Summer Habitat Suitability for Striped Bass in Chesapeake Bay: Reflections on a Population Decline

Abstract

We evaluated summer water temperatures, dissolved oxygen concentrations, distribution of striped bass Morone saxatilis subadults and adults, and juvenile abundance indexes in Chesapeake Bay to discern any influences of summer habitat suitability on historical changes in populations. Criteria for habitat suitability were those identified in freshwater reservoirs (temperature below 25°C and dissolved oxygen above 2–3 mg/L), which we confirmed for the York-Pamunkey estuary in the lower bay. Habitat suitability in the upper central basin in July declined significantly from 1962 to 1987, as did juvenile abundance indexes (mean catches per standard seine haul). Thickness of suitable temperature-oxygen habitat correlated significantly with Maryland juvenile indexes the following year. Relative reproductive performance of upper (Maryland) and lower (Virginia) bay stocks changed between 1967–1973 and 1980–1988 in parallel with reduction in upper bay summer habitat. The annual temperature-oxygen cycle in the bay revealed two key areas for striped bass subadults and adults: (1) a zone of cool water in north-central Chesapeake Bay near Annapolis, Maryland, where fish of these ages congregate in summer, and (2) a shallow sill across the lower bay near the mouth of the Rappahannock River, Virginia, where warm surface waters (>25°C) in summer impinge on the bottom and may block egress from the bay. The importance of year-round bay residents for reproduction in the upper bay may have been underestimated previously, at least for the recent low population levels. Reduced juvenile production at the head of the bay and population decline would be consistent with limitation of historically important habitat in summer and resultant physiological stresses of high temperature and low dissolved oxygen that affect reproductive competence the following year.