Fish-aggregating devices (FADs) are increasingly used worldwide to enhance tuna catches. Meanwhile, ecosystem-based management of this fishery is constrained by a paucity of information regarding the interaction of FAD-associated tuna aggregations with their local environment. This paper reports the results of a nine-month study around a FAD moored near Martinique Island, aimed at assessing the effects of the local environment on the variability of monthly estimates of proximate tuna biomass. Dual-frequency, active acoustics provided high-resolution quantitative data on the pelagic community around the FAD, from fish to micronekton forage. Geostatistics were used to compute biomass estimates of the tuna aggregation comprising most of the FAD-associated fish biomass, with a sampling error of 27%. Environmental variability was summarized by a small set of principal components (PCs) derived from profiles of temperature, salinity, and dissolved oxygen vs. depth; and maps of chlorophyll a derived from remotely sensed, sea surface colour. A generalized linear model was used to relate tuna biomass to environmental PCs and revealed a positive correlation between tuna abundance and: (i) a micronekton layer sensed at 38 kHz and potentially consisting of preferred prey at ∼180-m depth; and (ii) low subsurface salinity (60-80 m). These favourable environmental conditions may be related to the presence of North Brazilian Current eddies that migrating tuna follow when not temporally associated with the FADs.