Résumé
Habitat of pelagic species in the 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. Hence 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 line instrumented with hook timers and temperature-depthrecorders (TDR) from December 2004 to 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 depth where it occurred was determined using TDR data, the shape of the line (depending on the gear configuration) and the number of the hook on which the fish was caught. Results show the difference of habitat for the targeted or non-targeted species during daytime and night-time. Differences are observed for the depth of the catch, time of occurrences and temperature of the water layer. Results are discussed in terms of diversification of the fishing techniques and mitigation measures for reducing bycatch.