NOTE Hazin et al.: Distribution and relative abundance of Prionace giauca 



479 



(Fig. 2), therefore, is probably a consequence of ocean 

 depth, because almost all sets performed in this fish- 

 ing ground took place in shallow depths and in the 

 vicinity of oceanic banks. 



The monthly mean CPUEs of males and females 

 (Fig. 4) show that the higher relative abundance of 

 blue shark during the third and fourth quarters of 

 the year (Hazin et al., 1990) is mostly comprised by 

 males. These results indicate also that males and 

 females are segregated and that their migratory 

 movements are different. The different seasonal fluc- 

 tuation of CPUE for different male age groups indi- 

 cate that male specimens were also segregated by 

 size. The use of Stevens' (1975) growth equation to 

 calculate these ages from fork length may have limi- 

 tations because it is based on data from the North 

 Atlantic Ocean. Nevertheless, Amorim (1992) stud- 

 ied the growth of blue shark in the south-western 

 Atlantic and found a value of k- 0.1126, which ap- 

 proximates Stevens' value of 0.11. 



Differences in vertical distribution displayed by 

 male and female blue sharks (Fig. 8) indicate that 

 vertical sexual segregation likely occurred in the first 

 half of the year. They also suggest that the depth 

 range of male blue sharks may change seasonally. 



Acknowledgments 



We sincerely thank the Norte Pesca S/A, who pro- 

 vided all the data used in this research. We are par- 

 ticularly indebted to Alceu A. Couto and Anibal P. 

 Souza for their valuable help in data collection. We 

 also thank Francis G. Carey of the Woods Hole 



n= 320 Age (year) 



^J Feb. -May | | Jun.-Jul. I I Aug. 



~~ Sep. -Oct. | | Nov. -Dec. 



Figure 6 



Mean catch per 10,000 hooks of male blue shark, 

 Prionace giauca, by age classes, from February 

 through December 1990, in the southwestern equa- 

 torial Atlantic Ocean, n = 320. 



Oceanographic Institution who constructively criti- 

 cized the manuscript, and Sakutaro Yamada for his 

 technical assistance. Financial support was granted 

 by the Ministry of Education of the Japanese Gov- 

 ernment through the Mombusho Scholarship Pro- 

 gram and by the Brazilian Research Council (CNPq). 



May 



Temperature (°C) 

 14 18 22 



26 



30 



November 



Figure 7 



Typical temperature profiles for May and Novem- 

 ber, in the southwestern equatorial Atlantic Ocean, 

 and the calculated mean depths of the hooks 1 to 7 

 of the longline of the FV Argus. 



