Résumé
Classically, pelagic fish are supposed to disperse at night and aggregate in schools at daytime. This dynamics is usually attributed to pressure (lateral line) and light intensity signals acting through inter-individuals communication features. This implicates that trophic relationships have to fit these constraints. However, offshore, prey communities perform vertical migrations and are out of reach for most of pelagic fish during the day. To investigate this apparent contradiction, we studied the 3D spatial strategy of jack mackerel (Trachurus murphyi) according to the abiotic and biotic conditions of the habitat, through an integrative approach. Parameters taken into account were: temperature, salinity, water masses, oxygen and related gradients, prey distribution and dynamics during the diel cycle. Data came from three acoustic surveys performed in Central Chile in 1997, 1998 and 1999 during very different environmental conditions (El Nino - La Nina). Our results showed that jack mackerel presented an 'atypical' behavior being more aggregated during the night than during the day. This pattern was related to its nocturnal active foraging behavior on the migrant mesopelagic community. Actually, jack mackerel aggregative behavior could depends more on prey diel dynamics than on pressure and light thresholds. During the day its vertical distribution was related to specific hydrologic conditions, producing fluffy aggregations. During the night jack mackerel distribution appeared driven by prey independently of environmental abiotic conditions. It formed active dense schools and layers. In synthesis, we hypothesize that oceanographic conditions determine jack mackerel vertical distribution by day when "resting". By night, foraging activity determines jack mackerel distribution and behavior.