SCIENTIFIC COMMITTEE FOURTH REGULAR SESSION 11-22 August 2008 Port Moresby, Papua New Guinea ORIGIN OF YELLOWFIN TUNA (Thunnus albacares) IN THE HAWAIIAN ISLANDS: PRELIMINARY ASSESSMENT OF NATAL SIGNATURES IN OTOLITHS WCPFC-SC4-2008/BI-WP-2

The purpose of this project is to examine the potential of using tuna hard parts as natural markers to refine our understanding of population structure of tropical tuna. Yellowfin tuna (Thunnus albacares) were selected for study as tagging data and recent studies suggest that the movement patterns of yellowfin may be highly variable and more restricted than other tunas. Hawaiian yellowfin provide a good test case as local spawning is well documented but immigration of yellowfin from other areas is likely. This study will provide information on the source(s) of yellowfin recruits (age-1 and age-2) to Hawaii-based surface fisheries through the analysis of natural tracers (stable isotopes) in otoliths. Our first step will be to develop a baseline that describes the chemical signatures in the otoliths of age-0 yellowfin from putative spawning and/or nursery areas in Hawaii and the broader WCPO. The isotopic composition of age-0 bigeye tuna (T. obesus) will also be analyzed to provide a comparison analysis of a species that contrary to yellowfin, is not known to spawn in Hawaiian waters. Preliminary data from age-0 yellowfin tuna indicate stable isotopic composition in otoliths of individuals from the Hawaiian Islands, an offshore seamount area, and the Equatorial Western and Central Pacific were significantly different (MANOVA P< 0.01). In general, otoliths of yellowfin from the Equatorial regions were depleted relative to areas in and around Hawaii, with intermediate values observed for individuals collected from an offshore seamount in the Hawaii EEZ. Cross validated classification success from quadratic discriminant function analysis was 88%, indicating the approach has promise for identifying yellowfin tuna from different spawning sites. After building our baseline data set to include likely nursery areas, otolith core material from age-1 and age-2 yellowfin tuna from the Hawaii-based fisheries will be compared to baseline data using mix-stock procedures. This will allow us for the first time to investigate the relative contribution of locally spawned versus transient yellowfin tuna to Hawaii-based fisheries.