We conducted displacement experiments using acoustic tags to study the
orientation abilities of dolphinfish (Coryphaena hippurus) around FADs. Eleven fish were
passively monitored using coded transmitters and acoustic receivers attached
to FADs, while seven others were actively tracked with a vessel using
continuous acoustic tags equipped with pressure sensors. All these 18
dolphinfish were captured close to a FAD and released between 70 and 1720 m
from it. Five fish out of the 14 that were displaced at more than 360 m from
a FAD (estimated detection range of our acoustic receivers, defining the
target area during our experiment) returned to the target area. With one
individual successfully returning to the target area after having been
released at 1600 m from the FAD, the rate of return of fish released up to
this distance was significantly higher than expected if they were moving at
random, but many fish released at intermediate distances moved in other
directions. Based on detailed statistical analysis of the return score, we
estimated that dolphinfish are able to orientate towards a FAD area from at
least 820 m. The connection between our definition of the target area and
the association area of dolphinfish around a FAD is discussed, as well as
the impact of stress and motivation in the rates of return observed. More
experiments should be conducted to improve this first estimate. Specific
experiments should also be designed to more accurately determine the
association range of fish around FADs and their motivation to orientate
towards FADs.