Distribution and relative abundance 

 of the blue shark, Prionace glauca, 

 in the southwestern equatorial 

 Atlantic Ocean 



Fa bio H. V. Hazin 



Department of Marine Science and Technology 



Tokyo University of Fisheries 



5-7, Konan-4. Minato-ku, Tokyo 1 08, Japan 



The Brazilian Research Council 



CNPq (Conselho Nacional de Desenvolvimento Cientifico et Technologico) 



Avenida W3 Norte, Quadra 511. Bloco A., Ed. Birtar II 



Brasilia. D F, CEP-75000-000, Brazil 



Clara E. Boeckman 

 Elizabeth C. Leal 



Universidade Federal Rural de Pernambuco, Rua Dom Manoel de Medeiros 

 s/n. Dois Irmaos, Recife-PE, Brazil 



Rosangela R T. Lessa 



Brazilian Research Council, fCNPq) 



Avenida W3 Norte, Quadra 511, Bloco A , Ed Bittar II 



Brasilia, D F, CEP-75000-000, Brazil 



Kohei Kihara 

 Kazuyuki Otsuka 



Department of Marine Science and Technology 



Tokyo University of Fisheries 



5-7, Konan-4. Minato-ku, Tokyo 1 08, Japan 



The blue shark, Prionace glauca, 

 is one of the most abundant oce- 

 anic-epipelagic sharks and is prob- 

 ably the widest ranging chon- 

 drichthyian (Compagno, 1984). It 

 is frequently caught by tuna 

 longline fisheries in temperate, 

 subtropical, and tropical waters of 

 the world oceans (Pratt, 1979). 



Hazin et al. (1990) investigated 

 the distribution and abundance of 

 pelagic sharks caught from 1983 

 until 1988 by Brazilian longliners 

 in the southwestern equatorial 

 Atlantic. Blue shark and sharks of 

 the genus Carcharhinus were the 

 dominant species, together repre- 

 senting nearly 95% of the shark 

 catches (Hazin et al., 1990). They 



474 



reported that blue shark abun- 

 dance had a marked seasonal fluc- 

 tuation with the highest catches 

 taking place during the third and 

 fourth quarters of the year and the 

 lowest in the first quarter. 



The objective of the present 

 study is to further investigate the 

 distribution and relative abundance 

 of the blue shark, Prionace glauca, 

 in the south-western equatorial At- 

 lantic Ocean, including the follow- 

 ing aspects: a) seasonal fluctuation 

 of catch per unit of effort (CPUE) as 

 related to sea surface temperature; 

 b) sex, and size and age composition 

 of blue shark catches; c) vertical dis- 

 tribution as related to the vertical 

 temperature profile. 



Material and methods 



This study was based on shark 

 catches during 50 fishing cruises 

 by a commercial tuna longliner, FV 

 Argus, from August to December 

 1987 and from February 1990 to 

 December 1991. On 325 longline 

 operations during these cruises, a 

 total of 992 blue sharks were 

 caught. The commercial longline 

 consisted of 120 baskets, each with 

 7 branch lines. The bait was fro- 

 zen Brazilian sardine, Sardinella 

 brasiliensis. Average local time of 

 set, retrieval, and mean soaking 

 time of the longline is shown in 

 Table 1. Further details of longline 

 fishing gear and methods were 

 described in Hazin ( 1986). 



The fishing ground was located 

 between lat. 2°S and 7°S and long. 

 32°W and 38°W. Fishing areas 

 were divided into segments of 1° 

 latitude x 1° longitude. The cen- 

 tral positions of the 325 longline 

 sets ( Fig. 1 ) were calculated as the 

 average latitude and longitude of 

 the beginning and end of set and 

 retrieval. Distribution of fishing 

 effort by month is presented in 

 Table 2. Blue shark relative abun- 

 dance was expressed as average 

 catch, in number of fish, per 100 

 hooks (CPUE). The mean CPUE 

 was calculated as the total catch, 

 in number of fish, divided by the 

 total fishing effort, in 100 hooks. 

 The distribution of blue shark 

 mean CPUE by segments of 1° lati- 

 tude x 1° longitude was observed 

 and its relation to ocean depth 

 analyzed. Ocean depths for each 

 longline set were calculated as the 

 weighted mean of the three clos- 

 est values read from the nautical 

 chart number 50, issued by the 

 Brazilian Navy. 



The monthly fluctuation of 

 CPUE and sex ratio was analyzed 

 and compared to sea surface tem- 

 perature. An analysis of variance 



Manuscript accepted 21 October 1993 

 Fishery Bulletin 92: 474-480 ( 1994) 



