Abstract. Determination of 



stock structure for striped dol- 

 phins (Stenella coeruleoalba) in 

 the eastern Pacific has been prob- 

 lematic, because very few speci- 

 mens have been available for 

 study. We compared length data 

 obtained from vertical aerial pho- 

 tographs of 28 schools of striped 

 dolphins from the northern and 

 southern regions of the eastern 

 tropical Pacific and found no sig- 

 nificant differences in average 

 length for adult animals (> 180cm) 

 or for adult females, defined here 

 as dolphins closely accompanied 

 by a calf. Analyses of back-pro- 

 jected birth dates for dolphins 

 >155cm revealed a broad pulse in 

 reproduction extending from the 

 fall through the spring; however, 

 sample size was inadequate to 

 compare timing of reproduction 

 between the two areas. Striped 

 dolphins measured from aerial 

 photographs were longer on aver- 

 age than those killed incidentally 

 in fishing operations. We found a 

 pattern of segregation by size be- 

 tween schools that is analogous to 

 the separate schools of juveniles 

 and adults that are found in the 

 western Pacific. We hypothesized 

 that the specimen data base may 

 be biased because tuna purse- 

 seine fishermen in the eastern 

 tropical Pacific may selectively set 

 on schools composed of younger, 

 smaller dolphins. 



Examination of stock and school 

 structure of striped dolphin 

 (Stenella coeruleoalba) in 

 the eastern Pacific from 

 aerial photogrammetry 



Wayne L. Perryman 



Morgan S. Lynn 



Southwest Fisheries Science Center 



National Marine Fisheries Service. NOAA 



8604 La Jolla Shores Drive. La Jolla. Calif 92037 



Manuscript accepted 20 September 1993 

 Fishery Bulletin 92:122-131 (1994) 



Because striped dolphins, Stenella 

 coeruleoalba, are killed incidentally 

 in purse-seine fishing for yellowfin 

 tuna in the eastern tropical Pacific 

 (ETP), the National Marine Fisher- 

 ies Service (NMFS) is required by 

 the Marine Mammal Protection Act 

 (as amended in 1988) to monitor 

 trends in their abundance (Holt 

 and Sexton, 1989; Wade and 

 Gerrodette, in press). To satisfy 

 this congressional mandate, infor- 

 mation on stock structure is re- 

 quired. The determination of stock 

 structure for striped dolphins in 

 the ETP has been particularly dif- 

 ficult because of the small number 

 of animals killed in the tuna fish- 

 ery and, therefore, small number of 

 specimens available for study 

 (DeMaster et al., 1992). In the ab- 

 sence of morphological, life history, 

 or genetic data to provide evidence 

 of reproductive isolation, stocks of 

 striped dolphins have been identi- 

 fied provisionally based on 

 discontinuities in distribution. 

 With more sighting data from ob- 

 servers aboard fishing vessels and 

 research cruises, the number of 

 proposed stocks has decreased from 

 five or six (Smith, 1979 1 ; Holt and 

 Powers, 1982) to one (Dizon et al., 

 in press) pending availability of 

 additional data. 



For this report, we examined 

 length data to help clarify the issue 

 of stock structure. These data were 



extracted from vertical aerial pho- 

 tographs collected during line 

 transect surveys and are thus pre- 

 sumably free of any "sampling" bi- 

 ases associated with the fishery. 

 Here, we compare length samples 

 from aerial photographs of animals 

 from the northern and southern 

 stock regions proposed by Perrin et 

 al. (1985) for evidence of differences 

 in average length or timing of re- 

 production. Data were then com- 

 pared with measurements avail- 

 able from specimens killed inciden- 

 tally in purse-seine fishing. We also 

 examined the frequency distribu- 

 tion of lengths within individual 

 schools. These data were used to 

 test for size-age segregation, as 

 reported for dolphins taken in the 

 drive fishery on the Pacific coast of 

 Japan (Miyazaki, 1977; Miyazaki 

 and Nishiwaki, 1978). 



Methods 



Length measurements were made 

 on vertical aerial photographs of 28 

 schools of striped dolphins (Fig. 1). 

 We photographed the schools with 

 a KA-45A military reconnaissance 



1 Smith, T. D. (ed). 1979. Report of the sta- 

 tus of porpoise stocks workshop; 27-31 

 August, La Jolla, California. U.S. Dep. 

 Commer., NOAA, Natl. Mar. Fish. Serv., 

 Southwest Fish. Sci. Cent, P.O. Box 271, 

 La Jolla, CA 92038. Admin Rep., L.l-79- 

 41, 120 p. 



122 



