FISHERY BULLETIN: VOL. 79, NO. 4 



FIGURE 13.— Centrum 33 from a 287 cm TL. 59 k^ female 

 Alopias superciliosus iLJVC-0355i treated with the silver ni- 

 trate technique of Stevens il97.5). This method intensifies the 

 calcified ijrowth rings as shown, for easy visualization. At least 8 

 and probably 11 dark rings surround a central clear area of 14.1 

 mm. The e.xternal diameter of centrum 33 is 25.2 mm. Photo: 

 L. Compagno. 



Cadenat 1956; Bass et al. 1975), but primarily 

 Stillwell and Casey (1976), indicate that males 

 mature at about 300 cm TL, while females mature 

 at a larger size, probably 350 cm TL. To these data 

 are added the observation that all males over 307 

 cm TL examined by Stillwell and Casey had 

 calcified, elongate claspers, and mature sperm in 

 the epididymis. They noted that a smaller male of 

 289 cm TL had nearly mature testes. 



In contrast, of 13 females examined by Stillwell 

 and Casey (1976), only those over 350 cm TL 

 possessed mature ova and enlarged ovaries. 

 These data support earlier studies indicating that 

 mature (pregnant) females are all larger than 

 about 350 cm TL. Guitart Manday (1975) noted 

 that only the largest females captured in the 

 Cuban fishery were pregnant. Finally, the size of 



our pregnant female (356 cm TL) agrees with the 

 concept of female maturity at about 350 cm TL. 



ABUNDANCE, DISTRIBUTION, 

 AND HABITAT 



The early literature on the bigeye thresher 

 seemed to indicate that it is a widely distributed 

 but rare, subtropical to tropical pelagic shark 

 inhabiting relatively deep water. For example, 

 Telles (1970) believed that only 20 bigeye thresh- 

 ers had ever been recorded. Nakamura (1935), 

 Bigelow and Schroeder (1948), Cadenat (1956), 

 and others suggested that A. superciliosus was a 

 deepwater species, and Springer (1963) reported 

 that it never approached to within a few hundred 

 meters of the surface. More recent data based on 

 longline catches point to localized concentrations 

 of this species in considerable numbers, especially 

 in the western North-Central Atlantic from off 

 the north coast of Cuba and off North Carolina 

 (Guitart Manday 1975; Stillwell and Casey 1976), 

 and in the northwestern Indian Ocean (Osipov 

 1968; Gubanov 1972). Enough occur off Cuba to 

 have yielded a total commercial catch for 1975 of 

 3,400 kg (Guitart Manday^"). In the western 

 Central Atlantic the species occurs north at least 

 to off New York (Schwartz and Burgess 1975; 

 Stillwell and Casey 1976) and apparently is rela- 

 tively eurythermic. In the western North Atlantic 

 bigeye threshers are usually caught in waters 

 with the surface temperature from 16° to 25° C, 

 and with longlines fished at a depth from slightly 

 below the surface to 65 m depth where the tem- 

 perature falls to 14° C (Stillwell and Casey 1976). 



The bigeye thresher may be able to maintain 

 body temperatures higher than ambient water 

 temperature (Carey et al. 1971), which may equip 

 it for incursions into colder water. However, like 

 the shortfin mako, Isurus oxyrinchus, which is 

 also partly homeothermic, the bigeye thresher is 

 apparently a species preferring warm temperate 

 to tropical waters. From available distributional 

 data the bigeye thresher does not occur in cold 

 temperate waters and apparently has a narrower 

 temperature and latitudinal range than either the 

 blue shark, Pnonace glauca, or the great white 

 shark, Carcharodon carcharias , both of which 

 range from cold temperate seas into the tropics. 



'"Dario Guitart Manday Institute of Oceanology, Academy of 

 Science of Cuba, Havana. Cuba, pers. commun. to S. H. Gruber, 

 24 January 1978. 



632 



