Acclimation to a low oxygen environment alters the hematology of largemouth bass (Micropterus salmoides)

One of the most severe impacts of urbanization on aquatic systems is the increasing presence of low oxygen environments caused by anthropogenic sources of pollution. As urbanization increases nationally and globally, it is becoming exceedingly important to understand how hypoxia affects aquatic fauna, especially fish species. In an effort to better understand the impacts of prolonged hypoxia on fishes, largemouth bass were held at 3.0 and 9.0 mg L⁻¹ for 50 days, which has previously shown to be temporally sufficient to impart plastic phenotypic changes. Following the holding period, fish from each group were subjected to a low dissolved oxygen (DO) challenge of 2.0 mg L⁻¹ for 6 h, and their physiological and hematological parameters were compared with control fish held for 6 h with no change in DO. There were no differences in the physiological stress responses between the two holding groups; however, the low oxygen holding group had increased hemoglobin and hematocrit levels following the 6-h low oxygen challenge compared with the high oxygen group. These results suggest largemouth bass exposed to chronic low oxygen conditions, either naturally or anthropogenically, may possess a beneficial advantage of increased oxygen uptake capacity during periods of low oxygen.