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

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 [δ13C] or sulfur [δ34S] 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 ± SE isotopic ratios for the dorsal muscle tissue of age‐0 fish increased along the estuary, from −31.8 ± 0.3‰ for δ13C and −5.2 ± 0.7‰ for δ34S at the farthest upriver region to −21.8 ± 1.2‰ for δ13C and 10.3 ± 1.7‰ for δ34S 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.

Sulfur stable isotope indicators of residency in estuarine fish

“…But average standard deviations in the d 34 S values at stations in the upper estuary (all fish) and lower estuary (resident fish only) were identical at 1.1%, probably indicating that effects of fluctuating salinity in upper-estuary stations were not strong. Rather, it is likely that the fish isotope values reflect salinity and dietary regimes integrated over several months of fish growth preceding animal collection, and that this time integration helps stabilize the relationship between salinity and fish d 34 S. Generally, the period of time integration represented by fish d 34 S varies with fish size, growth, and metabolic rate (Tarboush et al 2006;Hoffman et al 2007). Recent isotope turnover models for larger estuarine fish show that isotope time integrations of 1-6 months are not uncommon (Buchheister and Latour 2010), and may apply to fish of this study that were mostly of moderate total lengths (5-50 cm) and were collected towards the end of summer growing seasons in estuarine waters that were near 30uC.…”

“…Use of multiple isotope tracers to study animal movement is increasing (Hobson and Wassenaar 2008;Hoffman et al 2010), but more research is needed to clarify the best mix of traditional tags and natural chemical markers for studying movement of estuarine fish. The chemical marker approaches have the disadvantage of less site specificity, but do apply more broadly to larger estuarine areas and can be measured in all animals without time-consuming mark-recapture efforts.…”

“…Commonly measured stable isotopes of hydrogen, carbon, nitrogen, oxygen, and sulfur all show strong salinity-related patterns in estuarine biota (Fry 2002), and using a combination of these markers to study fish movement will likely narrow the broadly resolved salinity zones identified in this study by sulfur isotopes alone. In this regard, it may be that hydrogen and oxygen isotopes are more useful additions to the sulfur isotope approach than are nitrogen and carbon isotope measurements, because the nitrogen and carbon isotopes strongly record food web dynamics (Hoffman et al 2010) and are thus somewhat confounded indicators of salinity signals in estuaries. Further research is needed to test whether hydrogen and oxygen isotope measurements in fish tissues and body water can improve salinity zone resolution achievable by d 34 S alone.…”

“…A more recent approach to habitat evaluation is to use chemical markers such as stable isotopes and trace metals to determine which habitats have relatively high importance for fish, especially habitats that have important nursery or productivity functions (Thorrold et al 2002;Wissel and Fry 2005). Residency is potentially an additional way to assess habitat quality, with low residency and high transience likely indicating suboptimal habitats (Hoffman et al 2007). The advantages residents may have include better avoiding local predation and better accessing local nutritious foods.…”

Estuarine consumers utilize marine, estuarine and terrestrial organic matter and provide connectivity among these food webs

“…For individuals, the isotopic turnover period depends on both somatic growth and metabolic turnover rates; in organisms that grow rapidly, somatic growth rates essentially determine the isotopic turnover period (Martínez del Rio et al 2009). Zooplankton turnover rates are generally <1 mo (Hoffman et al 2007), thus reflecting temporal environmental variability. However, zooplankton collected in the brackish estuary might have been displaced downstream, especially during high dis charge conditions.…”