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

Abstract

1. In South Africa, the humpback dolphin (Sousa plumbea) has been recognized as the most endangered marine mammal, with a low abundance, a discontinuous distribution, and numerous threats. This research was initiated in 2008 to estimate the number of individual humpback dolphins in Algoa Bay, as well as studying their residency patterns and distribution.2. The last boat-based study on humpback dolphins, conducted 24 years ago, formed an important reference against which to compare current findings. This study reveals that since the 1990s the number of identified animals decreased from 70 to 50 individuals, and the mean group size of humpback dolphins has also decreased, from seven to three animals.3. Humpback dolphin behaviour was predominantly that of foraging and travelling, although the overall frequency of foraging still seems to have decreased since early 1990s. 4. Although the species was seen almost all year round, the number of sightings per survey was lowest in January, March, and April. The number of sightings per kilometre surveyed substantially decreased from 0.018 sightings per kilometre in 2008 to 0.004 in 2011, and the number of animals per kilometre also decreased from 0.042 to 0.009. The re-sighting rates were low, ranging from one to eight times, with 52% of identified dolphins seen only once throughout the study period. Only 6% of the individuals were seen more than three times. 5. Reduced humpback dolphin numbers and sighting rates could be indicative of the rise in anthropogenic activities, such as shipping and recreational boating, as well as a rise in pollution from vessels and urban development.6. Conservation strategies, such as a Biodiversity Management Plan for the species, are urgently needed in Algoa Bay, especially in the south-west part of the bay, between Cape Recife and the Port Elizabeth Harbour, where the species occurs most frequently.

Science Alone Won’t Do It! South Africa’s Endangered Humpback Dolphins Sousa plumbea Face Complex Conservation Challenges

“…Notably Gui et al (2016) and Aznar-Alemany et al (2019) have shown that levels of PCBs and flame retardants are above the likely effect threshold for impairment of immune function for the species, while several studies have suggested a declining population. Bouveroux et al (2018) corroborated data from , showing significant decreases in sighting rates, group sizes, the number of overall identified animals and an increase in the occurrence of solitary individuals in Algoa Bay. Similar trends have been reported from Plettenberg Bay (Greenwood, 2013) and Richard's Bay (Atkins et al, 2013(Atkins et al, , 2016.…”

“…Interestingly, humpback dolphins are one of the better studied cetaceans in southern Africa. Research since the late 1960s (Elwen et al, 2011) has covered a wide range of topics, including socio-ecology (Saayman et al, 1972;Saayman and Tayler, 1979;Cockcroft, 1999;Karczmarski and Cockcroft, 1999;Karczmarski, 1999;Karczmarski et al, 1999a,b;, diet (Barros and Cockcroft, 1999), anatomy (Plön et al, 2012(Plön et al, , 2018, distribution, habitat use, and movements (Atkins et al, 2004;Bouveroux et al, 2018;Vermeulen et al, 2018), population abundance estimates (Keith et al, 2002;James et al, 2015), vulnerability to direct human impacts such as bycatch in shark nets (Cockcroft, 1990(Cockcroft, , 1994Atkins et al, 2013Atkins et al, , 2016, vessel traffic (Karczmarski et al, 1997;, health assessment, and pollutant loads (Cockcroft, 1999;Lane et al, 2014;Gui et al, 2016;Aznar-Alemany et al, 2019), acoustics (Frainer et al, 2019), and global population structure (Mendez et al, 2011(Mendez et al, , 2013. This species naturally occurs in small populations (Braulik et al, 2015a) and in the late 1990s, it was estimated that fewer than 1,000 individuals existed in South African waters (Karczmarski et al, 1999b).…”

The biogeography of group sizes in humpback dolphins (Sousa spp.)

“…We do not take some other critical factors like environmental changes and human activi-ties into our analysis, but they indeed disrupt social bonds and influence social parameters. For instance, the mean group size of humpback dolphins observed in the Algoa Bay, South Africa, has been confirmed to decrease from 7 in 1990s to 3 in 2010s, and such a trend was most likely due to the sharp increase of human-caused disturbance such as shipping, fishing, and entertainment in this region during the last several decades (Karczmarski 1999;Koper et al 2016;Bouveroux et al 2018b). Thus, additional research is warranted to better understand how humpback dolphins vary their group sizes and social dynamics in response to the increase of anthropogenic pressure in their shallow-water habitats.…”

“… 2016 ; Bouveroux et al . 2018b ). Thus, additional research is warranted to better understand how humpback dolphins vary their group sizes and social dynamics in response to the increase of anthropogenic pressure in their shallow‐water habitats.…”

Low mitochondrial genetic diversity in the Indian Ocean humpback dolphin Sousa plumbea in South African waters

“…However, more recent assessments suggest that the South African humpback dolphin population likely comprises fewer than 500 individuals (Vermeulen et al 2017). In addition to a decline in abundance at some study sites (James et al 2015, Vermeulen et al 2017, average group size has decreased from 6 or 7 individuals to 3 individuals, and changes in behaviour and movement patterns, such as increased time spent travelling, have been observed in Algoa Bay (Koper et al 2016, Bouveroux et al 2018. These changes are likely a result of increasing anthropo genic threats (Plön et al 2016) and suggest that a shift in Red List threat classification may be required for this population.…”