Independent and Interactive Effects of Long-Term Exposure to Hypoxia and Elevated Water Temperature on Behavior and Thermal Tolerance of an Equatorial Cichlid

Hypoxia and climate warming are pervasive stressors in aquatic systems that may have interactive effects on fishes because both affect aerobic metabolism. We explored independent and interactive effects of dissolved oxygen (DO) and temperature on thermal tolerance, behavior, and fitness-related traits of juvenile F-1 offspring of the African cichlid Pseudocrenilabrus multicolor. Fish were reared in a split-brood design with four treatments (low or high DO, cool or hot temperature); thermal tolerance, growth, and condition were measured after 1 mo in the rearing treatments, following which behavioral traits were measured over 3.6 mo. Critical thermal maximum was higher in fish reared under hot conditions but was not affected by hypoxia. There was an interactive effect of DO and temperature on agitation temperature (temperature at which fish show behavioral signs of thermal stress) and the thermal agitation window (degrees C between the onset of agitation and final loss of equilibrium). Fish reared and tested under hot, normoxic conditions showed a higher agitation temperature, while fish reared and tested under hot, hypoxic conditions showed the largest thermal agitation window. Fish grew more quickly in length under hot than cool conditions and more quickly under normoxic than hypoxic conditions. Fish reared under cool, normoxic conditions were characterized by higher condition than other groups. Both cool and hypoxic rearing conditions reduced activity and aggression. These results highlight the importance of integrating physiological tolerance measures with sublethal behavioral effects of hypoxia and high temperature to gain a fuller understanding of species responses to multiple stressors.