Anchored or drifting FADs
It must be recalled that there is a frequent confusion between drifting FADs exploited by industrial ships and anchored FADs exploited by small-scale fisherfolk. A seiner can capture up to 200 tons of fish in a single set of seine lasting a half-day. It is what 200 martinican vessels land per year with anchored FAD fishing. Additionally, under a FAD, tunas organize in multispecific levels but homogeneously as far as size is concerned. Small tuna of several species are close to the surface and turn next to the FAD. Medium tuna are found a bit deeper and the biggest even deeper but over a larger surface than the others. The disadvantage of using a seine (those used by industrial vessels can cover up to 21 ha) is that it captures all juveniles at the surface. A seine cannot be used with anchored FADs because the mooring rope would prevent it from closing. Fisherfolk only capture a few juveniles that they use as baits. One must thus remember that the impact of anchored FADs differs from that of drifting ones. However, it remains essential to frame and assess the impact of any fishing activity on fish resources.
A FAD is an ecological trap
That hypothesis was emitted for drifting FADs. Biologists call ecological trap a situation in which individuals of a species, trusting misleading clues generally associated to human activity, colonize a habitat that turns out to be unfit for their survival. By following artificial rafts used by fisherfolk, tuna would indeed get away from their usual migrating paths and end up in unfavorable areas lacking food resources. Studies conducted on that topic could not confirm that that drifting FADs have a negative impact on tunas’ entire life cycle and are a real ecological trap. However, it seems reasonable not to deploy FADs in areas where tuna juveniles tend to concentrate. It would avoid that those young fish be lead out of the favorable areas.
To learn more
HALLIER J.P AND GAERTNER D., Drifting fish aggregation devices could act as an ecological trap for tropical tunas species, Marine ecology progress series, Vol. 353: 255–264, 2008.
FONTENEAU A, ARIZ J, GAERTNER D, NORDSTROM V, PALLARES P., Observed changes in the species composition of tuna schools in the Gulf of Guinea between 1981 and 1999, in relation with the fish aggregating device fishery. Aquatic Living Resources, 13: 253–257, 2000.