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Fishery Bulletin 93(3). 1995 



Very few neonates, pregnant females with near- 

 term fetuses, or lactating females, were collected even 

 though we found strong evidence for a calving mode 

 during the sampling period. We considered three 

 possible factors to explain the inconsistency: 1) dif- 

 fering entanglement rates by various age and sex 

 strata, 2) temporal disproportion in sampling effort, 

 especially during the projected peak of the calving 

 season, and 3) population segregation. 



Following the same rationale developed for sam- 

 pling L. borealis in Ferrero and Walker (1993), we 

 did not consider the potential for bias as a result of 

 the sampling method (i.e. incidental take in drift 

 nets) to be a major factor. Delphinus delphis entangle- 

 ments were scattered randomly along the length of 

 the nets. Multiple entanglements were rare. The 

 entanglements, therefore, appear to arise as indi- 

 vidual events rather than as whole school encoun- 

 ters. Consequently, the sample likely reflects the com- 

 position of the population in the area rather than 

 the composition of an individual school. There is also 

 no evidence to assume that neonates or pregnant fe- 

 males avoid entanglement more than other age or 

 sex strata. 



Regarding the second possibility, the temporal dis- 

 tribution of the sample does not reflect low sampling 

 effort during the late spring and early summer 

 months. The peak in sampling effort occurred in 

 June, and over 75% of that effort was accomplished 

 in March-June. Thus, the absence of neonates and 

 parturient females in the sample did not result from 

 inadequate collection effort at that time. 



Population segregation appears to be a likely fac- 

 tor in contributing to the small number of neonates 

 and parturient females in our samples, and its effect 

 is reflected in our sampling where collection locations 

 tracked the northerly progress of the fishery. Our only 

 advanced-term fetus was collected at the southern 

 end of the study area in March. As the sampling ef- 

 fort moved northward, the parturient or lactating 

 females and calves probably remained to the south 

 and were unavailable for sampling. The remaining 

 part of the female population appeared to have been 

 represented in the sample. 



The age and length distributions also reflect the 

 likelihood that a significant portion of the female 

 population was missing from the sample. The sex 

 ratios calculated for specimens collected in areas 

 south of lat. 34°N were not significantly different 

 from 1.0; however, north of 34°N the ratio signifi- 

 cantly differed from 1.0, favoring males in the 10 to 

 15 year-old age group. Furthermore, in the 15+ age 

 group the sex ratio was not significantly different 

 from 1.0 which is inconsistent with data for other 

 delphinid species characterized by a greater propor- 



tion of mature females in the population (see Table 8 

 in Perrin and Reilly, 1984). Our age and length dis- 

 tributions, which suggest a male-dominated ratio, 

 better describe only the part of the D. delphis popu- 

 lation inhabiting the northern portion of the study 

 area during the fishing season. These distributions 

 and the sex ratio, therefore, should not be consid- 

 ered indicative of the overall population structure. 



Schooling segregation by age, sex, and reproduc- 

 tive status has been demonstrated for numerous spe- 

 cies of delphinid cetaceans (Perrin and Reilly, 1984); 

 Delphinus delphis is among this group. Schooling 

 segregation of D. delphis has been documented in 

 the Black Sea where during calving and early lacta- 

 tion, females occur predominantly in offshore waters 

 (Kleinenberg, 1956; Tomlin, 1957). 



Our preliminary estimate of 11.1 months for ges- 

 tation is similar to published accounts for the com- 

 mon dolphin. Gestation periods ranging from 10 to 

 11 months have been reported for D. delphis 

 (Kleinenberg, 1956; Tomlin, 1957; Harrison, 1969). 



Growth curves for North Pacific D. delphis have 

 not been published and little definitive information 

 on age and length at the onset of sexual maturation 

 is available. Gurevitch and Stewart 3 reported that 

 male D. delphis in the eastern tropical Pacific Ocean 

 (central tropical population) reach sexual maturity 

 around 6-7 GLG's, at a mean length of 200 cm. Hui 

 (1979) presented data on Delphinus from southern 

 California where females were reported to attain 

 sexual maturity around 7-14 GLG's, and males be- 

 tween 8 and 12 GLG's. Length at onset of sexual 

 maturation was 165-182 cm for females and 175- 

 190 cm for males. Our preliminary estimates of age 

 and length at sexual maturation for both sexes fall 

 within the ranges presented in Hui ( 1979). 



However, comparisons of our findings on the short- 

 beaked D. delphis from the North Pacific transition 

 zone with those from the southern California sample 

 of Hui (1979) are potentially misleading, given re- 

 cent evidence for the occurrence of two sympatric 

 species of common dolphin in the southern Califor- 

 nia area (Heyning and Perrin, 1994). These two spe- 

 cies, D. bairdii and D. delphis, are documented to 

 differ markedly in overall size, and it is possible that 

 Hui's (1979) sample contained members of both these 

 species, accounting for the wide ranges in lengths at 

 sexual maturation. In addition, differences in ages 

 at onset of sexual maturation between our sample 



3 Gurevitch, V. S., and B. S. Stewart. 1978. Structure of kill of 

 the common dolphin, Delphinus delphis, from the eastern tropi- 

 cal Pacific in 1977. Final Rep. for Contract 03-78-M02-0101, 

 Southwest Fish. Sci. Cent., Natl. Mar. Fish. Serv., NOAA, 8604 

 La Jolla Shores Dr., La Jolla, CA 92038, 19 p. 



