Poststocking Survival of Hybrid Striped Bass after Changes in Water Hardness and Temperature

Abstract

AbstractMortality of hybrid striped bass (striped bass Morone saxatilis × white bass M. chrysops) after stocking can be high and is often attributed to changes in hardness between hatchery water and natural systems. Two studies were conducted to examine the effect of (1) rate of reducing water from high to low hardness and (2) abrupt changes in water hardness and temperature on poststocking survival of hybrid striped bass. In the first study, small (<30 g) and large (>50 g) fingerling hybrid striped bass in high‐hardness water (365 mg/L CaCO3) were tempered to low‐hardness water (42 mg/L) over 35 or 60 min and compared with a control group exposed to no change in hardness. In the second study, small and large fingerlings from high‐hardness water (365 mg/L; 17°C) were subjected to (1) an acute 5°C increase in water temperature; (2) an acute 326‐mg/L decrease in water hardness; (3) a simultaneous acute 5°C increase in temperature and 326‐mg/L decrease in water hardness; or (4) no change in hardness or temperature. All groups were monitored for 1 week poststocking, and mortality was summarized. In the first study, no differences in mortality were observed between the two tempering rates or the control for small or large fingerlings. In the second study, a 326‐mg/L water hardness decrease alone, a 5°C temperature increase alone, and these two treatments in combination did not significantly affect mortality for either size‐class. These studies suggest that temperature and hardness changes occurring alone or simultaneously do not explain observed poststocking mortality, but additional environmental factors not tested might act synergistically with hardness and temperature changes to reduce hybrid striped bass poststocking survival.