GRANT NUMBER: NA07FD0176

REPORT TITLE: Assessment of natal origin and stock structure of Atlantic bluefin tuna using otolith elemental fingerprints

AUTHOR: Rooker, Jay R.

PUBLISH DATE: April 15, 2003

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ABSTRACT

Understanding population structure and trans-oceanic movement of Atlantic bluefin tuna is critical to optimize utilization of this highly migratory species. Due in part to increased evidence of trans-Atlantic migrations, there has been increased scrutiny by scientists and resource representatives of the two-stock hypothesis, which guides stock assessments and management projections for western Atlantic bluefin tuna. There is strong belief by commercial and recreational sectors of the bluefin tuna fishery that the harvest of undersized (sublegal) juveniles in the Eastern Atlantic is curtailing the recovery of Western stock bluefin tuna, despite stringent harvest limits that have been promulgated upon North American fisheries. An Inter-sessional Conference of ICCAT on mixing will address methods to incorporate pan-oceanic migrations by Atlantic bluefin tuna in future stock assessments under the current two stock concept. There is also a call to investigate other population structures, such as metapopulations (populations linked through migration) that currently guide Pacific salmon conservation efforts in the U.S. Whichever population structure emerges as most reasonable or useful, there is a clear need for empirical methods to directly estimate the contributions of recruits originating from eastern (Mediterranean) and western nurseries (Gulf of Mexico) to the fisheries that depend upon these recruits.

The goal of this study is to examine the two-stock issue for Atlantic bluefin tuna (Thunnus thynnus) using otolith chemistry. Specifically, otolith chemistry (trace elements in otoliths) of juveniles from eastern and western Atlantic stocks (i.e. nurseries) is being used to assess the discriminatory power of otolith elemental fingerprints for stock identification. To date, we have examined otolith chemistry of young bluefin tuna (age-0 & age-1) from the eastern (Mediterranean Sea) and western Atlantic, and assumed that no transoceanic migration activity occurred. Spatial stability of otolith elemental fingerprints was also examined on a smaller scale by measuring the otolith elemental composition of individuals from putative sub-nurseries within the Mediterranean Sea. In addition, the temporal stability and predictive potential of these natural tags were investigated by contrasting otolith chemistry of two year-classes of age-0 T. thynnus. Finally, age-0 and age-1 T. thynnus collected from the same nursery were compared to assess age-specific differences in otolith chemistry.