FISHERY BULLETIN: VOL. 84, NO. 2 



northern offshore stock of spotted dolphins (as de- 

 fined by Smith 1983) is treated in this analysis. The 

 geographic boundary used to divide it from a south- 

 ern stock is lat. 1°S (Henderson et al. 1980). 



Life History Data 



Data and specimens were collected by biological 

 technicians aboard tuna purse seine vessels in the 

 ETP. Biological data used in this analysis are body 

 length, color phase, reproductive condition (preg- 

 nant, lactating, or resting), and corpora counts for 

 each specimen (see Perrin et al. [1976] for a descrip- 

 tion of collection and examination procedures). 

 Although there is no certainty that all ovarian cor- 

 pora persist for life in all delphinids (Perrin and 

 Re illy 1984), corpora counts were used with age to 

 estimate ovulation rates. Counts included corpora 

 albicantia (CAs), corpora lutea (CLs), and in some 

 cases corpora atretica (atretic follicles). Only 

 specimens that had both ovaries examined were in- 

 cluded in the ovulation rate analyses. 



Age Estimates 



Ages were estimated for about 800 specimens 

 (from 1973 to 1978 and 1981 samples) by counting 

 growth layer groups (GLGs, Perrin and Myrick 1980) 

 in the dentine and cementum of decalcified and 

 hematoxylin-stained thin sections (Myrick et al. 

 1983). Tooth readings were made independently by 

 two readers (A. C. Myrick and A. A. Hohn), without 

 referring to field or laboratory data on size or repro- 

 ductive condition. For the 1973-78 sample, a tooth 

 of each specimen was read at least three times by 

 each reader. Age estimates by each reader were sig- 

 nificantly different (Reilly et al. 1983). To minimize 

 the differences, the mean of the multiple age esti- 



mates by each reader was calculated and the average 

 of the two means was used as the estimate of a 

 specimen's aga For the 1981 sample a tooth from 

 each specimen was read once by each reader after 

 calibration tests showed that differences in estimates 

 between readers were acceptably small (Reilly et al. 

 1983). An average of these two readings was used 

 for specimen age. 



We consider the method we used to estimate ages 

 improved over that used by Perrin et al. (1976) 

 because 



1) the preparation technique we used provides 

 superior resolution of GLGs (Myrick et al. 1983); 



2) the new method of reading utilizes GLGs in 

 the cementum as well as in dentine and allows a more 

 accurate estimate of maximum age for adults 

 (Myrick et al. 1983; see also Kasuya 1976); 



3) calibration of GLGs in tetracycline-labeled 

 teeth of Hawaiian spinner dolphins, Stenella longi- 

 rostris (Myrick et al. 1984), has provided a basis for 

 interpreting dental layering within an absolute-time 

 framework (Myrick et al. 1983; Myrick et al. 1984). 

 Perrin et al. (1976) used the term tooth layers in lieu 

 of known time units. 



RESULTS AND DISCUSSION 



Composition of Samples 



Chi-square (contingency) tests were used to evalu- 

 ate whether fractions of mature, pregnant, and lac- 

 tating females in the 1973-78 aged sample were a 

 representative subset of the overall sample for those 

 years. For all three tests, differences were not sig- 

 nificant (P > 0.05). 



Reproductive statistics showed some differences 

 between years (Table 1). Chi-square tests were 

 carried out for homogeneity between 1973-74 and 



Table 1.— Number of sexually mature, pregnant only, lactating only, simultaneously pregnant and 

 lactating, and "resting" female spotted dolphins, and the proportion of the sample pregnant or 

 lactating in the aged and overall samples. The proportion pregnant and proportion lactating in- 

 clude the simultaneously pregnant and lactating specimens. 



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