Acute and Sublethal Effects of Ammonia on Striped Bass and Hybrid Striped Bass

Bioassays in static water (mean ± SD; temperature, 20–22 C; pH, 8.2–8.4; alkalinity, 205 ± 10 mg/L CaCO 3 ; total hardness, 220 ± 10 mg/L CaCO 3 ) were used to determine median lethal concentrations (LC 50 ) of un‐ionized ammonia (NH 3 ‐N) for striped bass Moronc saxatilis and hybrid striped bass M. saxatilis × M. chrysops . The 96 h LC 50 for striped bass was 1.01 ± 0.24 mg/L NH 3 ‐N 3 and was significantly higher than the LC 50 for hybrid striped bass (0.64 ± 0.05 mg/L NH 3 ‐N). The effects of sublethal ammonia were evaluated after fish were exposed for 96 h to 0.0, 0.25, or 0.5 mg/L NH 3 ‐N or to additional exposure to oxygen depleted water (about 2.0 mg O 2 /L). Plasma ammonia of striped hass did not change as sublethal ammonia increased, but exposure to oxygen depletion caused a decrease in plasma ammonia. In contrast, plasma ammonia of hybrid striped bass increased as environmental ammonia increased, and increased further after exposure to oxygen depletion. Plasma cortisol levels of striped bass were significantly higher and more variable than cortisol levels of hybrid striped bass; additional exposure to oxygen depletion increased this variability, but these responses may be due to the stress of handling and confinement. Mean differences also existed between species for hemoglobin and hematocrit, while differences in variability occurred for osmolality and oxygen depletion rates. Striped bass tolerated ammonia better than hybrid striped bass but were more susceptible than hybrid striped bass to the additional stress of oxygen depletion. Most changes in physiological characteristics were relatively independent of environmental ammonia, but they were affected by oxygen depletion challenge.