300 



Fishery Bulletin 88(2). 1990 



Tyutyunnikov 1968, in Perrin and Reilly 1984) and 

 43.6% and 40.4% for the western north Pacific (Kasuya 

 and Izumisawa 1981, in Perrin and Reilly 1984; Kasuya 

 1985). 



Gross annual reproduction rate (GARR) (Perrin and 

 Reilly 1984) calculated from catch statistics and the 

 range of APR values yields estimates between 0.043 

 and 0.065. Although biases in the catch will influence 

 these GARR estimates, they are useful for comparative 

 purposes. The former GARR estimate is greater than 

 that calculated for an unexploited stock of Tursiops 

 from eastern Australian waters, although this was 

 based on an unreliable technique of estimating calf 

 numbers from aerial surveys (Lear and Bryden 1980, 

 in Perrin and Reilly 1984). The latter GARR figure, 

 although probably an overestimate, is some 40% and 

 500% lower than those estimated for exploited popula- 

 tions of Tursiops off Iki Island, Japan (Kasuya 1985) 

 and in the Black Sea (Danilevsky and Tyutyunnikov 

 1968, in Perrin and Reilly 1984), respectively. 



The probable biases in the calculated APR and GARR 

 estimates suggest that an assessment of the theoretical 

 maximum natural rate of increase (ROI) of the Natal 

 bottlenose dolphin population would be more practical. 

 Assuming a calving interval of 2-3 years, age at first 

 breeding of 10 years, and an annual survival rate of 

 less than 0.97, an ROI of 4-6% can be calculated (Reilly 

 and Barlow 1986). The ROI makes allowances for adult 

 and calf mortality not accounted for by a GARR esti- 

 mate, which, therefore, infers that even the greater 

 GARR figure may be an underestimate. Given an an- 

 nual increase of as much as 6% of the estimated 900 

 population, the mean annual mortality of bottlenose 

 dolphins in shark nets— 32 dolphins per year including 

 about 4 reproductive females— in conjunction with 

 whatever other sources of man-induced mortality, such 

 as the probable death of first-born neonates through 

 pollutant toxicity (Cockcroft et al. 1989), implies that 

 mortalities may be close to or exceed the likely replace- 

 ment rate of this population. However, this conclusion 

 should be viewed with some caution, as it is based on 

 an estimated population of only some 900 dolphins, 

 although biases of aerial counts suggest that numbers 

 may be greater (Cockcroft et al. In press). Additional- 

 ly, other factors may also influence understanding of 

 the reproductive capacity of this population. If bottle- 

 nose dolphins on the Natal coast are geographically 

 separated for long periods (Cockcroft et al. 1989) with 

 little mixing even of adjacent groups (Cockcroft et al. 

 In press), then reproductive parameters for females in 

 different areas may vary and have a profound effect 

 on calculated replacement potentials. 



The incidental mortality and probable depletion of 

 long-lived dolphins, that invest many years in the care 

 and socialization of their young and are resident in 



areas with which they are familiar, is of concern. The 

 future management of the Natal bottlenose dolphin 

 population requires accurate population figures and an 

 unbiased estimate of age and sex structure. Regular 

 aerial, boat, and shore-based surveys along the Natal 

 coast are needed to define the former. The latter is best 

 obtained through a combination of intensive field obser- 

 vational work on free-ranging dolphins and a continued 

 monitoring of captured animals. 



Acknowledgments 



We gratefully acknowledge Shantal Koch and Sabine 

 Klages for many hours spent in preparing histological 

 slides, counting dentine layers, and sectioning ovaries. 

 We appreciate the help of Dr. T. Kasuya for reading 

 a selection of teeth so that we could calibrate our own 

 techniques. Our thanks to the Director and staff of the 

 Natal Sharks Board for their cooperation in collecting 

 animals from the nets. 



Citations 



Arvy, L. 



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