Abundance trends of French tropical tuna purse seine fisheries bycatch species in the Eastern Atlantic Ocean

Accurately assessing the abundance trends of bycatch species impacted by tropical tuna fisheries is a major challenge due to limited data and the rarity of many species. This study used observer data collected from French tropical tuna purse seine vessels operating in the East Central Atlantic Ocean between 2013 and 2022 to develop standardised abundance indices and assess temporal trends for 16 major bycatch species, including sharks, rays, billfishes and sea turtles. For each bycatch species, a delta-modeling approach was applied, with a first component modeling bycatch occurrence and a second component to model bycatch abundance conditional on presence. Each model was fitted using a Generalised Additive Mixed Model (GAMM). The results revealed stable or declining trends for most species, while silky sharks, devil rays and olive ridley turtles showed increases. These standardised indices provide to our knowledge the most extensive, concurrent fleet-specific fishery-dependent abundance estimates for multiple tropical Atlantic pelagic bycatch species caught on dFADs, and they represent valuable inputs for future stock assessments, ecological risk assessments, and eventually bycatch mitigation planning.