730 



Fishery Bulletin 89(4). 1991 



Spanish fishermen operating in the Atlantic Ocean and 

 Mediterranean Sea according to their experience and 

 customs. 



A total of 706 baskets were deployed during 16 noc- 

 turnal fishing operations in April 1985, located between 

 10°27'-11°42'N lat. and 17°17'-17°45'W long. Each 

 fish was identified, sexed, and measured. The hook 

 number within each basket was also recorded. A sum- 

 mary of the species caught and their abundances will 

 be reported in a separate paper (Rey and Munoz- 

 Chapuli, in press). 



Maximum depth reached by the deepest hook (posi- 

 tion 17) was estimated to be between 370 and 460 m, 

 using the theoretical procedure described by Yoshihara 

 (1951, 1954). 



Hook catch rates (HR) per 100 hooks were calculated 

 using: 



compared catches of regular (to 150 m depth) and deep 

 (250-300 m depth) tuna longlines in the western and 

 central equatorial Pacific Ocean. They recorded a 

 higher HR for swordfish using shallower gear. Carey 

 and Robison (1981) observed, through radio-tracking, 

 that in the Atlantic Ocean swordfish follow isoluminic 

 trajectories. They were located in depths of 400-600 m 

 during the day and 0-1 70 m at night. However, in the 

 Pacific Ocean the depth range recorded for swordfish 

 was 50-100 m during the day and 0-70 m at night. This 

 could explain the slightly deeper maximum depth of 

 fishing efficiency we recorded in the tropical eastern 

 Atlantic Ocean, compared with that of Suzuki et al. 

 However, Yang and Gong (1987) found a higher sword- 

 fish catch per unit of effort around 150 m in the cen- 

 tral Atlantic (0°-10°N). 



HR = (number of fish caught/number of hooks set in the jth position) x 100. 



A total of 1412 hooks were set in posi- 

 tions 1-16, while 706 hooks were set in 

 position 17. 



Results and discussion 



Table 1 shows the hook catch rates (HR) 

 obtained for total catch, swordfish, and 

 mako shark which was the most valuable 

 species in the bycatch. Composition of 

 bycatch is also presented in this table. A 

 maximum HR of more than 5 fish/100 

 hooks for the total catch was observed 

 between hook positions 3 and 5 (Fig. 2A). 

 Values were generally lower beyond posi- 

 tion 10, where the HR values were 2.8- 

 4.6 fish/100 hooks. 



The highest HR for swordfish was 

 recorded at hook positions 3-13 (Fig. 2B). 

 Mako sharks were captured mainly be- 

 tween hook positions 5 and 8 (Fig. 2C). 

 They were never captured at hook posi- 

 tion 15 or 17. 



Table 1 includes the proportion of by- 

 catch (total catch less swordfish and 

 mako shark). As can be observed in Fig- 

 ure 2D, hook positions 14-17 show the 

 highest HRs for the bycatch proportion. 

 This indicates that these lower hooks are 

 less effective for catching both swordfish 

 and mako shark, which are caught in 

 higher numbers on hooks 3-13. 



Few data exist in the literature to com- 

 pare with our results. Suzuki et al. (1977) 



Table 1 



Hook catch rates of total fish, swordfish Xiphias gladius, and mako shark Isurus 

 oxyrinchus during 16 nocturnal fishing operations in the tropical eastern Atlan- 

 tic Ocean. Right column shows proportion of "other species" in the total catch. 

 Correlation coefficients (r) from the linear relationship between hook position 

 and HR/proportion other species are also shown. HR = no. hooks with 

 catch/no. hooks set in jth position x 100 (see text). 



* 55.9% Car char hinus signatus, 17.0% C. falciformis, 9.3% Alopias superci- 

 liosus, 7.8% Prionace glauca, 5.5% Mobula sp., 1% Sphyrna lewini. Less 

 than 1%: Sphyrna couardi, S. mokarran, S. zygaena, Centrophorus granu- 

 losus, Galeocerdo cuvieri, Isurus paueus, Carcharhinus plumbeus, Thunnus 

 obesus, Lepidocybium solanderi, Alepisaurus ferox. 

 "P<0.01. 



