Vertical Behavior and the Observation of FAD Effects on Tropical Tuna in the Warm-Pool of the Western Pacific Ocean

Abstract

Archival and acoustic tagging were carried out in the exclusive economic zone (EEZ) of Papua New Guinea (PNG) in the western Pacific Ocean during 2006–07 to investigate the vertical behavior of tropical tuna found in association with large arrays of anchored FADs. Industrialized purse-seine fishing on anchored FADs has existed in the PNG EEZ for more than a decade. Archival tags were implanted in bigeye (n = 40; length 40–90 cm FL) and yellowfin (214; length 42–126 cm FL) tuna in theBismarck and Solomon Seas. Acoustic tags were released in the same areas in 195 tuna (10 bigeye, 116 yellowfin, 69 skipjack). In addition, 27 tuna (eight bigeye, 19 yellowfin) received both an archival and an acoustic tag. Archival tag data from 32 recaptures were categorized into the three distinct verti- cal behavior modes for bigeye, and the three distinct modes for yellowfin that have been described in the published literature. The depth distribution for each of the cat- egories was then calculated to examine potential vulnerability to industrial purse- seine capture in this region. A region-specific analysis was considered important as oceanographic conditions in this region are distinctly different to the conditions reported in the published literature from other locations. Analysis of acoustic data reveals short residence times at FADs and strong school cohesion. Vertical behav- ior of skipjack, yellowfin and bigeye tuna that were simultaneously present at the same FAD, as determined by depth transmitting acoustic tags, suggested some ver- tical separation of these species. However, there was a high degree of depth overlap, particularly during early morning hours when purse seining on floating objects nor- mally occurs in this region. This overlap limits the potential for targeting particular species or size classes of tuna through fishing depth selection. This observation was confirmed from the archival tag depth records for yellowfin and bigeye during the same time period regardless of behavior type exhibited. The recapture of bigeye and yellowfin tuna implanted with both acoustic and archival tags allowed the observa- tion of the natural vertical behavior of these fish when they were close to anchored FADs equipped with acoustic receivers. Occupation of shallow (<100 m) depths over a 24 h period was identified as the dominant behavior exhibited on FADs.