Foraging in a lethal environment: Fish predation in hypoxic waters of a stratified lake
Development of low oxygen conditions (hypoxia) in bottom waters of stratified lakes has a profound effect on the distribution of aquatic organisms. Fish avoid hypoxic waters by moving into oxygenated water and suffer massive die—offs when migration is not possible. However, central mudminnows (Umbridae: Umbra limi) were captured regularly in severely hypoxic bottom waters of a lake in northern Wisconsin, USA. Caging experiments confirmed the lethality of this environment to mudminnows. The presence of freshly ingested phantom midge larvae (Diptera: Chaoborus) in the stomachs of mudminnows during the day coupled with the observation that Chaoborus are restricted to hypoxic waters during the day indicate that mudminnows routinely ventured into a lethal environment to forage. This conclusion was supported by caging experiments whereby mudminnows denied access to hypoxic water during the day did not contain Chaoborus in their stomachs. In laboratory experiments, ingestion of air bubbles enhanced the ability of mudminnows to forage in hypoxic water, but even in the absence of surface access, mudminnows readily foraged in hypoxic waters. These data help explain reports of other fish venturing into hypoxic waters and suggest that foraging in hypoxic waters may be common among some fishes when food abundance is low in surface waters.