Abstract. — Age and reproduc- 

 tive parameters were determined for 

 bottlenose dolphins Tursiops tr-un- 

 cahis captured in shark nets off Natal, 

 on the southeast coast of southern 

 Africa. Calibration of dentinal and 

 cemental growth-layer-group (GLG) 

 readings, using a known-age captive- 

 born dolphin, indicated that dentinal 

 and cemental GLGs are deposited 

 annually, at least up to an age of 6 

 years. Female growth in length and 

 mass and male growth in length are 

 best described by von Bertalanffy 

 growth curves. Male growth in mass 

 is characterized by a growth spurt at 

 puberty. Most growth occurs during 

 the suckling years. Both sexes reach 

 physical maturity and their asymp- 

 totic size— 243 cm and 176 kg for 

 males, and 238 cm and 160 kg for 

 females— between 12 and 15 years. 

 Both males and females may reach 

 ages in excess of 40 years, based on 

 counts of GLGs in teeth. Females at- 

 tain sexual maturity between 9 and 

 11 years of age, 2 or 3 years earlier 

 than males. Mating and birth are 

 seasonally diffuse, although there is 

 a peak of births in summer. The calf 

 is born at a mean length and mass 

 of 103 cm and 13.8 kg, respectively, 

 after a gestation period of about 1 

 year. Lactation lasts between 18 

 months and two years, although 

 there is evidence of an extended 

 mother and calf association of up to 

 3 years. Postpubertal female ovula- 

 tion rate is 0.28/year, and the esti- 

 mated calving interval is approx- 

 imately 3 years. There is no evidence 

 that females become reproductively 

 senescent with increasing age. Esti- 

 mates of population replacement 

 suggest that man-induced mortality 

 may equal or exceed the replacement 

 rate. 



Age, Gro\A^h, and 

 Reproduction of Bottlenose 

 Dolphins Tursiops truncatus from 

 the East Coast of Southern Africa 



Victor Gavin Cockcroft 

 Graham James Berry Ross 



Port Elizabeth Museum, P.O Box 13147 

 Humewood 6013, Republic of South Africa 



Manuscript accepted 6 December 1989. 

 Fishery Bulletin, U.S. 88:289-302. 



Many populations of small inshore 

 delphinids are exploited either direct- 

 ly or indirectly (Mitchell 1975, Bed- 

 dinglon et al. 1985, Northridge and 

 Pilleri 1986), and their continued sur- 

 vival depends on adequate manage- 

 ment based on knowledge of their life 

 history. In this context, knowledge of 

 the reproductive parameters of a 

 species is important in formulating 

 management and conservation pro- 

 posals. In partictilar, the reproduc- 

 tive biology of females is crucial to an 

 understanding of the likelihood of a 

 species' survival. 



An estimated 900 bottlenose dol- 

 phins Tursiops truncatus inhabit 

 Natal, along a stretch of some 400 

 km of coastal waters, southeast of 

 southern Africa (Ross et al. 1989). 

 Between January 1980 and Decem- 

 ber 1987, a minimum of 212 of these 

 animals were captured in noncom- 

 mercial inshore nets set to catch and 

 deplete the numbers of large sharks 

 off bathing beaches (Cockcroft and 

 Ross In press). The catch of dolphins 

 in anti-shark nets is of concern in the 

 continued survival of these animals 

 off Natal (Ross et al. 1989, Cockcroft 

 and Ross In press). 



Feeding studies on the bottlenose 

 dolphins off Natal have shown that 

 groups are segregated by sex and 

 size. Lactating females and their 

 calves frequent and feed in the near- 

 shore zone, adolescents feed slightly 

 further offshore, while resting fe- 

 males and adult males feed still fur- 



ther offshore (Cockcroft and Ross 

 1990). Consequently, the catch of 

 these animals in the shark nets is 

 biased, with calves less than 2 years 

 old and lactating females constituting 

 almost 60% of the total (Cockcroft 

 and Ross In press). Other age and sex 

 classes, particularly pregnant females 

 and adolescents, are, therefore, prob- 

 ably underrepresented. 



This study was undertaken to es- 

 tablish the reproductive potential of 

 bottlenose dolphins off Natal, as part 

 of a more extensive investigation of 

 the natural history of these animals 

 and severity of the threat to the pop- 

 ulation through capture in shark nets. 

 Where appropriate, recognition is 

 given to the effects of biases in sex 

 and size composition, noted above, 

 which reduce the accuracy of deduc- 

 tions made from catch data (Perrin 

 and Reilly 1984). 



Materials 

 and methods 



Routine necropsies were performed 

 on all bottlenose dolphins retrieved 

 from the Natal shark nets. Biological 

 and morphological parameters, in- 

 cluding sex, total length, and mass 

 were recorded for each animal, based 

 on the recommendations of Norris 

 (1961) and Mitchell (1975). Collected 

 organs were preserved in 10% for- 

 malin and later transferred to 50% 

 ethyl alcohol for storage. 



289 



