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

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 the Bismarck 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 vertical 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 categories was then calculated to examine potential vulnerability to industrial purseseine 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 behavior of skipjack, yellowfin and bigeye tuna that were simultaneously present at the same FAD, as determined by depth transmitting acoustic tags, suggested some vertical separation of these species. However, there was a high degree of depth overlap, particularly during early morning hours when purse seining on floating objects normally 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 observation of the natural vertical behavior of these fish when they were close to anchored B. Leroy (B) Oceanic Fisheries Programme, Secretariat of the Pacific Community, B.P. D5, 98848 Noumea Cedex, New Caledonia e-mail: [email protected] J.L. Nielsen et al. (eds.), Tagging and Tracking of Marine Animals with Electronic Devices, Reviews: Methods and Technologies in Fish Biology and Fisheries 9, DOI 10.1007/978-1-4020-9640-2 10, C © Springer Science+Business Media B.V. 2009 161