius spp.), to such an extent that the scars eventually obliterate the whale’s natural pigmentation over large areas of the flanks and belly (Best 1977).
Ecosystem and cultural services: Marine mammals integrate and reflect ecological variation across large spatial and long temporal scales, and therefore they are prime sentinels of marine ecosystem change; migratory mysticete whales may be used to investigate broadscale shifts in ecosystems (Moore 2008). The inshore Bryde’s population is the largest, resident predator in South African coastal waters. They are principally dependent on our small pelagic fish stocks for their prey, and suitable/safe habitat for breeding. Unlike most other large baleen whales that migrate between disparate feeding and breeding areas, the South African inshore form may have largely sympatric feeding and breeding areas with its range. Ecosystem services include revenue through tourism, food web stabilization and ecosystem indicators.
Population Trend
Recent data on the inshore form (Penry 2010) suggest that the population size is small: closed population = 125 individuals (95% CI 107-155, CV = 0.09); open population individuals = 196 (95% CI 117–437, CV 0.37). However, the actual estimates are based on mark-recapture data collected in the coastal waters in and around Plettenberg Bay on the south coast (Penry 2010). The surveys covered a distance of approximately 55 km along the coast and ranged up to 10 km offshore. Their applicability to the total population is thus unknown. Additionally, molecular analyses suggest that the apparently resident South African inshore form is significantly differentiated from the seasonal offshore form, which is thought to occur at higher population numbers (Best 1996).
Between 1911 and 1967, more than 2,000 Bryde’s Whales were documented as caught by whaling operations off the Cape region of South Africa. This included 1,300 individuals caught between 1947 and 1967 (IWC 2006a), which are expected to have been from the inshore population of Bryde’s Whales (IWC 1980a, 1980b). Subsequently, in 1983, the South African inshore population was approximated at 582 ± 184 individuals (Best et al. 1984). Unfortunately, no population estimates are currently available for the rest of the South Atlantic, however, between 1969 and 1976 the majority of the 2,536 “Sei” Whales caught by the pirate whaling ship Sierra in the South Atlantic are, in fact, assumed to be Bryde’s Whales from the offshore stock (IWC 1980b; Best 1996). This suggests that the offshore population was probably more abundant than the inshore form.
Bryde’s Whales were not consistently distinguished from Sei Whales in International Whaling Statistics, but in some cases a breakdown of “Sei” Whale catches into the two species can be determined from original records, or approximated, based on current knowledge of the geographical and seasonal occurrence of Sei and Bryde’s Whales, or from the compositions of later catches in the same area and season (IWC 1997, 2006b). Population estimates of Bryde’s Whales in the southern hemisphere are in urgent need of reassessment. Historic estimates of abundance revealed the following approximations: in the southern Indian Ocean – 13,854; in the western South Pacific – 16,585; and in the eastern South Pacific – 13,194 (IWC 1981). These estimations were not founded on recently approved survey methods, and consequently a “zero catch limit” was set by the IWC f
Threats
Both inshore and offshore stocks were subjected to some whaling in the past, but no specific estimates of depletion exist. The Bryde’s Whale was the incidental beneficiary of IWC area restrictions on factory ship whaling that were originally designed to protect the low-latitude winter breeding grounds of other baleen whale species (Tønnessen and Johnsen 1982). This benefit was lost to the offshore population in 1969, when an illegal and unregulated pelagic operation commenced whaling in the southeast Atlantic (Best 1996). No direct takes are known in the area since 1976.
Like most cetaceans, Bryde’s Whales are occasionally by-caught in fishing gear, but they do not appear to be especially susceptible within South African waters. In 2014, a Bryde’s Whale (presumably from the inshore population) was entangled and subsequently died in experimental octopus fishery lines in Plettenberg Bay, and a second in June 2016 in False Bay in the same fishery gear (G. Penry unpubl. data). The possible expansion of this fishery is a cause for concern along the South African coastline. Records of vessel strikes are also rare in South Africa. The inshore form potentially competes with pelagic fisheries (sardine, anchovies) (sensu Clapham et al. 1999). Given the suspected effects of seismic surveys on fish (Gordon et al. 2003, Koper and Plön 2012), it is possible (but completely unsupported by any data) that prey availability to this population may have been adversely affected. Disturbances from ecotourism ventures during the “Sardine Run”, while Bryde’s Whales are feeding on bait balls (roughly spherical, tightly packed formations of fish), have also been reported. Although increased water temperatures may be favourable for Bryde’s Whales, which prefer subtropical and tropic waters (MacLeod 2009), climate change and natural environmental stochasticity may exacerbate existing prey depletions (Burns and Baker 2000).
