Relation between Hook Depth and Fishing Efficiency in Surface Longline Gear
Surface longline gear can operate at a range of depths, and hooks placed at different depths can have different fishing efficiencies, depending on the target species and its behavior. 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 species. However, little data exist on this subject in the literature, probably due to the difficulty of obtaining information about the differential catch rates of each hook and in estimating absolute depths. Yoshihara (1951) reports that the vertical distributions of Thunnus orientalis and Germo germo caught by longline are bell-shaped, suggesting different water temperature preferences by each species. Suzuki et al. (1977) state that the same fishing methods and longline gear are frequently 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 Japanese 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 behavior and depth range of swordfish Xiphias gladius (Carey and Robison 1981) and blue shark Prionace glauca (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 between 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 swordfish 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.