The following provides a quick snapshot of key findings. A more in-depth discussion and references to the scientific literature can be found here.
Hypoxia can potentially reduce fish growth, survival, reproductive success and, ultimately, population growth. Rapid changes in oxygen concentrations may trap fish in hypoxic waters and lead to direct mortality. While such direct mortality due to low DO is possible, a more common immediate fish response to hypolimnetic hypoxia is avoidance of bottom waters which can shift them away from preferred diets, increased total metabolic costs, and reproduction by occupying warmer waters and migrating long distances. However, documenting these effects has proven difficult because hypoxia persists for a short time period, making it difficult to distinguish hypoxia impacts from other seasonal processes. In addition, while nutrient additions can exacerbate hypoxia, they can also increase productivity and be beneficial to fish.
Laboratory studies have demonstrated the potential for some Lake Erie fish to be negatively affected by direct exposure to low DO concentrations. For example, yellow perch consumption and growth rates decline under hypoxia and hypoxia can reduce the abundance of zooplankton prey. However, growth and survival rates of some preferred benthic prey are largely unaffected by low DO conditions.
Potential impacts on mobile fish appear to be indirect and vary among species. For example, hypoxia-intolerant rainbow smelt avoid hypoxic waters by migrating horizontally or moving up into a thin layer of the water column just above the hypoxic zone. By contrast, some yellow perch move horizontally away from the hypoxic region, but others move higher in the water column to avoid low DO and undertake short feeding forays into the hypoxic zone.
Hypoxia may also either reduce or increase the overlap between predator and prey as both are squeezed into the same area of the water column. For example, intolerant, cold-water rainbow smelt preferred the benthic Chironomidae during oxygenated periods, but consumed almost entirely zooplankton during hypoxia. In contrast, the diets of emerald shiner, a warm-water epilimnetic zooplanktivore, seemed unaffected or perhaps increased because zooplankton are forced into the epilimnion.
These species-specific responses to hypoxia are generally supported by seasonal trends in fish condition. While condition of emerald shiner improved from summer into fall, rainbow smelt condition declined during hypoxia. The both long-term and short-term measures of condition in the more tolerant yellow perch did not decrease during the height of hypoxia.
While these direct observations and laboratory measurements show the complexity of responses, models can help sort them out, and some of that work is discussed here.