Impact of DFAD density on tuna associative behavior and catchability in the Indian Ocean

Ecosystems and biodiversity across most of the world are
being altered by human activities. Habitat modification and
degradation is among the most important drivers of biodiversity
loss. These modifications can have an impact on
species behavior, which can in turn impact their mortality.
The use of Drifting Fish Aggregating Devices (DFADs) by
purse seine fisheries is a major concern and offers a good
case study to assess the impact of habitat modifications
on species behavior and mortality. Because several pelagic
fish species, such as tuna, associate with floating objects,
fishers have started deploying their own floating objects
– DFADs – in the early 1990s to increase tuna catchability.
The massive deployment of DFADs has modified tuna
habitat, by increasing the density of floating objects, with
potential consequences on tuna associative behavior. In
this study we use an individual-based model, based on a
correlated random walk calibrated on passive acoustic tagging
data, to determine a general relationship between FAD
density and the time tuna spend between two associations
with a FAD. Using this general relationship and fisheries
data in the Indian Ocean (IO), we predict that tuna spend
a high percentage of their time (up to 85 %) associated to
DFADs in the western IO, where purse seine fishing pressure
on DFADs is highest. Hence, purse seine fisheries modify tuna habitat by increasing DFAD density which in turn
impacts tuna mortality, through a modification of their associative
behavior. As DFAD density is directly linked to
tuna fishing mortality, there is an urgent need to continue
regulation efforts on DFAD deployments.