ENVIRONMENTAL CONTROLS AND BIOLOGICAL CONSTRAINTS ON RECRUITMENT OF STRIPED BASS MORONE SAXATILIS IN CHESAPEAKE BAY

Abstract

The causes of >30-fold recruitment variability in striped bass Morone saxatilis were investigated in Chesapeake Bay. Factors that affect survival and growth of early-life stages were evaluated through 1) field surveys in 2001-2003 to document spatial and temporal variability in larval and juvenile abundances, 2) synthetic data analyses to provide a longer-term perspective and 3) trophodynamic and growth analyses to document how environmental variability controls and regulates variability in year-class strength. Daily discharge from the Susquehanna River in spring months controlled the distribution and apparent survival of striped bass and other anadromous fish larvae. Control of recruitment in upper Chesapeake Bay includes both direct and indirect effects of hydrological variability on egg and larval survival. In dry years (1999 and 2002), direct effects of biophysical controls resulted in low abundances of striped bass feeding-stage larvae, a consequence of reduced retention of eggs and yolk-sac larvae at the salt front and Estuarine Turbidity Maximum (ETM). The strongest year classes are produced in wet years that have high retention of eggs and yolk-sac larvae (i.e. direct effects), and a spatial and temporal match between feeding larvae and zooplankton prey (i.e. indirect effects). In moderate and high freshwater flow years (1996, 1998, 2001, 2003), indirect trophodynamic effects were most important. Striped bass larvae were strongly associated with the ETM in wet years when zooplankton prey levels also were highest in the ETM. A higher percentage of feeding larvae was observed in a wet year (2003) than in a dryer year (2001) (93% versus 35%) and faster larval growth (58% higher) occurred in the wet year. Inter-annual variability in growth and its effect on larval stage duration and cumulative mortality were sufficient to generate variability of the magnitude observed in juvenile recruitment. A forecasting model developed for age-0 striped bass recruitment that included spring flow and spring temperatures as independent variables has strong predictive capability. Growth of age-0 juveniles was density dependent, leading to diminished juvenile survival in years of high abundance through size-selective overwinter mortality. Through this mechanism, age-0 abundance levels that vary >10-fold are reduced to 3-4-fold variability at age-3. Density dependence regulates dynamics of age-0 juveniles, compensating for coarser controls over recruitment generated by environmental factors during the egg and larval stages.