Tracking Nursery Habitat Use in the York River Estuary, Virginia, by Young American Shad Using Stable Isotopes

—We developed and applied a stable isotope turnover model to estimate how long age-0 American shad Alosa sapidissima reside within tidal freshwater and brackish-water habitats in the York River estuary, Virginia. The residence time was estimated by modeling the changing stable isotope ratio (either the carbon [dC] or sulfur [dS] stable isotope ratio) of an age-0 American shad as it migrates seaward from its present habitat to a new habitat and determining the minimum time required to acquire the isotopic signature of its new habitat. A sensitivity analysis of our turnover model indicates that the results are robust at relatively fast turnover rates, such as those experienced by young fish, but that at slow turnover rates the model can yield biologically meaningful differences with relatively small changes in variables. The average 6 SE isotopic ratios for the dorsal muscle tissue of age-0 fish increased along the estuary, from 31.8 6 0.3% for dC and 5.2 6 0.7% for dS at the farthest upriver region to 21.8 6 1.2% for dC and 10.3 6 1.7% for dS in the lower estuary, and were significantly different among regions. To account for these distinct signatures along the estuary, the turnover model predicts that age-0 fish remain in discrete regions (;10 river kilometers) of the tidal freshwater portion of the river for at least 15–45 d and in the lower estuary for at least 32–66 d. Juveniles, therefore, are spatially segregated, and probably migrate slowly downstream during the summer and early fall, accumulating in the lower freshwater and oligohaline portions of the estuary before their oceanic migration. American shad Alosa sapidissima, the largest North American anadromous clupeid, migrate from their freshwater nursery habitat to the ocean in their first year of life. Investigators propose both temperaturebased migration, consistent with fall migration (e.g., Leggett and Whitney 1972; Williams and Bruger 1972; O’Leary and Kynard 1986; Hoffman et al., in press) and size-based migration, migration occurring as early as June (Marcy 1976; Limburg 1996). A detailed analysis of the age-structure of Hudson River age-0 American shad suggests that age-0 fish move about 120 km downriver in about 2 weeks (Limburg 1996), implying rapid movement through their freshwater habitat. Given the coast-wide concerns regarding American shad population declines (ASMFC 1999), which are, in part, due to habitat degradation (e.g., Limburg et al. 2003), further measures of age-0 American shad habitat use, including the spatial scale, duration, and rate of seaward migration, are needed. Stable isotopes have been applied to document movement patterns in a variety of animals (Hobson 1999). Certain stable isotope ratios of consumers, such as the carbon (C) stable isotope ratio, tend to closely resemble their prey (DeNiro and Epstein 1978; Fry and Sherr 1984). Stable isotopes, therefore, offer the potential to track habitat changes when either (1) movement is related to ontogenetic niche (diet) shifts or (2) the habitats of interest possess distinct biogeochemical properties, such as a switch from fresh to marine waters, and the rate of movement between habitats is faster than the rate of change of the isotope ratio in the animal’s tissue (Hobson 1999). Thus, the animal arrives at the new habitat with an isotopic composition indicative of its previous habitat. After settling into the new habitat, the animal’s tissues will acquire the isotope ratio specific to that habitat. The ability to quantify the timing of these movements (or diet shifts) is dependent on the turnover rate in the tissue being analyzed (Tieszen et al. 1983; Hobson and Clark 1992). The turnover rate, in turn, depends on growth and metabolism. Growth turnover occurs as new tissue is added, diluting the pool of older tissue derived from the previous diet. Metabolic turnover occurs as older tissue is broken down and new tissue synthesized. In both slowly and rapidly growing fish, growth dominates stable isotope turnover (Fry and Arnold 1982; Hesslein et al. 1993; Herzka and Holt 2000; MacAvoy et al. 2001; Sakano et al. 2005) and metabolism can accelerate the turnover rate beyond that exerted by growth alone (Frazer et al. 1997; Vander Zanden et al. 1998; Herzka et al. 2001; Trueman et al. 2005). * Corresponding author: [email protected] 1 Present address: U.S. Environmental Protection Agency, National Health and Ecological Effects Research Laboratory, Mid-Continent Ecology Division, 6201 Congdon Boulevard, Duluth, Minnesota 55804, USA. Received October 5, 2006; accepted May 3, 2007 Published online August 2, 2007 1285 Transactions of the American Fisheries Society 136:1285–1297, 2007 Copyright by the American Fisheries Society 2007 DOI: 10.1577/T06-223.1 [Article]