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Survival, oxygen consumption (inline image), total plasma cortisol and glucose levels and gill heat-shock protein 70 (hsp70) expression were measured in 10 and 50 g juvenile Atlantic cod Gadus morhua during an acute temperature increase (2° C h−1) to their critical thermal maximum. Ninety three per cent of the fish in both size classes survived to 24° C; however, mortality was 100% within 15 min of reaching this temperature. The inline image for both size classes increased significantly with temperature, reaching peak values at 22° C that were c. 2·8-fold those of control (10° C) fish. Resting plasma cortisol and glucose levels were lower in 10 g as compared to 50 g fish. Plasma glucose levels were highly variable in both size classes, and significant increases were only seen at >22° C for the 10 g fish. In contrast, plasma cortisol showed an exponential increase with temperature starting at 16° C in both size classes, and reached maximum levels at 22° C that were 19-fold (10 g fish) and 35-fold (50 g fish) higher than their respective control groups. Both the constitutive (73 kDa) and inducible (72 kDa) isoforms of hsp70 were detected in both size classes using the widely utilized mouse monoclonal antibody. Expression of these isoforms, however, did not change when Atlantic cod were exposed to elevated temperature, and the 72 kDa isoform was not detected using salmonid-specific antibodies. These results indicate that juvenile Atlantic cod are very sensitive to acute increases in water temperature. In addition, they (1) show that inline image and plasma cortisol, but not plasma glucose or gill hsp 70 levels, are sensitive indicators of thermal stress in Atlantic cod and (2) support previous reports that the upper critical temperature for this species is 16° C.

Although free swimming Atlantic cod held in thermally stratified water move to preferred temperatures, sea-caged fish are limited in their movement in the water column, and are thus exposed to large seasonal (~ 0 to 20oC) and daily (+ 10oC) temperature fluctuations in Atlantic Canada. Thermal biology and tolerance are important physiological traits that determine whether cod survive these temperature challenges, and define suitable areas for cage-site aquaculture production. Thus, we determined the acute thermal tolerance (both upper and sub-lethal) of cod of various life stages (juvenile to adult), whether thermal tolerance is plastic, and when various physiological systems are negatively impacted by high water temperatures. In our first experiment, we measured the critical thermal maximum (CTM) of 8 and 12oC acclimated juvenile cod, and 12oC acclimated juvenile haddock (a closely related gadoid preferring warmer temperatures) by increasing water temperature by 2oC h-1, while simultaneously measuring routine oxygen consumption (Rr)(Fig. 1). Acclimating cod to 12 vs 8oC increased the CTM from 21.4 to 23.0 oC, and the CTM of 12oC acclimated haddock was slightly higher than measured in cod (23.9oC) acclimated to the same temperature. Further, in all groups Rr plateaued approx. 2oC prior to the groups CTM. These data suggested that there is some, but limited, plasticity in the acute thermal tolerance of gadoids, and that cod experience sub-lethal stress approx. 2oC before CTM. We followed this initial experiment with measurements the CTM in 10oC adult cod, while simultaneously measuring both Rr and cardiovascular parameters. The CTM for these adult fish (22.2C) was similar to that measured for juvenile cod, when acclimation temperature is taken into account, and Rr and cardiac function were negatively influenced at 2 and 4oC, respectively, prior to CTM. Finally, we performed CTM experiments on 10 and 50 g cod, to examine the usefulness of plasma glucose and cortisol, and heat shock protein 70 (hsp 70) as indicators of thermal stress. Plasma cortisol levels showed an exponential increase with temperature, starting at approx. 16oC (~ 6oC prior to their CTM), although the max. levels achieved were very different between the two size classes (10g, ~200 ng ml-1; 50 g, ~ 450 ng ml-1). In contrast, neither plasma glucose, nor gill hsp 70 levels were useful indices of thermal stress. Plasma glucose levels varied considerably and were only significantly elevated at 22and we did not detect changes in constitutive or inducible hsp70 levels at any temperature using rabbit or salmonid specific antibodies., World Aquaculture Society meeting, February 2007, San Antonio, Texas, USA