The Biology of FAD-Associated Tuna: Temporal Dynamics of Association and Feeding Ecology

Introduction Most tropical tunas captured by commercial and recreational fishers worldwide are “associated.” Sometimes this association is with natural structures such as seamounts or reefs or drifting logs, but increasingly these associations are with man-made fish aggregating devices—FADS. In fact, some researchers estimate that over 50%, by weight, of total tuna catch in the Indian and Pacific Oceans is now taken near FADs. FAD deployment has increased steadily over the last 25 years, and has become rampant over the last decade. Until recently, there has been very little consideration of the ecological impact of the practice. Understanding the amount of time that tuna spend in association with FADs and the trophic dynamics of FAD-associated tuna are critical to evaluating the impact of FADs on tuna distribution, their vulnerability to fishing gear, and the impact of FADs on their overall well being. To address these issues, we have embarked on a comprehensive investigation of the trophic ecology of FAD-associated tuna in Hawaiian waters and have combined this with experiments using sonic telemetry to observe the movements of tuna within the network of anchored FADs in Hawaiian waters. This report presents preliminary results and data analysis of these interrelated projects. Data presented here were obtained using three techniques: gut content analyses, isotope ratio analyses, and sonic telemetry. The telemetry involves placing coded acoustic pingers in fish captured at FADs and detecting the implanted pingers with data logging hydrophones attached to the mooring lines of the FADs.