We adapted a visual census method, mainly used in demersal and reef fish
studies, to characterize fish communities associated to drifting fish
aggregating devices (FADs) in the Western Indian Ocean. Drifting FAD
associated fishes from both equatorial (Seychelles) and tropical waters
(Reunion Island) were examined by divers. A total of 32 species (belonging
to 16 families) were observed associated with drifting FADs in equatorial
waters, and 24 species (14 families) were found around FADs in tropical
waters. Twenty species were found in both regions. The highest number of
species observed at a single FAD was 18 (12 ± 2, mean±SD) in
equatorial and 13 (10 ± 3) in tropical waters, not counting
circumnatant species loosely associated with the FAD. Some species like
Kyphosus vaigiensis, Canthidermis maculata, Elagatis bipinnulata,
Acanthocybium solandri and Coryphaena hippurus were observed on all or most
of the surveys. In this study, the contribution in biomass of the 18 common
species associated with drifting FADs (but excluding circumnatant species),
represents more than 98% of the biomass. The overall biomass values of
closely associated species remains well below tuna biomass estimates for
circumnatant tuna schools at FADs, estimated as high as 200 tons. The
species that most significantly contribute to the by-catch in tuna
purse-seines logically match those that showing the highest biomass values
in our surveys (Carcharhinus spp., Elagatis bipinnulata, Coryphaena
hippurus, Canthidermis maculata, and Acanthocybium solandri). One of the
most abundant and ubiquitous species in our study was the spotted oceanic
triggerfish Canthidermis maculata that sometimes formed massive schools of
many thousands individuals around the drifting FADs. Future research is
needed to explore the role of such non tuna species in the attraction and
aggregation processes of tuna around drifting FADs.