Using Trace Elements in Pectoral Fin Rays to Assess Life History Movements in Sturgeon: Estimating Age at Initial Seawater Entry in Klamath River Green Sturgeon PETER J. ALLEN* 1 Department of Wildlife, Fish, and Conservation Biology, University of California at Davis, 1 Shields Avenue, Davis, California 95616, USA JAMES A. HOBBS Bodega Marine Laboratory, University of California at Davis, 2099 Westside Road, Bodega Bay, California 94923, USA JOSEPH J. CECH,JR. Department of Wildlife, Fish, and Conservation Biology, University of California at Davis, 1 Shields Avenue, Davis, California 95616, USA JOEL P. VAN EENENNAAM AND SERGE I. DOROSHOV Department of Animal Science, University of California at Davis, 1 Shields Avenue, Davis, California 95616, USA Abstract.—To investigate the timing of the initial entry of green sturgeon Acipenser medirostris into seawater, we examined the ratios of strontium and barium to calcium in pectoral fin rays via laser ablation– inductively coupled plasma–mass spectrometry. In a reference group of six hatchery fish that was reared 1–2 years in freshwater and 1–3 years in seawater, we found a significant increase in the Sr:Ca ratio, a decrease in the Ba:Ca ratio, and an increase in the Sr:Ba ratio in calcified growth zones during the transition from freshwater to seawater. In 10 wild adults captured in the Klamath River, California, combined evaluation of the Sr:Ca and Ba:Ca ratios provided a clearer understanding of fish movements than the Sr:Ba ratio, probably because wild fish have a more complex life history. The Ba:Ca ratio dropped significantly between growth zones 1 and 2 (ages 0.5–1.5), indicating a transition into saline waters, such as the Klamath River estuary. The Sr:Ca ratio increased slightly in the same location but attained the values found in reference fish held in seawater between growth zones 3 and 4 (ages 2.5–3.5). The Sr:Ca, Ba:Ca, and Sr:Ba ratios in growth zones 4– 6 (ages 4.5–6.5) of wild fish were similar to those found in reference fish held in seawater and corroborated the results of previous physiological studies as well as the limited field data. These results demonstrate the usefulness of trace element analyses of green sturgeon pectoral fin rays and confirm an early age of entry into seawater for this species. Trace element analysis of calcified structures in fishes has proven useful for retracing life history changes in anadromous species (Secor et al. 1995; Zimmerman and Reeves 2000; Elsdon and Gillanders 2003). This type of analysis has the potential to be particularly useful with fishes that have been difficult to study through classical field techniques. Samples from rare or difficult-to-sample fishes can be analyzed for trace elements to reconstruct life history–based movements. Threatened and endangered species par- ticularly lend themselves to these types of studies because reference samples may be all that are available for use. The transition between freshwater and seawater habitat use has been identified by changes in trace elements in the otoliths of a number of different species (Secor and Piccoli 1996; Thorrold et al. 1998; Crook et al. 2006; Clarke et al. 2007). Strontium : calcium ratios have typically been analyzed because of the large variations in Sr between freshwater and seawater and the relative abundance of this trace element in seawater. Barium, an element often found in higher ratios in freshwater, has also proven useful for differentiating freshwater and estuarine residence from seawater residence (Secor and Piccoli 1996; Crook et al. 2006). Trace element studies aimed at retracing changes in life history have been very limited in sturgeon species (Acipenseridae). Sturgeon otoliths are unique in that * Corresponding author: allenp@cc.umanitoba.ca 1 Present address: Department of Biological Sciences, Duff Roblin Building, University of Manitoba, Winnipeg, Man- itoba R3T 2N2, Canada. Received April 9, 2008; accepted August 8, 2008 Published online February 23, 2009 240 Transactions of the American Fisheries Society 138:240–250, 2009 Ó Copyright by the American Fisheries Society 2009 DOI: 10.1577/T08-061.1 [Article]