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Fishery Bulletin 92(1), 1994 



very near the surface. Dolphins were not measured 

 if either the rostrum or tail flukes were not clearly 

 visible or if they were surfacing, diving, or jumping, 

 which would make them appear shorter when 

 viewed from above. Because there was from 80 to 

 90% overlap between adjacent photographs, the 

 same dolphin could often be measured in two to four 

 photographs. If more than one length was available 

 for a dolphin, the largest length was selected, as- 

 suming it was the best determination of true length. 

 This helped to minimize the reduction in apparent 

 length caused by the normal swimming movements 

 of the dolphins (Scott and Perryman, 1991; 

 Perryman and Lynn, 1993). 



We measured each dolphin from the tip of the 

 rostrum to the trailing edge of the tail flukes (Fig. 

 2). These points were selected because the fluke 

 notch that is used to determine standard length 

 (Norris, 1961) was very difficult to see in most of the 

 images. For adult specimens, this measurement 

 should exceed standard length by 2-2.5 cm (Chivers, 

 1993 2 ). The measurements were made on sections 

 of the original black and white negatives that we 

 captured with a high-resolution video camera and 

 transferred to a Macintosh Ilci computer. Image 

 enhancement and length measurements were made 

 with the aid of the digital image processing and 

 analysis program, Image (version 1.37), which was 

 developed by the National Institute of Health (W. 

 Rasband, Research Services, Bethesda, Maryland). 

 The length of each dolphin was determined by mul- 

 tiplying its length on the image by the scale of the 

 photograph ( scale= A/lens focal length ). 



Data analysis 



Perrin et al. (1985) compared the mean lengths of 

 physiologically adult male and female dolphins from 



2 S. Chivers. 1993. Southwest Fisheries Science Center, La Jolla, 

 California 92037. unpubl. data. 



■Photo Length- 



putative geographic stocks of several species to pro- 

 vide supporting morphological evidence for repro- 

 ductive isolation. For our analyses, we used length 

 as the criteria for eliminating the youngest dolphins 

 from our sample. Based on the length data for adult 

 striped dolphins in Perrin et al. (1985) and a review 

 of our length sample, we estimated that the mini- 

 mum length for adult female striped dolphins in the 

 eastern Pacific is about 180 cm. We used this length 

 as our first cut-off point, and tested for differences 

 U-test) between the means of our length samples 

 (<180 cm) from the northern and southern regions 

 (Fig. 1). Since the selection of this value was some- 

 what arbitrary, we repeated the tests on data sets 

 with minimum values of 185 and 190 cm. 



Based on behavioral arguments reviewed in Per- 

 ryman and Lynn ( 1993), we assumed that the larger 

 dolphin swimming closely alongside a calf was an 

 adult female. Since this determination was based on 

 behavior and not on examination of sexual charac- 

 ters, we qualify the term in quotation marks, "adult 

 female," whenever we are referring to a length 

 sample based on this assumption. A <-test was used 

 to compare the mean lengths of "adult females" from 

 the northern and southern regions. We also per- 

 formed a power analysis to determine what range 

 of differences between means we could expect to 

 detect (probability of type II error < 0.10) for this 

 analysis and the ones described in the paragraph 

 above. 



Calf birth dates 



We examined the length data from striped dolphins 

 estimated to be one year old or less for evidence of 

 pulses in reproduction (see Barlow 1 1984], for spot- 

 ted and spinner dolphins; Perryman and Lynn 

 (1993], for common dolphins). Ninety centimeters 

 was used as the best estimate of average length at 

 birth and 155 cm for average length at one year for 

 striped dolphins in the eastern Pacific (Gurevich and 

 Stewart, 1979 ;i ). We as- 

 sumed postnatal growth was 

 linear during the first year 

 and back-projected the birth 

 dates for all dolphins <155 

 cm in length. Our goal here 

 was not to determine the ex- 



Photo 



Length 



■Standard Length- 



Figure 2 



Illustration of the difference between points used to determine standard 

 length and length as measured from our vertical photographs. 



1 Gurevich, V. S., and B. S. Stewart. 

 1979. A study of growth and re- 

 production of the striped dolphin 

 iStenella coeruleoalba). U.S. Dep. 

 Commer., NOAA, Natl. Mar. Fish. 

 Serv., Southwest Fish. Sci. Cent., 

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

 Final Rep to NOAA, SWFC Con- 

 tract 03-78-D27-1079, 29 p. 



