Largest reported groups for the Indo‐Pacific bottlenose dolphin (Tursiops aduncus) found in Algoa Bay, South Africa: Trends and potential drivers

Abstract

This study investigates how group size of Indo-Pacific bottlenose dolphins (Tursiops aduncus) changes temporally, spatially, and/or with predominant behavior at two discreet sites along the Eastern Cape coastline of South Africa: Algoa Bay and the Wild Coast. The mean group size of bottlenose dolphins was large with an average of 52 animals. Significantly larger groups were observed in Algoa Bay ( x = 60, range = 1-600) than off the Wild Coast ( x = 32.9, range = 1-250). In Algoa Bay, the mean group size increased significantly over the study period, from an average 18 animals in 2008 to 76 animals in 2016. Additionally, the largest average and maximum group sizes ever reported both in South Africa and worldwide, were recorded in Algoa Bay (maximum group size = 600). Neither season nor behavior had a significant effect on mean group size at both sites. Similarly environmental variables such as the depth and substrate type also had no influence on group size. It remains unclear which ecological drivers, such as predation risk and food availability, are leading to the large groups observed in this area, and further research on abundance and distribution of both predators and prey is necessary.

Abundance of Indo-Pacific bottlenose dolphins (Tursiops aduncus) along the south coast of South Africa

“…The declining group size trend is further supported by a shore-based group size estimate of 140 individuals from the early 1970s [55]. It is worth noting, however, that our group sizes for 2014-2016 are comparable to those elsewhere in the species range (generally less than 30 individuals [56]) but smaller than past estimated and the mean group size commonly observed in Algoa Bay [23,53].…”

“…Our total population estimate (2,155; 1,873-2,479) for the study area between the Goukamma and Tsitsikamma MPAs on the south coast of South Africa is roughly thirteen times smaller than that derived for Algoa Bay (28,482; 16,220-40,744) from data collected in the early 1990s [23]. Although the current size of the Algoa Bay T. aduncus population could be very different, large group sizes (ranging from 200-600 individuals) still occur here [53]. We observed smaller group sizes, and our entire study area population size estimate is approximately a third of that obtained during a previous study [24] that focused only on Plettenberg Bay (6,997; 5,230-9,492 in 2002-2003) based on 637 individually identified animals.…”

“…Although the current size of the Algoa Bay T . aduncus population could be very different, large group sizes (ranging from 200–600 individuals) still occur here [ 53 ]. We observed smaller group sizes, and our entire study area population size estimate is approximately a third of that obtained during a previous study [ 24 ] that focused only on Plettenberg Bay (6,997; 5,230–9,492 in 2002–2003) based on 637 individually identified animals.…”

Another dolphin in peril? Photo‐identification, occurrence, and distribution of the endangered Indian Ocean humpback dolphin (Sousa plumbea) in Algoa Bay

“…During recent fieldwork conducted in Algoa Bay, there have been two occasions where humpback dolphins were observed leaving the area when large groups of bottlenose dolphins arrived (Bouveroux, unpublished data). Since 2008, the group sizes of bottlenose dolphins in Algoa Bay have significantly increased, from an average group size of 18 dolphins to an average of 76 individuals per group in 2016 (Bouveroux, Caputo, Froneman, & Plön, in press), which could result in a habitat shift for humpback dolphins. Therefore, direct competition between the two dolphin species for prey availability and/or habitat use, as well as the increase in anthropogenic activities and pollution, may be responsible for the decline in the abundance and/or shift in the distribution of humpback dolphins in Algoa Bay.…”

Estimates of abundance and change in abundance of the Indo-Pacific bottlenose dolphins (Tursiops aduncus) along the south coast of South Africa

“…The Bitou municipality (which includes Plettenberg Bay) is the fastest growing municipality in the Western Cape Province, with an average annual population growth of 4.8% from 2001 to 2013 and tourism brings in much revenue to the area [22]. However, while it may be tempting to link the decline in T. aduncus numbers and group sizes with the increasing population and associated pressures in the area, a considerable increase in the mean group size of the same species in the more developed Algoa Bay to the east has been shown, from 18 to 76 individuals between 2008 and 2016 [53], which can be a consequence of a shift of the population’s preferred habitat in recent years.…”