725 Transactions of the American Fisheries Society 130:725–741, 2001 q Copyright by the American Fisheries Society 2001 Identification of Freshwater and Anadromous Inconnu in the Mackenzie River System by Analysis of Otolith Strontium KIMBERLY L. HOWLAND* AND WILLIAM M. TONN Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada JOHN A. BABALUK AND ROSS F. TALLMAN Department of Fisheries and Oceans, Freshwater Institute, 501 University Crescent, Winnipeg Manitoba R3T 2N6, Canada Abstract.—Because trace elements from the environment are incorporated into the otoliths of fish as they grow, otolith microchemistry can be used to reconstruct key aspects of their environ- mental history, such as migratory patterns. We used scanning proton microprobe analysis on otoliths of inconnu Stenodus leucichthys from the Mackenzie River system (Northwest Territories, Canada) to determine whether patterns of strontium (Sr) distribution supported the hypothesis that some populations are anadromous while others remain in a low-Sr freshwater environment. The otoliths of inconnu from the Slave River had low, flat Sr profiles, indicating that these fish spent their entire lives within a freshwater environment. In contrast, while the otoliths of inconnu from the Arctic Red River varied substantially, they generally had low, flat profiles near the core followed by oscillating patterns of high and low Sr through to the outer edge of the otolith. These latter profiles indicate that these fish spent their first 1–2 years in a freshwater environment and then moved annually into a higher-Sr environment, such as the Mackenzie estuary or Beaufort Sea. A subset of fish from the Arctic Red River, primarily males, revealed smaller annular maxima or ranges of Sr concentrations and were considered partially anadromous. Because the salinity of the environment consistently correlates strongly with Sr levels in otoliths, otolith Sr can provide a reliable tool for examining the life history and migratory patterns of diadromous species that are otherwise difficult to study directly. The otoliths of teleost fish form through the de- position of alternating layers of calcium carbonate (in the form of aragonite crystals) and the protein otolin from the surrounding endolymphatic fluid (Mugiya et al. 1981; Gauldie and Nelson 1988). Trace elements from the environment, such as strontium (Sr), become incorporated into otoliths as they grow (Kalish 1989). Otolith microchem- istry, which involves the analysis of these trace elements, allows researchers to reconstruct the en- vironmental histories of fish (e.g., Secor 1999; Thresher 1999). Analysis of otolith Sr levels has proven partic- ularly useful in determining migratory patterns of fishes (Kalish 1990; Rieman et al. 1994 ; Halden et al. 1995, 1996) and is especially appealing for investigating such questions in large river systems or with widely ranging species for which tagging studies alone may not provide sufficient infor- mation (Limburg, in press). The Sr concentration of water is known to increase in direct proportion to its salinity (Ingram and Sloan 1992), ranging * Corresponding author: howlandk@dfo-mpo.gc.ca Received June 21, 2000; accepted March 19, 2001 from 0.06 mg/L in freshwater to 8.0 mg/L in full- strength seawater (Rosenthal et al. 1970). It has been assumed that the Sr content of otoliths reflects the salinities encountered by a fish throughout its lifetime (Secor 1992; Halden et al. 1995); recent studies have found strong correlations between otolith Sr levels and both the salinity (Fowler et al. 1995a, 1995b; Secor et al. 1995) and Sr content (Mugiya and Tanaka 1995; Farrell and Campana 1996) of the surrounding environment. We analyzed otolith Sr content to address ques- tions regarding the life history and migratory be- havior of inconnu Stenodus leucichthys in the Mac- kenzie River system, Northwest Territories, Can- ada. The inconnu is a highly migratory whitefish (coregonine) commonly found in the large north- ern rivers and associated lakes of northwestern North America and northern Eurasia (McPhail and Lindsey 1970). This species shows a high degree of interpopulation variation throughout its range, with both anadromous and resident (nonanadrom- ous) populations known to occur in Siberia (Berg 1948) and Alaska (Alt 1977, 1988). By analogy, it was suggested that inconnu in the Mackenzie River system also comprise both life history types