3. BIONOMICS AND LIFE HISTORY 

 3.1 Reproduction 



3.11 Sexuality 



The sexes of the black marlin are separate. The 

 males and females cannot be distinguished by exter- 

 nal characters. In the waters around Formosa, the sex 

 ratio is 53/414 male throughout a size range of 20 to 

 200 kg in body weight (Nakamura, 1944a). Females 

 become larger than males (Nakamura, 1944b). 



3.12 Maturity 



Age at which sexual maturity is reached is not 

 known. 



3.13 Mating 



The mating of the black marlin has not been 

 observed. 



3.14 Fertilization 

 External. 



3.15 Gonad 



Black marlin are densely distributed in the 

 northwestern part of the Coral Sea between October 

 and December. Almost all fishes caught in this area 

 have well-developed gonads (Table 1); thus these 

 fishes are believed to be from spawning schools in this 

 area. The skewed sex ratios may also be indicative of 

 this possibility (Ueyanagi, 1960). 



Merrett (1970) studied histological gonad develop- 

 ment in billfishes including this species from the In- 

 dian Ocean. 



3.16 Spawning 



There is very little information relating to spawning 

 grounds and spawning seasons of the black marlin. 

 Nakamura (1941, 1942, 1944b) surmised that spawn- 

 ing occurs in the vicinity of Hainan Island and the 

 South China Sea in May or June. Ueyanagi (1960) 

 also presumed spawning to occur in the northwestern 

 part of the Coral Sea between October and December. 



The sex ratio of the black marlin varies with area 

 and season (Table 2). Variations in sex ratios seem to 

 be related to spawning (Nakamura, 1942). 



3.17 Spawn 



The egg of the black marlin seems to be pelagic and 

 nonadhesive, but details are not known. 



3.2 Preadult Phase 



3.21 Embryonic phase 

 No data. 



3.22 Larvae phase 



The embryological development of the black marlin 

 is not known. Some larvae have been obtained by 

 research vessels. Morphological features of these lar- 

 vae have been described by Ueyanagi (1964). 



The back of the larvae is slightly concave where the 



Figure 3. — Distribution and fisheries of black marlin. Shaded areas show good fishing grounds for commercial longline fisheries. 

 Horizontal barred area shows harpoon fishing ground. Dotted areas show sport fishing grounds. Solid line indicates northern and 

 southern limits of distribution of black marlin based on data from the longline catches. Dotted line shows movement of black marlin 

 from the Indian Ocean into the Atlantic Ocean. 



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