Abstract. -We examined the av- 

 erage age at attainment of sexual 

 maturity (ASM) and several other 

 reproductive parameters for evidence 

 of density compensatory responses 

 in two stocks of the spotted dolphin, 

 Stenella attenuata. The northern off- 

 shore and southern offshore stocks 

 were compared because both have 

 been exploited by the yellowfin tuna 

 purse-seine fishery, but at different 

 rates. We predicted decreasing 

 trends in the ASM and increasing 

 trends in the pregnancy rate for each 

 stock because both have declined in 

 abundance. A lower ASM and a 

 higher pregnancy rate were pre- 

 dicted for the sample from the north- 

 ern offshore stock because it has 

 been exploited to a greater extent 

 than the southern offshore stock. No 

 statistically significant trends were 

 found in the ASM, but the increase 

 in the proportion of sexually mature 

 females simultaneously pregnant 

 and lactating for the northern off- 

 shore stock and the decrease in the 

 proportion of mature females in the 

 population for the southern offshore 

 stock over time were statistically 

 significant. The mean estimate of 

 ASM was significantly higher for the 

 northern offshore stock, 11.1 years 

 (SE=0.236), than for the southern 

 offshore stock, 9.8 years (SE=0.264). 

 No significant differences between 

 stocks were found in the mean esti- 

 mates of reproductive parameters. 

 Our analyses of temporal trends in 

 several biological parameters did not 

 provide conclusive evidence for 

 compensatory responses having oc- 

 curred, and therefore some possible 

 explanations are considered. 



Comparison of age at sexual 

 maturity and other reproductive 

 parameters for two stocks of 

 spotted dolphin, Stenella attenuata 



Susan J. Chivers 

 Al C. Myrick Jr. 



Southwest Fisheries Science Center, National Marine Fisheries Service, NOAA 

 PO Box 27 1 , La Jolla, CA 92038-027 I 



Manuscript accepted 17 June 1993. 

 Fishery Bulletin 91:611-618 (1993). 



As population abundance declines, 

 compensatory responses, such as in- 

 creases in survival and pregnancy 

 rates and a decrease in the average 

 age at attainment of sexual matu- 

 rity (ASM), are thought to occur 

 (Eberhardt, 1977; Eberhardt and 

 Siniff, 1977). Each situation must be 

 carefully evaluated to determine 

 whether compensatory responses will 

 be detected because the magnitude 

 of the parameter response and the 

 range of population densities over 

 which measurements are made are 

 likely to be species specific, environ- 

 mentally variable, and subject to pa- 

 rameter measurement error (Good- 

 man, 1981; Fowler, 1981, 1988; 

 DeMaster, 1984a; York, 1987). For 

 species that are difficult to observe, 

 unbiased estimates of survival rates 

 are difficult to obtain, precluding 

 their use as potential biological indi- 

 ces (Barlow and Boveng, 1991). Re- 

 productive rates, however, are gen- 

 erally more easily measured and, if 

 their relationship to population den- 

 sity is determined, may provide indi- 

 ces of compensatory responses for 

 populations (Perrin and Donovan, 

 1984). For instance, Barlow (1985) 

 reported that in the spotted dolphin 

 (Stenella attenuata), the fraction of 

 mature females pregnant, and the 

 fraction of mature females simulta- 

 neously pregnant and lactating, may 

 correlate well with per capita re- 

 source availability because these pa- 

 rameters were found to be relatively 



free of collection biases. The identifi- 

 cation of biological indices (i.e., bio- 

 logical parameters that correlate with 

 population abundance) would provide 

 a useful management tool to distin- 

 guish between trends in population 

 abundance resulting from exploita- 

 tion or changes in environmental con- 

 ditions (Hanks, 1981; Gerrodette and 

 DeMaster, 1990). 



The spotted dolphin is an ideal 

 choice for this analysis because a 

 lengthy time series of biological and 

 population abundance data are avail- 

 able. This species is used as a cue by 

 purse-seine vessel operators to find 

 yellowfin tuna in the eastern tropi- 

 cal Pacific (ETP) (Perrin, 1969). Dol- 

 phins are incidentally killed during 

 fishing operations, and the spotted 

 dolphin population has had the larg- 

 est number of animals killed in al- 

 most every year since 1959 (Smith, 

 1983; DeMaster et al., 1992). Two 

 stocks of spotted dolphin, the north- 

 ern offshore and southern offshore 

 stocks, are recognized as separate 

 management units in the ETP 

 (Perrin et al., 1985). During the first 

 decade of the purse-seine fishery, the 

 1960s, the large numbers of dolphins 

 killed caused the abundance of these 

 two stocks to decline. The northern 

 offshore spotted dolphin was esti- 

 mated to be between 35% and 50% 

 of their pre-exploitation abundance 

 (circa 1959) by 1979 (Smith, 1983), 

 while the southern offshore stock 

 was estimated to be at 92-98% of its 



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