From April 2003 to April 2004 monthly sea surveys were
conducted around 2 fish aggregating devices (FADs) moored at 2000 and 2500 m
depth in Martinique (Lesser Antilles). The use of a dual frequency splitbeam
echosounder combined with an underwater camera and fishing methods allowed
assessment of average space and time distribution of pelagic fish aggregated
beneath the FADs, as well as identification of their overall size and
species composition. At daytime, 4 fish aggregations were identified at each
FAD, representing 4 distinct types: i) an aggregation of small juvenile tuna
(mean fork length, FL: 30 cm) observed very close to the surface in 25%
of daytime periods; ii) a small surface aggregation dominated by carangids,
Caranx crysos, present in 65% of daytime periods; iii) a large sub-surface aggregation
observed during all daytime periods: this aggregation appeared to be
primarily comprised of 58 cm FL blackfin tuna (Thunnus atlanticus), mixed with yellowfin
(Thunnus albacares) and skipjack (Katsuwonus pelamis) tunas of the same size; and iv) sub-surface scattered large predators (mainly blue marlin, Makaira nigricans) present in 10% of daytime periods. A smaller sub-surface aggregation comprised of medium tuna mixed with
“extranatants” (fishes which remain within 10 to 50 m of a FAD) was observed
in 75% of night-time periods, whereas unidentified scattered fishes were
detected from 70 to 400 m depth. The low daytime vulnerability of medium
sub-surface tunas to applied line techniques leads us to assume that their
feeding motivation was low during daytime. These tunas could preferentially
feed on mesopelagic organisms during night-time and transition periods
around Martinican moored FADs. Local fishermen mainly targeted the large
scattered predators using very small tunas as living bait. The sub-surface
tuna aggregation hence appeared to be currently unexploited by local
fisheries, though it represented the large majority of the pelagic biomass
around the moored FADs.