Relation between Hook Depth 

 and Fishing Efficiency in 

 Surface Longline Gear 



Juan Carlos Rey 



Spanish Institute of Oceanography, Coastal Center of Fuengirola 

 P.O. Box 285, Fuengirola, Malaga, Spain 



Ramon Murioz-Chapuli 



Department of Animal Biology, Faculty of Science 

 University of Malaga, 29071 Malaga, Spam 



Surface longline gear can operate at 

 a range of depths, and hooks placed 

 at different depths can have differ- 

 ent fishing efficiencies, depending 

 on the target species and its behav- 

 ior. With better knowledge of the 

 relationship between hook depth 

 and hook catch rates, catch rates 

 could be improved by placing the 

 majority of hooks at the depth 

 range preferred by the target spe- 



cies. However, little data exist on 

 this subject in the literature, prob- 

 ably due to the difficulty of obtain- 

 ing information about the differen- 

 tial catch rates of each hook and in 

 estimating absolute depths. Yoshi- 

 hara (1951) reports that the vertical 

 distributions of Thunnus orientalis 

 and Germo germo caught by long- 

 line are bell-shaped, suggesting dif- 

 ferent water temperature prefer- 



Figure 1 



Scheme of the longline used in this study. Numbers show hook position. 



ences by each species. Suzuki et al. 

 (1977) state that the same fishing 

 methods and longline gear are fre- 

 quently used regardless of the areas 

 and species. These authors also 

 remark that there have been no 

 systematic studies on the vertical 

 distribution of tunas and billfishes. 

 A well-known exception was in the 

 1970s when the Korean and Japa- 

 nese commercial longliners changed 

 from regularly using the gear near 

 surface to fish for bigeye tuna to 

 using it at greater depths to target 

 yellowfin tuna Thunnus albacares 

 (Saito 1975, Suzuki et al. 1977, 

 Yang and Gong 1987). There is also 

 some information about the behav- 

 ior and depth range of swordfish 

 Xiphias gladius (Carey and Robison 

 1981) and blue shark Prionace glau- 

 ca (Sciarrota and Nelson 1977) in 

 the literature. 



Our aim was to obtain a simple, 

 statistical relationship between the 

 fishing efficiency of each hook in a 

 basket (a stretch of longline be- 

 tween two floats) and its relative 

 depth. We used data from a tropical 

 eastern Atlantic Ocean fishing trip, 

 where commercial longline gear 

 was regularly used by southern 

 Spanish fishermen to catch sword- 

 fish as a primary target species and 

 mako shark Isurus oxyrinchus as 

 the most valuable bycatch species. 

 The study was carried out on the 

 gross catch, in which the target 

 species, swordfish and mako shark, 

 comprised only 5-21% of the total 

 catch. 



Methods 



The longline gear employed in this 

 study consisted of "baskets" be- 

 tween floats (Fig. 1). Float lines 

 measured about 7m, while the main 

 line within each basket measured 

 about 1200m. Branch lines (33 per 

 basket) were 15 m long. This stan- 

 dard gear design is modified by 



Manuscript accepted 1 July 1991. 

 Fishery Bulletin, U.S. 89:729-732 (1991). 



729 



