Critical Thermal Maxima of Two Geographic Strains of Channel and Hybrid Catfish

Critical thermal maxima have been used extensively to provide physiologically and ecologically valuable reference points that identify early signs of thermal stress. In catfish pond culture, daily temperature maxima up to 36 degrees C and daily fluctuations of as much as 6 degrees C are observed. These extreme conditions will probably be exacerbated by the effects of global climate change. Channel Catfish Ictalurus punctatus have a broad natural distribution from southern Canada to northern Mexico. If regional genetic differences could cause strains with a southern distribution to have greater thermal tolerance than strains with a northern distribution, and consequently a greater critical thermal maximum, then hybrid catfish (Channel Catfish x Blue Catfish I. furcatus) strains might be expected to have greater critical thermal maxima than their respective Channel Catfish strains because Blue Catfish have a more southern range of distribution. To examine this, we quantified differences of acute thermal tolerance in two geographically distinct strains of Channel Catfish and their hybrid crosses with an industry standard strain of Blue Catfish. The catfish were subjected to water temperature increases of 2.0 +/- 0.1 degrees C center dot h(-1) until they lost equilibrium. Standard length, which ranged from 162 to 320mm, had a significant effect on survival, survival being greater in larger fish. The critical thermal maximum ranged from 38.6 degrees C to 40.3 degrees C. Southern Channel Catfish tolerated higher temperatures than northern Channel Catfish did, and both strains of Channel Catfish tolerated higher temperatures than their hybrid catfish strains did. This study indicates that geographically distinct catfish strains differ in acute thermal tolerance and suggests heritability for this trait, as evidenced by similar responses in Channel Catfish and their corresponding hybrid crosses with Blue Catfish. Received August 5, 2013; accepted October 14, 2013