HOHN and HAMMOND: POSTNATAL GROWTH OF SPOTTED DOLPHIN 



>■ 



o 



z 



UJ 



o 



UJ 

 DC 



12 

 8 

 4 

 



- I FETUSES 

 CALVES 



Ira 



70 72 74 76 78 80 82 84 86 88 90 92 94 96 



LENGTH (cm) 



100 r 



< 

 < 



0) 



o 



a. 



LU 



o 



UJ 



a 



Average length 

 at birth 



I I I I I I i"r I f I f I f ri I I I I I I I I I I 



70 72 74 76 78 80 82 84 86 88 90 92 94 96 



LENGTH (cm) 



Figure 3— Length-frequency data for specimens from sets with kill >40 grouped in 1 cm intervals 

 for 21 fetuses and 15 calves from the southern offshore population of spotted dolphins, and the logistic 

 model fitted to the percentage of animals that were postnatal. 



accuracy of this estimate For these reasons, we take 

 the estimate from all southern specimens of 83.2 cm 

 as our provisional best estimate of length at birth 

 for southern offshore spotted dolphins while 

 recognizing that this estimate may be biased 

 downwards because of a possible overrepresentation 

 of calves in the sample. 



ESTIMATION OF LENGTH-AT-AGE 



USING ANALYSES OF 



MONTHLY DISTRIBUTIONS OF LENGTH 



Perrin et al. (1976) used the technique of fitting 



a growth curve to the means of normal distributions 

 fitted to length-frequency data by month to estimate 

 the length of the offshore spotted dolphin at 1 yr 

 of age Perrin and Henderson (1984) used the same 

 technique for the spinner dolphin. The technique is 

 based on the assumption that breeding in these 

 dolphins is seasonal and that a cohort of animals 

 born at approximately the same time is characterized 

 by a distribution of lengths, identifiable as a mode 

 in the overall length distribution, which can be 

 followed from month to month as mean length of the 

 cohort increases. If there are sufficient data in each 

 month, mean lengths can be followed from birth until 



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