VARIABILITY, TRENDS, AND BIASES IN REPRODUCTIVE RATES OF 

 SPOTTED DOLPHINS, STENELLA ATTENUATA 



Jay Barlowi 



ABSTRACT 



Tfemporal changes were examined in three parameters that affect reproduction of spotted dolphin popula- 

 tions in the eastern Pacific Of mature females, percent pregnant decreased markedly from the period 

 1971-73 to the period 1974-83. Within the period 1974-83, percent pregnant remained relatively constant. 

 Of pregnant females, percent lactating increased during the period 1971-83. The percentage of sexually 

 mature females did not change Potential biases in the measurement of the three parameters were iden- 

 tified by examining the effects of sampling conditions. The percentage of mature females that are preg- 

 nant and the percentage of pregnant females that are lactating were found to be robust to sampling con- 

 ditions. The percentage of mature females in a sample was found to depend significantly on the number 

 of dolphins killed per set, and annual variability was too large to be explained by random sampling error. 

 Comparisons between two populations show that the more exploited population has a lower percent preg- 

 nant, although the opposite might be expected from density compensatory effects. Percent lactating and 

 percent immature were higher in the more exploited population. 



Changes in the reproductive parameters of cetacean 

 populations can be used to make inferences about 

 the status or general "health" of a population. For 

 instance, increases in pregnancy rates and decreases 

 in the age at attainment of sexual maturity were link- 

 ed to reductions in Antarctic whale populations 

 (Gambell 1975). Re-analysis of these data, however, 

 revealed unsuspected biases, and Gambell's results 

 are now being questioned (Mizroch 1983). The pur- 

 pose of this paper is to examine potential biases in 

 measuring reproductive rates of spotted dolphins, 

 Stenella attenuata. This species is taken incidentally 

 in the tuna purse seine fishery in the eastern tropical 

 Pacific (Smith 1983). The intent is to determine 

 whether reproductive rates can be measured with 

 sufficient precision to monitor intrapopulation 

 changes or to make interpopulation comparisons. 

 Previous studies of female reproduction in spot- 

 ted dolphins of the eastern Pacific have shown an 

 apparent decrease in pregnancy rates from 1973 to 

 1975 (Perrin et al. 1977), from 1973 to 1978,^ and 

 from 1971 to 1978 (Hester 1984). Hester (1984) sug- 

 gested that this decline in pregnancy rates is related 

 to the decline in fishing-related dolphin mortality 

 during the same time period. 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, La Jolla, CA 92038. 



^Henderson, J. R., W. F Perrin, and R. B. Miller. 1980. Rate 

 of gross annual reproduction in dolphin populations {Stenella spp. 

 and Delphinics delphis) in the eastern tropical Pacific, 1973-78. 

 Admin. Rep. LJ-80-02, 51 p.; available from Southwest Fisheries 

 Center La Jolla Laboratory, National Marine Fisheries Service, 

 NOAA, P.O. Box 271, La Jolla, CA 92038. 



Three indices of the reproductive status of female 

 spotted dolphins are examined in the present paper: 

 1) the fraction of sexually mature individuals that 

 are pregnant, 2) the fraction of pregnant females 

 that are lactating, and 3) the fraction of females that 

 are sexually mature These measures have been used 

 previously in calculating what has been termed the 

 gross annual reproductive rate (GARR) of spotted 

 dolphins (Perrin et al. 1976). This paper reexamines 

 data from 1971 to 1978 plus additional data from 

 1979 to 1983 to determine whether the previously 

 noted trends in reproductive rates are real, and if 

 so, whether they are continuing. Also, factors are ex- 

 amined which may be biasing estimates of reproduc- 

 tive rates and which could be causing spurious 

 changes in apparent pregnancy rates and GARR. 

 Finally, differences in these reproductive indices be- 

 tween two geographic stocks of spotted dolphin are 

 discussed in view of their different histories of in- 

 cidental fishing mortality. 



MATERIALS 



Reproductive data were collected from a sample 

 of the dolphins killed in tuna purse-seining opera- 

 tions in the eastern tropical Pacific (ETP). Three 

 stocks of spotted dolphins are recognized in this area 

 based on morphological differences (Perrin et al. 

 1979). Samples considered here include two of these: 

 the northern offshore stock which has been subject 

 to tuna fishing since 1959 and the southern offshore 

 stock which has been subject to exploitation since 



Manuscript accepted January 1985. 

 FISHERY BULLETIN: VOL. 83, NO. 4, 1985. 



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