Effects of Fish Aggregating Devices (FADS) on Tuna Movement

Abstract

Tuna are often found in association with floating objects and remain near them. Taking advantage of this behavior, man-made floating objects, referred to as fish aggregating devices (FADs) are commonly used in tuna purse seine sheries. As much as half of the worlds tuna catch now comes from schools associated with FADs as their use continues to rise. Despite the prevalence of FAD, little is known about FADs effects on tuna movement. In this dissertation, I modified the Advection-Diffusion Reaction (ADR) model with the inclusion of FAD effects by modeling the attraction and hindering tendencies of FAD to tuna. In my ADR-FAD model, the advection and diffusion are modulated by FAD distribution. To verify the ADR-FAD model, more than 10,000 simulations were conducted using conventional tag release-recapture data, shing e ort data and FAD sets data as a proxy for FAD distribution. Skipjack tag data from the Pacific Tuna Tagging Programme was analyzed using the ADR-FAD model and the original ADR model. Both models show similar results but the ADR-FAD model fits signiffcantly better than the ADR model. Differences in the effects of FAD between the open and coastal ocean were found, suggesting FAD use should be managed and regulated differently between the coastal and open ocean. I also applied the ADR-FAD model to two different skipjack tag datasets from the Regional Tuna Tagging Programme (1989-1992) and the Pacific Tuna Tagging Programme (2006-present) to compare the effects of FAD over the two periods. The density of FAD increased three folds since the 1990's, and the spatial distribution differs between the two periods. FAD induced skipjack movement is significantly higher in the 2000's, while diffusive movement in the 2000's is reduced when compared to the 1990's. Even though tuna movement is tied to environmental and trophic conditions, model results indicate rapid expansion in FAD use exerts considerable influences on tuna movement.