Population Structure of Albacore Tuna in the Eastern Pacific Ocean

North Pacific albacore tuna (Thunnus alalunga) support the most lucrative fishery for a highly migratory species on the U.S. West Coast, and currently are assessed and managed as a single stock.  Population structure of North Pacific albacore; however, may be more complex than the current single stock hypothesis, given the apparent regional differences in growth rates and movement patterns in the eastern Pacific Ocean (EPO).  Accurately characterizing population structure and stock mixing is critical to effective management and a suite of techniques have proved highly effective including genetics, electronic tags, and natural tags (i.e. otoliths, tissues).  Otolith chemistry is one approach to investigate population structure of tunas and other fish with the principal assumption that the otolith acts as a natural tag because the chemical composition of the otolith is related to the physicochemical conditions of the water mass inhabited.  Water mass chemistry has been shown to regionally differ in the EPO, allowing scientists to retrospectively examine movement patterns of other fish species.  As such, the purpose of this study is to examine chemical signatures in the otoliths of albacore collected in two regions of the EPO that have shown limited mixing: northern region (offshore Oregon and Washington, > 40°N) and southern region (offshore California, < 40°N) and to evaluate annual and daily growth rates using otolith-based techniques.  Samples from juvenile and sub-adult (ages 1-4) albacore were collected from each region through recreational and commercial fishing vessels in 2009 and 2010.  Preliminary results suggest otolith d¹³C and d¹⁸O stable isotopes, in combination with several other elements (Ba, Ca, Li, Mg, Mn, P, Sr), were useful in discriminating similar age classes between regions with 100% classification success.  We are currently incorporating our age and growth work with otolith chemistry results with the aim of providing valuable information on the life history and population structure of this economically important species.