In summary, few immediate threats to the South African inshore Bryde’s Whale are apparent. This is not to say that there are none, just that they have not been well assessed. Apart from the potential reduction in prey resources, the relatively small size of the population and its genetic isolation is probably its greatest threat due to demographic, genetic and environmental stochasticity.
Uses and Trade
Although the Bryde’s Whale complex may be split into a number of species or subspecies pending further genetic analyses, this assessment differentiates between an inshore and offshore form in South African waters, focusing most specifically on the inshore form. The coastal, inshore stock is suspected to consist of fewer than 1,000 mature individuals based on data from a 1982 line transect, ship-based survey that covered the known southern hemisphere summer distribution of the population and resulted in an estimate of 582 ± 184 animals where inclusion of secondary sightings might have increased this estimate by 29%. A mark recapture study in Plettenberg Bay produced an abundance estimate of between 150 and 250 individuals between 2005 and 2008, but how this relates to the total population is unknown. Based on the assumption that the population remains limited to fewer than 1,000 mature individuals, we list the inshore population as Vulnerable D1.
There are currently no assessments available for the offshore stock, which was exploited by land-based whalers from at least 1911 to 1967 as an integral part of the catch, and more extensively between 1969 and 1976 by illegal unregulated pelagic whaling. Being largely restricted to the southern African shelf edge from Cape Point to at least the equator; its environment is likely to have been exposed to oil and gas exploration activities for the last two decades. Given the suspected effects of seismic surveys on fish, it is possible (but completely unsupported by any data) that prey availability to this population may have been adversely affected. There are no population estimates either before or after exploitation or indications of trend and the offshore stock must be listed as Data Deficient.
This assessment echoes the global situation where the taxonomy (number and identity of species) is not yet resolved. If there is more than one species, the less abundant species may be threatened. If it is all one species, then it should be reclassified as Least Concern. Taxonomic resolution and current estimates of population size and trends for both forms are required and should result in a reassessment once such data are available.
Regional population effects: Currently, the inshore form is thought to exhibit non-migratory behaviour, and remains year-round over the continental shelf of South Africa (Best 2001); while the offshore form, occurring off the west coast of Southern Africa, appears to migrate northwards in autumn (Best 2001). There are no apparent barriers to the dispersal of either population.
Conservation
Although the Bryde’s Whale complex may be split into a number of species or subspecies pending further genetic analyses, this assessment differentiates between an inshore and offshore form in South African waters, focusing most specifically on the inshore form. The coastal, inshore stock is suspected to consist of fewer than 1,000 mature individuals based on data from a 1982 line transect, ship-based survey that covered the known southern hemisphere summer distribution of the population and resulted in an estimate of 582 ± 184 animals where inclusion of secondary sightings might have increased this estimate by 29%. A mark recapture study in Plettenberg Bay produced an abundance estimate of between 150 and 250 individuals between 2005 and 2008, but how this relates to the total population is unknown. Based on the assumption that the population remains limited to fewer than 1,000 mature individuals, we list the inshore population as Vulnerable D1.
There are currently no assessments available for the offshore stock, which was exploited by land-based whalers from at least 1911 to 1967 as an integral part of the catch, and more extensively between 1969 and 1976 by illegal unregulated pelagic whaling. Being largely restricted to the southern African shelf edge from Cape Point to at least the equator; its environment is likely to have been exposed to oil and gas exploration activities for the last two decades. Given the suspected effects of seismic surveys on fish, it is possible (but completely unsupported by any data) that prey availability to this population may have been adversely affected. There are no population estimates either before or after exploitation or indications of trend and the offshore stock must be listed as Data Deficient.
This assessment echoes the global situation where the taxonomy (number and identity of species) is not yet resolved. If there is more than one species, the less abundant species may be threatened. If it is all one species, then it should be reclassified as Least Concern. Taxonomic resolution and current estimates of population size and trends for both forms are required and should result in a reassessment once such data are available.
Regional population effects: Currently, the inshore form is thought to exhibit non-migratory behaviour, and remains year-round over the continental shelf of South Africa (Best 2001); while the offshore form, occurring off the west coast of Southern Africa, appears to migrate northwards in autumn (Best 2001). There are no apparent barriers to the dispersal of either population.
Bibliography
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