Table 3. — Larval occurrences of shortbill spearfish and blue marlin during 

 winter and summer months in the western subtropical Pacific Ocean (lat. 

 15°-25°N) (Ueyanagi, 1962). 



Season 



No. of 

 hauls 



Species 



Percent 

 occurrence 



Average no. 

 per haul 



Dec. -Feb. 69 Shortbill spearfish 14.5 0.25 



Blue marlin 4.4 0.16 



May-Aug. 351 Shortbill spearfish 8 0.12 



Blue marlin 37.3 1.29 



2.3 Determinants of Distribution Changes 



To determine the vertical distribution of billfishes 

 in the equatorial Indian Ocean, Merrett (1971) relates 

 the percent catch rates of each species taken by the 

 longline fishery to depth soundings and states that no 

 T. angustirostris was caught in waters shallower than 

 500 fathoms. The highest catch rate for this species 

 was obtained in the 501-1,000 fathom range and the 

 catch rate decresed in deeper waters. Data are too 

 sparse to be really significant, however. 



2.4 Hybridization 



2.41 Hybrids 



No information is available. 



2.42 Influence of natural hybridization in 

 ecology and morphology 



No information is available. 



3 BIONOMICS AND LIFE HISTORY 



3.1 Reproduction 



3.11 Sexuality 



Hermaphroditism, heterosexuality, intersex- 

 uality: Heterosexual. 



Nature and extent of hermaphroditism: No example 

 of hermaphroditism of T. angustirostris has been 

 reported thus far. 



Sexual dimorphism: Despite many morphological 

 studies, there is no report dealing with sexual 

 dimorphism. Concerning the difference in size by sex- 

 es, Koga (1959) reports that larger fish are more fre- 

 quently female than male in the size composition of 

 longline catches made in the southeastern Pacific 

 Ocean from September through November. Merrett 

 (1971) states that no limitation of size by sex is 

 observed in the longline catch from the equatorial 

 western Indian Ocean. 



3.12 Maturity 

 Size and weight at sexual maturity: The size of fish 



in running ripe condition reported thus far are as 

 follows: a 1,524-mm specimen (measured from bill tip 

 to origin of lateral keels on caudal peduncle) from the 

 waters adjacent to Taiwan (Nakamura, 1937), a 

 1,638-mm specimen (measured from bill tip to caudal 

 fork) from the central Pacific (Royce, 1957), and a 

 1,390-mm specimen (measured from center of orbit to 

 shortest caudal ray) from the equatorial western In- 

 dian Ocean (Merrett, 1971). The specimen reported 

 by Merrett was in the ovulation stage, the maturity 

 stage was VI, and the fish weighed 18.6 kg (41 lb). 



3.13 Mating 



Monogamous, polygamous, promiscuous: Although 

 the evidence is not certain, the billfishes seem to be 

 promiscuous. 



3.14 Fertilization 

 External. 



3.15 Gonads 



Merrett (1970) describes the ovulation for T. 

 angustirostris, the final phase in the course of 

 oogenesis which follows a point at which the eggs at- 

 tain 750 yum in size. He states that immediately prior 

 to this, the ovary becomes more vascular and the 

 follicles, both theca externa and granulosa layers, 

 then burst. On bursting, the released eggs swell by the 

 uptake of liquid. The increase in size is a mean 

 diameter of from 845 to 1,442 Aim. Clearly, ovulation 

 is not an all or none process, since barely half of the 

 eggs in the ovary are estimated to be shed. Ovulation 

 is observed to take place first centrally, and then 

 radially outwards. His observations further suggest a 

 continuous availability of spermatozoa in mature 

 males by the differential maturation of the testicular 

 lobules in conjunction with the possession of a 

 muscular seminal vesicle. The male cycle appears dis- 

 similar to the cycle in other teleosts while the female 

 cycle is similar to other teleosts, as maturation is syn- 

 chronous throughout the ovary. 



Merrett (1971) estimated the mean fecundity of the 

 Indian Ocean istiophorids and found that con- 

 siderable variation occurs in the fecundity of 

 Istiophorus platypterus and that such variation is ap- 

 parently related to fish size. Although the relation 



45 



