The Effects of Constant and Fluctuating Thermal Environments on Life History Traits of the Fathead Minnow, Pimephales promelas

Abstract

Climate change can impact the life history of organisms through elevating temperatures And the increasing intensity of daily fluctuations. Most experiments addressing the effects of climate change do so with constant temperature treatments. However, the use of constant temperatures as opposed to fluctuating temperatures in an experiment can greatly affect the trait under observation, as fluctuating conditions have been shown to produce different results. In order to determine the effects of constant versus fluctuating temperature environments on fish, as well as to determine the validity of a growth-measuring technique called growing degree days, I raised fathead minnows, Pimephales promelas, in four fluctuating and four constant temperature regimes. I found that paired constant and fluctuating treatments that shared similar degree days showed no difference in growth at the lower temperatures but did at higher, more stressful temperatures. Conversely, the paired treatments reared at higher temperatures showed no difference in critical thermal maximum (CTmax), the temperature at which the fish lose control of motor functions. For the temperature pairings at lower temperatures, however, the fluctuating treatments demonstrated a significantly higher CTmax than the constant treatments. Therefore, in the context of climate change experiments two conclusions were drawn: primarily, the use of growing degree days (GDD) to measure environments with varying levels of environmental fluctuations should be done with caution. Secondly, extreme fluctuating and constant temperature environments do indeed produce different growth results, and further research should be carried out to determine the severity of these differences.