Trade-offs in the response of mayflies to low oxygen and fish predation.

We examined how mayfly larvae (Ephemeroptera,Callibaetis montanus) balance the conflicting demands of avoiding both benthic hypoxia and fish predators. Using vertical oxygen and temperature gradients typical of ice-covered lakes, we observed the behavior of mayflies in the presence and absence of fish. In the absence of fish and with adequate oxygen, mayflies spent most of the time on the bottom substrate. As benthic oxygen concentration declined, mayflies increased their activity and moved up in the water column. In the presence of fish and with adequate oxygen, mayflies spent even more time associated with the bottom substrate and reduced their activity levels. As benthic oxygen concentrations declined, mayflies increased their activity and moved up in the water column, but to a lesser extent than when fish were absent. Because of this depression in activity and reluctance to leave the bottom substrate, mayflies endured lower oxygen concentrations in the face of predation threat relative to when fish were absent. Despite this trade-off, benthic hypoxia resulted in increased mortality due to fish predation. Because benthic invertebrates vary in their ability to tolerate hypoxia and in their vulnerability to fish predators, periods of benthic hypoxia could lead to selective predation on some taxa and be an important force structuring benthic invertebrate assemblages.