Life History Parameters of Indian Ocean Humpback Dolphins Sousa plumbea Off KwaZulu-Natal, South Africa

Abstract

For the effective conservation of coastal cetaceans, such as Indian Ocean humpback dolphin, Sousa plumbea, currently listed as “Endangered” by the International Union for the Conservation of Nature (IUCN), information on its life history is urgently required. To fill this gap, the age of 119 humpback dolphins (74 males and 45 females) incidentally caught in bather protection nets (BPN) off the east-coast of South Africa over a 40-year period was determined by counting tooth growth layer groups (GLGs). Estimated length at birth using Scott's equation was 111.57 cm for males and 104.33 cm for females. A Gompertz growth curve indicated that physical maturity was reached at 268.03 cm in males and 246.12 cm in females, corresponding to 24 and 13 years, respectively. Histological analyses of reproductive organs of 45 males and 28 females showed that sexual maturity in males is attained at 7.5 years, and 210 cm total body length. In females, sexual maturity occurred around 8 years and a body length of 220 cm. The ovulation rate was 0.2 per year, indicating a calving interval of approximately 5 years. These results will aid in assessing the status of the South African population of Indian Ocean humpback dolphins.