Résumé
The vertical habitat of pelagic species in longline fisheries is a major question in terms of gear selectivity and stock assessment and so is essential to be understood for the sustainable use of the marine resources. In the longline fishery, knowledge of the habitat gives fishermen the opportunity to set their line at a specific depth depending on the time of the day and environmental conditions. It maximizes their chance to catch targeted species and limit as much as possible bycatch. For that purpose, longline fishing experiments were conducted as part of the CAPPES (Capturabilite des grands pelagiques a la palangre dans les eaux des Seychelles) project using a instrumented longline, equipped with hook timers and temperature-depth-recorders (TDR) from December 2004 to May 2006. Conductivity, temperature and depth profiles were also recorded. These experiments collected data on the vertical habitat of pelagic fishes during daytime and night. The lines were set in two different ways: shallow at night (setting after sunset and hauling before sunrise) to target swordfish as traditionally done by local fishermen; deep during the day (setting before sunrise, hauling at noon) to target tuna. For each fish caught (target or bycatch) the basket number, hook number where recorded. The depth of captured where later determined. Results show capture depths for the target species : yellowfin, bigeye and swordfish during both daytime and night-time sets. While yellowfin tuna and swordfish were mainly caught in the first 150m (both for day and night sets), bigeye tuna were captured close to the surface during night and below 150m during daytime.