Overwintering habitats of migratory juvenile American shad in Chesapeake Bay

Abstract

We describe overwintering habitats of age-0 American shad in the lower Chesapeake Bay estuary through analyses of multiple, complementary data sets, including bottom-trawls of the Virginia portion of Chesapeake Bay and its tributaries, stable isotope analysis of American shad and common prey items, and stomach content analysis. This is the first detailed description of overwintering habitats used by young American shad during their first migration to the Atlantic Ocean. American shad generally migrated from their freshwater rearing habitat during November and December and migrated to the ocean during February through March. American shad were captured in all of Virginia’s tributaries and along Chesapeake Bay’s western coast. These fish were caught in relatively cool waters (5–9°C) over a wide range of salinities (0.1–27.5). Strong selection for certain temperatures or salinities was not apparent. Stomach content and stable isotope analyses demonstrated that juveniles were feeding in the estuary, growing on a diet of estuarine calanoid copepods, mysid shrimps, and larval fishes. The stable isotope data were used to describe temperature- and size-cued migration from fresh water. Temperature was an important cue affecting both the timing and the rate of migration. Further, American shad exhibited at least three different size-related migration behaviors: most juveniles emigrated from the freshwater rearing habitat at 2–5 g (ca. 55–75 mm fork length); other juveniles emigrated at a size of 2 g or less and rapidly moved into the lower estuary; and finally, a few juveniles remained in the upper estuary and did not emigrate until they were 5 g or larger. A few American shad were captured with anomalous stable isotope signatures, which may be explained by migration into the Chesapeake Bay estuary from an adjacent system.