Associative behavior of yellowfin Thunnus albacares, skipjack Katsuwonus pelamis, and bigeye tuna T. obesus at anchored fish aggregating devices (FADs) off the coast of Mauritius

Abstract

Anchored fish aggregating devices (FADs) are deployed by fishermen worldwide to facilitate the capture of pelagic fish. We investigated the associative behavior of yellowfin Thunnus albacares, skipjack Katsuwonus pelamis, and bigeye tuna T. obesus in an array of anchored FADs off the coast of Mauritius (southwestern Indian Ocean) using passive acoustic telemetry. Our results suggest that yellowfin and bigeye tuna have longer FAD residence times than skipjack tuna. The survival curves based on the continuous residence times for bigeye and skipjack tuna were best explained by single exponential models, indicating time-independent associative processes and characteristic timescales of 4.3 and 0.9 d, respectively. Continuous residence times of yellowfin tuna were best explained by time-dependent power law models, but the single exponential model (characteristic timescale of 6.5 d) also fit the data well. The analysis of absence times (time between 2 FAD associations) revealed that single exponential models fit the data for all 3 species (characteristic timescales of 1.3, 5, and 2.7 d for yellowfin, skipjack, and bigeye, respectively), with a time-dependent sigmoidal component at short timescales for skipjack and bigeye tuna, ascribed to diel behavior and the short inter-FAD distances of the array. Our results are consistent with those of previous studies but also reveal common behavioral patterns among species and suggest that inter-FAD distances affect absence times but not residence times. In other words, high densities of FADs tend to decrease the amount of time tuna spend unassociated with FADs.