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

Abstract

Abstract 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. 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. Humpback dolphin behaviour was predominantly that of foraging and travelling, although the overall frequency of foraging still seems to have decreased since early 1990s. 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. 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. 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.