PERRIN ET AL.: GROWTH AND REPRODUCTION OF SPOTTED PORPOISE 



fully understood but certainly has to do with the 

 latitude of the area, its position relative to the 

 rest of the Pacific and to the American continents, 

 and the shapes of the adjacent land masses. These 

 factors interact with general global oceanic and 

 atmospheric circulation to produce a water mass 

 with relatively high surface temperature, low 

 surface salinity, a strongly developed, shallow 

 thermocline (usually within 100 m of the surface), 

 and a pronounced, thick oxygen minimum layer 

 just below the thermocline. The effect is to create 

 a very extensive but shallow warm habitat with a 

 sharp oxythermal floor. To the west, these condi- 

 tions tail off along a divergence centered on lat. 

 10°N (Wyrtki 1964). The conditions must be caus- 

 ally interrelated, but one of the more striking 

 correlations with the occurence of the mixed- 

 species aggregation is in the thickness of the oxy- 

 gen minimum layer (Figure 1). 



The occurrence of the aggregation is not tightly 

 correlated with the geographic distributions of 

 the major prey species of the participating pred- 

 ators. Major shared prey items are the omma- 

 strephid squid Dosidicus gigas, an unidentified 

 ommastrephid (probably Symplectoteuthis sp.), a 

 scombrid fish Aj/x/s sp. (A. thazard or A. rochei), 

 and the exocoetid fish Oxyporhamphus microp- 

 terus (Perrin et al. 1973). Dosidicus gigas is 

 primarily equatorial but migrates sporadically as 

 far as California and southern Chile, far beyond 

 the limits of the distribution of the mixed-species 

 aggregation (Clarke 1966; Young 1972). Species of 

 Symplectoteuthis occur widely in the tropical 

 Pacific and Indian oceans (Clarke 1966). Auxis 

 thazard occurs in "tropical and subtropical wa- 

 ters of the Indo-Pacific and Atlantic oceans," and 

 A. rochei in "tropical and subtropical waters of 

 the Indo-Pacific and Atlantic oceans, including 

 the Mediterranean Sea" (Richards and Klawe 

 1972). The genus Oxyporhamphus is also pantropi- 

 cal (Bruun 1935). At least some of the several 

 myctophid fishes in the aggregate apparently are 

 a mainstay of the diet of the spinner porpoise in 

 mixed schools (Perrin et al. 1973) and are not re- 

 stricted to the tropics but occur also in temperate 

 waters of the eastern Pacific (Moser and Ahlstrom 

 1970) and elsewhere. These facts, combined with 

 the pantropical distributions of the cetaceans, 

 tunas, and birds, suggest that the multispecies 

 aggregation does not have its roots in the dis- 

 tribution of the component species or their prey 

 but rather in the peculiarities of the physical 

 oceanography of the region. 



The Sample 



In 1971 and early 1972, when more specimens 

 were decked than could be processed in the time 

 available (the limit per net set was usually about 

 35 to 40 specimens), adult females were selected 

 for measuring and dissection. The intention was 

 to insure that sample sizes would be large enough 

 to allow estimation of pregnancy rate with 

 adequate precision. The information on age struc- 

 ture of the catch for that period is limited to the 

 coloration phase data. The observer program sub- 

 sequently expanded, and beginning in October 



1972 no selection was practiced in determining 

 which animals were to be dissected; the first 35 to 

 40 specimens of both sexes and all ages that came 

 to hand were set aside for measuring and dissec- 

 tion and the remainder discarded. The length 

 data for 1968 and for October 1972-December 



1973 are presumably cross-sectional with respect 

 to the kill. 



The sample of animals at least measured in- 

 cluded 3,504 postnatal animals and associated 

 fetuses fi'om known localities and 23 from impre- 

 cisely known localities (Figure 2). Coloration 

 phase and sex data were collected for another 

 6,150 specimens. In addition, some data were 

 available for 45 other specimens collected by 

 other research agencies, museums, and private 

 individuals. Because of the seasonal nature of the 

 tuna fishery, the sample is heavily biased toward 

 the early months of the year, with minimal cov- 

 erage of the latter part of the year and practically 

 no specimens from the summer months (Table 1). 



Two races of S. attenuata exist in the eastern 

 tropical Pacific — a large coastal form and a 

 small offshore form (Perrin 1975, in press). This 

 paper deals only with the offshore form. The es- 

 timates of life history parameters cannot be as- 

 sumed to apply also to the coastal form. 



Table L— Samples of postnatal spotted porpoise by month 



for all years. 



231 



