Tuna, floating objects, and habitat suitability: an analysis of the indicator-log hypothesis in the Western Indian ocean

Tropical tunas are known to associate with floating objects, forming large aggregations around them. Taking advantage of this behavior, fishers deploy artificial floating objects (known as Fish Aggregating Devices, or FADs) worldwide to facilitate their fishing activities. The lack of understanding of why tuna associate with these devices hinders the assessment of their ecological impacts. The “indicator-log” hypothesis posits that floating objects of natural origin (NLOGs) concentrate in productive areas and that tunas associate with them to reach and remain in these areas. This study tests the validity of the indicator-log hypothesis, by testing if the presence and abundance of NLOGs are positively correlated with environmental variables that serve as proxies for habitat preference and suitability for tropical tunas. Using data (1,550 observations of NLOGs) recorded by scientific observers on-board purse seine vessels from 2014 to 2022 in the Western Indian Ocean, along with Lagrangian simulations of NLOG densities in the same area, we found no relationship between NLOG abundance and environmental variables in oceanic regions. We did find a weak correlation only in more coastal areas (Mozambique Channel). Consequently, at both spatial and temporal scales considered in this analysis (2°/month and 1°/week), NLOGs do not appear to constitute environmental cues that can be used by tuna to identify productive oceanic areas. Additional research is needed to explore other hypotheses regarding tuna associative behavior, which may also imply potential detrimental impacts of FADs on tuna populations.