Transactions of the American Fisheries Society 136:1285–1297, 2007 [Article] Ó Copyright by the American Fisheries Society 2007 DOI: 10.1577/T06-223.1 Tracking Nursery Habitat Use in the York River Estuary, Virginia, by Young American Shad Using Stable Isotopes 1 J. C. HOFFMAN,* D. A. BRONK, AND J. E. OLNEY Virginia Institute of Marine Science, Post Office Box 1346, Gloucester Point, Virginia 23062, USA Abstract.—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 [d13C] or sulfur [d34S] 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 À31.8 6 0.3% for d13C and À5.2 6 0.7% for d34S at the farthest upriver region to À21.8 6 1.2% for d13C and 10.3 6 1.7% for d34S 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 as the carbon (C) stable isotope ratio, tend to closely American anadromous clupeid, migrate from their resemble their prey (DeNiro and Epstein 1978; Fry and freshwater nursery habitat to the ocean in their first Sherr 1984). Stable isotopes, therefore, offer the year of life. Investigators propose both temperature- potential to track habitat changes when either (1) based migration, consistent with fall migration (e.g., movement is related to ontogenetic niche (diet) shifts Leggett and Whitney 1972; Williams and Bruger 1972; or (2) the habitats of interest possess distinct O’Leary and Kynard 1986; Hoffman et al., in press) biogeochemical properties, such as a switch from fresh and size-based migration, migration occurring as early to marine waters, and the rate of movement between as June (Marcy 1976; Limburg 1996). A detailed habitats is faster than the rate of change of the isotope analysis of the age-structure of Hudson River age-0 ratio in the animal’s tissue (Hobson 1999). Thus, the American shad suggests that age-0 fish move about animal arrives at the new habitat with an isotopic 120 km downriver in about 2 weeks (Limburg 1996), composition indicative of its previous habitat. After implying rapid movement through their freshwater settling into the new habitat, the animal’s tissues will habitat. Given the coast-wide concerns regarding acquire the isotope ratio specific to that habitat. American shad population declines (ASMFC 1999), The ability to quantify the timing of these move- which are, in part, due to habitat degradation (e.g., ments (or diet shifts) is dependent on the turnover rate Limburg et al. 2003), further measures of age-0 in the tissue being analyzed (Tieszen et al. 1983; American shad habitat use, including the spatial scale, Hobson and Clark 1992). The turnover rate, in turn, duration, and rate of seaward migration, are needed. depends on growth and metabolism. Growth turnover Stable isotopes have been applied to document occurs as new tissue is added, diluting the pool of older movement patterns in a variety of animals (Hobson tissue derived from the previous diet. Metabolic 1999). Certain stable isotope ratios of consumers, such turnover occurs as older tissue is broken down and new tissue synthesized. In both slowly and rapidly growing fish, growth dominates stable isotope turnover * Corresponding author: [email protected] (Fry and Arnold 1982; Hesslein et al. 1993; Herzka and 1 Present address: U.S. Environmental Protection Agency, National Health and Ecological Effects Research Laboratory, Holt 2000; MacAvoy et al. 2001; Sakano et al. 2005) Mid-Continent Ecology Division, 6201 Congdon Boulevard, and metabolism can accelerate the turnover rate beyond Duluth, Minnesota 55804, USA. that exerted by growth alone (Frazer et al. 1997; Received October 5, 2006; accepted May 3, 2007 Vander Zanden et al. 1998; Herzka et al. 2001; Published online August 2, 2007 Trueman et al. 2005). 1285 1286 HOFFMAN ET AL. Two different models are commonly used to model turnover. Fry and Arnold (1982) proposed the following power model: c Rf ¼ Rf þðRi À Rf Þ 3ðWi=WtÞ ; ð1Þ 13 where Rt is the stable isotope ratio (e.g., d C) of the animal or tissue at time t, Rf is the stable isotope ratio at equilibrium, Ri is the initial stable isotope ratio, Wi and Wt represent the initial weight and weight at time t, respectively, and c is the turnover coefficient. The model simplifies to dilution-only turnover (i.e., growth only) when c ¼1 and can be used to date ecologically significant events associated with changes in isotopic ratios (Fry and Arnold 1982), such as age at settlement of fish larvae (Herzka et al. 2001). Hesslein et al. (1993) used an exponential model with terms for a specific growth rate (k ¼ loge[Wt/Wi]/t) and a metabolic rate (m), as follows: ðkþmÞt Rt ¼ Rf þðRi À Rf Þ 3 e ; ð2Þ where i indicates the ratio at t ¼ 0 and f the equilibrium ratio. The advantages of this model are that time, t,is an explicit term and that m can be estimated using a curve-fitting procedure if k is known. If Rf and Ri are both known (e.g., the isotopic signatures of two FIGURE 1.—A comparison between (a) Hesslein et al.’s habitats with distinct isotopic distributions), then the (1993) turnover model and (b) the model proposed in this equation can be solved directly for the equilibrium time study. For both models, results are shown over 100 d for a population in freshwater at t ¼ 0 with a d13C ¼28.0% ( R ;a (teq) theoretically required for the isotopic signature of i the population in the first habitat to resemble that in the typical riverine value) that moves into a marine habitat with an equilibrium value of À20.0% (Rf). The combined growth and second (Rf) upon moving into the second habitat. However, the time required for R to exactly equal R is metabolic rates are 0.05/d. In panel (a), the isotopic turnover t f ‘‘half-life’’ is 14 d; in panel (b), the time required to reach infinite because the model has an asymptote at R f equilibrium (t ) is 56 d. In the proposed model, SD ¼ 1 for R (Figure 1a). eq i and Rf, and n ¼ 25 fish. The figure shows the mean value of To resolve this problem, the turnover ‘‘half-life’’ d13C at time t (R ; solid line) and the SDs around the mean (i.e., the time required to reach one-half the difference t (dashed lines). The probability distribution function of Rf is between the initial [Ri] and final [Rf] isotopic ratios; shown to the right. The Kolmogorov–Smirnov (K–S) test was Figure 1a) is often estimated. It would be better, run at each daily time step (P-value: gray line). however, to have a biologically meaningful estimate of the teq than a convenient reference point along the transition. To achieve this, we propose a statistical turnover rate model. To evaluate the model, a method (Figure 1b). If Rf and Ri are variables with some mean and standard deviation, then we can define sensitivity analysis was done by calculating the effect on the estimate of the time to equilibrium, t ,by teq as the time at which the distribution of Rt is not eq changing the model variables (i.e., R , R , and k þ m) significantly different from that of Rf at some i f and population variables (i.e., the number of fish probability level (e.g., a ¼ 0.05). Now teq is not infinite. It represents both the minimum residence time sampled [n] and the standard deviations of Ri and Rf). 13 required to reach approximate equilibrium and the We surveyed the d C of age-0 American shad during period during which we can estimate how long ago the their summer residence in the upper York River organism (or population) arrived in its new habitat estuary, Virginia, and subsequent migration to Ches- (after which the organism will have an isotopic apeake Bay to determine whether habitat-specific signature similar to that of its new habitat). signatures were present. We then measured the d13C Our objective was to estimate habitat-specific and sulfur stable isotope ratios (d34S) of age-0 residence times (teq) by applying a stable isotope American shad at selected locations and times to apply NURSERY HABITAT USE BY AMERICAN SHAD 1287 FIGURE 2.—The Mattaponi River, Virginia. The inset shows its location with respect to Chesapeake Bay (CB).