BARLOW: REPRODUCTIVE RATES OF SPOTTED DOLPHINS 



80 t- 



UJ 



H 

 < 



2 



UJ 



o 



tu 



0. 



70 - 



60 

 50 

 40 

 30 



(995) 



(1149) 



(1013) 



(1215) 



(593) 



(564) . 



(509) 



(465) 



(579) 



(751) 



(137) 



± 



1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 



YEAR 



Figure 4.— Percentages of all females that were mature for the northern offshore stock of spotted dolphins from 



1971 to 1983. Sample sizes are in parentheses. 



Table 8.— Multiway tests of factors affecting 

 percent mature. Log-likelihood chi-square 

 was used to calculate the probability that 

 percent mature is independent of the stated 

 factor(s) using the log-linear model. I\/latura- 

 tion state (mature/not mature) is implicit as 

 the first factor in each comparison. 



showed a decrease in ASM without any change in 

 the length at which maturity is attained. Laws (1956) 

 predicted an inverse relationship between ASM and 

 early growth rates for marine mammals. Spotted 

 dolphins appear to show an increase in length at 

 maturation with no change in ASM, and thus do not 

 follow predicted patterns. 



Trends in Percent Pregnant 



Annual pregnancy rates, calving interval, and per- 

 cent pregnant all measure essentially the same thing. 

 Annual pregnancy rate and calving interval require 

 knowledge of gestation times. Because density com- 

 pensatory responses have not been shown in ceta- 

 cean gestation times, it is more straightforward to 

 deal directly with percent pregnant. 



What appeared to be a rapid decline in dolphin 

 pregnancy rates from 1973 to 1978 (Henderson et 

 al. fn. 2), now appears as two eras with distinctly 

 different pregnancy rates. The fraction of pregnant 

 females in the 1971-73 samples was quite high. The 

 10 years since 1973 show a lower and relatively con- 

 stant fraction of pregnant females. This difference 

 in results is due largely to use of a larger sample 

 size and a longer time series. 



There is no evidence of the sort of slow, long-term 

 trends in pregnancy rates that might be associated 

 with changes in population sizes. The reason for the 

 dramatic change in pregnancy rates between 1973 

 and 1974 is not known. At least three hypotheses 

 could be used to explain this change: 1) it was the 

 result of a naturally high pregnancy rate in 1971-73; 

 2) it was the result of heavy fishing-related morta- 

 lity of nursing calves prior to 1974 that resulted in 

 artificially high pregnancy rates; or 3) it was the 

 result of a bias in the sampling by tuna vessels. 



The first hypothesis suggests that changing en- 

 vironmental conditions result in annual changes in 

 pregnancy rates. In the ETP the largest environ- 

 mental perturbations are associated with "El Nino" 

 events which occur on the time scale of from 5 to 

 10 yr (Rasmusson and Carpenter 1982). El Nifio con- 

 ditions prevailed in 1972 (moderate), 1975-76 (weak), 

 and 1982-83 (very strong). These dates do not help 

 explain the change in pregnancy rates that occurred 

 between 1973 and 1974. 



The second hypothesis is that heavy dolphin mor- 

 tality in the 1960's and early 1970's may have some- 

 how affected dolphin pregnancy rates. Large reduc- 



665 



