Hypoxia is a common phenomenon in aquatic environments which most
frequently occurring during late summer or late winter. Hypoxic events are increasing in intensity as water temperatures increase, and it is important to understand how hypoxia affects aquatic ecosystems, particularly fish. Additionally, increases in ammonia concentrations have been shown to occur during periods of hypoxia. Ammonia is converted to nitrate in aquatic environments through nitrification, via the nitrogen cycle. However, this process is carried out by aerobic bacteria and is depressed by hypoxia. Ammonia has been shown to be more toxic to fish during hypoxia than during normoxia, indicating that it could have large implications on fish foraging activity in hypoxic waters. While fish have reduced growth in hypoxic water, the simultaneous accumulation of ammonia may result in death. Therefore, evaluation of how hypoxia affects fish simultaneously exposed to ammonia is essential to understand how fish are affected during natural hypoxic events, such as those that occur in the hypolimnion of Lake Erie.

Juvenile yellow perch were reared under varied oxygen conditions to examine how hypoxia affects growth and survival of fish. Yellow perch are a physoclistous fish which can result in irreversible failure to inflate the swim bladder. Therefore, fish with both inflated and uninflated swim bladders were tested at different concentrations of oxygen for survival, growth, and oxygen consumption to gain a better understanding of how hypoxia affects yellow perch. Fish were stocked into 12 tanks with three levels of iii oxygen: 3, 4, and >7 mg O2/L (33, 45, & >78% saturation at 21°C). Fish were fed ad libitum with live brine shrimp nauplii over a 14-day period. Oxygen consumption was measured at the end of the experimental trial. Survival was not affected by lowered dissolved oxygen, but growth was reduced significantly in fish with both inflated (at 32% oxygen saturation) and uninflated swim bladders (at 33% and 48% oxygen saturation). This indicated that fish without swim bladders were less tolerant of hypoxia. Oxygen consumption was dependent on oxygen level, but did not differ between fish with inflated and uninflated swim bladders. The critical oxygen level causing a decrease in oxygen consumption of yellow perch was 3.5 mg/L O2. Results from this study indicated that larval/juvenile yellow perch are able to survive lowered dissolved oxygen levels, but are faced with 50% and 24% reduced growth in fish with uninflated and inflated swim bladders, respectively.

Elevated ammonia is commonly associated with hypoxia in aquatic environments, and it is important to study the interactive effects of simultaneous exposure of hypoxia and ammonia on fish. Hypoxia has been shown to cause gill remodeling in some species of fish, which involves reduction of interlamellar cell mass to increase respiratory surface area. However, ammonia has been shown to cause severe damage to gills, limiting the respiratory, ionregulatory, and excretory efficiency of the gills. In this study, common carp (Cyprinus carpio) were exposed simultaneously to ammonia and a range of oxygen concentrations to assess how oxygen concentrations might affect ammonia toxicity. Histology of gill tissue was examined to observe how the gills of carp responded to hypoxia, hypoxia and ammonia simultaneously, and how gills recovered in normoxic iv conditions. Carp were exposed to three levels of oxygen 8.8 ± 1.1, 2.7 ± 0.5 and 1.6 ± 0.6 mg O2/L. The 24-h LC50 for the three respective levels of oxygen were 0.66 (0.55-0.81), 0.50 (0.38-0.66), and 0.45 (0.35-0.52) NH3-N/L. The histological results showed that gill remodeling was not present in carp. Gills however did undergo severe histopathological changes when exposed to ammonia and hypoxia simultaneously, emphasizing the importance of monitoring water quality when the effects of lowered dissolved oxygen on fish are examined.

This work has serious ecological implications for distribution and growth of yellow perch in the Great Lakes due to changes in hypolimnion oxygen and ammonia levels under current conditions and simulations that forecast more frequent and severe hypoxic events.