Acute Thermal Stress
In this study critical thermal maxima is used as a physiological and ecological reference point to identify early signs of thermal stress. Since catfish production ponds in the Southeastern United States can have daily temperature maxima up to 36°C and daily fluctuations of as much as 6°C, it is valuable to assess whether regional genetic differences or hybridization influence survival or growth. We quantified differences of acute thermal tolerance in two geographically distinct strains of Channel catfish Ictalurus punctatus and their hybrid crosses with an industry standard strain of Blue catfish I. furcatus. The catfish were subjected to water temperature increases of 2.0±0.1°C per hour until they lost equilibrium. The southern strain of Channel catfish tolerated higher temperatures than the northern strain, and both strains of Channel catfish tolerated higher temperatures than their hybrid catfish strain.
Effects of acute thermal stress on ion and acid-base regulation in Channel catfish
To assess the physiological response of fish to daily temperature variations, hematological, metabolic, ionic and acid-base regulation were investigated in Channel catfish subjected to acute temperature increases from 28ºC to either 32ºC or 36ºC. Several biological parameters were analyzed from blood sampled from fish in each treatment. Arterial blood pH, bicarbonate (HCO3–), partial pressure of carbon dioxide (PCO2), partial pressure of oxygen (PO2 ), hematocrit, hemoglobin, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), glucose, lactate, total proteins, chloride (Cl–), and osmolality all increased when Channel catfish were subjected to acute temperature change from 28ºC to 36ºC. With the exception of PCO2 and HCO3– there was no significant change between fish at 28o C and 32o C. The changes observed are very typical of acute stress in fish and suggests that acute temperature increase to 36ºC may by very stressful to Channel catfish and potentially the underlying physiological factor influencing critical thermal maxima (38.6-40.3ºC).
Chronic Thermal Stress
To gain a more complete perspective of temperature effects on catfish physiological performance and growth it is important to examine long term effects as well as acute. To do this, we conducted a 6 week growth experiment where catfish were subjected to daily cycling temperatures of either 27-31°C or 32-36°C. These ranges were chosen to mimic daily pond fluctuations. Two geographically distinct strains of Channel catfish and a corresponding hybrid catfish were used. Hybrid catfish had the highest survival at both temperatures, and both strains of Channel catfish had greater growth at 27-31°C than at 32-36°C.