There is currently inadequate scientific information to conclude if deployments of fish-aggregating devices (FADs) induce adverse impacts on tropical tunas, and thus function as “ecological traps.” This workshop was structured with the following three sections:
1) potential changes in habitat due to FADs, 2) effects of FADs on tuna behavior and movements (by retaining them or by carrying them into locations that were not part of their original migratory patterns), and 3) effects of FADs on the biology of tunas through changes in diet, growth, reproductive success, etc.
Natural floating objects (e.g. logs, branches, etc.) have always been components of the habitat of tunas, with densities varying by area and season. In the recent years, fishers have deployed a large number of FADs in the ocean and it is essential to evaluate the degree to which this FAD deployment practice has changed the habitat of tunas (as opposed to natural floating objects) in each oceanic region and during each season. A first step in this process requires the estimation of the densities of floating objects within the habitat of tropical tunas, including distances between adjacent floating objects, and compare data for natural logs with data for all types of floating objects within the same spatial strata.
Some data tend to show that FADs may modify the large-scale movement behavior of tunas, but it is not yet possible to identify whether FADs are the main factor responsible for these changes, or if they are also due to environmental factors. There is limited scientific information on the processes involved with the associative behavior of tropical tunas, which impairs our ability to assess whether FADs can impact the large-scale movements of tunas. The commonalities and differences between anchored and drifting FADs in terms of behavioral responses were underlined and should be considered in the design of the future experiments.
Differences in the condition factors of tunas in free-swimming schools and those associated with floating objects have been observed, but with opposing results depending on the areas investigated. Therefore it remains uncertain if or how FADs may affect the biology of tunas, and a more accurate assessment of condition would be necessary.
Major gaps in our knowledge on this topic were identified during the workshop. Research dedicated to investigate this topic is needed if one wants to assess whether the deployments of FADs adversely affects the ecology of tunas.
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