For fish to survive large acute temperature increases (i.e. >10.0°C) that may bring them close to their critical thermal maximum (CTM), oxygen uptake at the gills and distribution by the cardiovascular system must increase to match tissue oxygen demand. To examine the effects of an acute temperature increase (~1.7°C·h -1 to CTM) on the cardiorespiratory physiology of Atlantic cod, we (1) carried out respirometry on 10.0°C acclimated fish, while simultaneously measuring in vivo cardiac parameters using Transonic ® probes, and (2) constructed in vitro oxygen binding curves on whole blood from 7.0°C acclimated cod at a range of temperatures. Both cardiac output (Q) and heart rate (fH) increased until near the fish's CTM (22.2±0.2°C), and then declined rapidly. Q 10 values for Q and fH were 2.48 and 2.12, respectively, and increases in both parameters were tightly correlated with O 2 consumption. The haemoglobin (Hb)-oxygen binding curve at 24.0°C showed pronounced downward and rightward shifts compared to 20.0°C and 7.0°C, indicating that both binding capacity and affinity decreased. Further, Hb levels were lower at 24.0°C than at 20.0°C and 7.0°C. This was likely to be due to cell swelling, as electrophoresis of Hb samples did not suggest protein denaturation, and at 24.0°C Hb samples showed peak absorbance at the expected wavelength (540·nm). Our results show that cardiac function is unlikely to limit metabolic rate in Atlantic cod from Newfoundland until close to their CTM, and we suggest that decreased blood oxygen binding capacity may contribute to the plateau in oxygen consumption.

Cold-water fishes are becoming increasingly vulnerable as changing thermal conditions threaten their future sustainability. Thermal stress and habitat loss from increasing water temperatures are expected to impact population viability, particularly for inland populations with limited adaptive resources. Although the long-term persistence of cold-adapted species will depend on their ability to cope with and adapt to changing thermal conditions, very little is known about the scope and variation of thermal tolerance within and among conspecific populations and evolutionary lineages. We studied the upper thermal tolerance and capacity for acclimation in three captive populations of brook trout (Salvelinus fontinalis) from different ancestral thermal environments. Populations differed in their upper thermal tolerance and capacity for acclimation, consistent with their ancestry: the northernmost strain (Lake Nipigon) had the lowest thermal tolerance, while the strain with the most southern ancestry (Hill's Lake) had the highest thermal tolerance. Standard metabolic rate increased following acclimation to warm temperatures, but the response to acclimation varied among strains, suggesting that climatic warming may have differential effects across populations. Swimming performance varied among strains and among acclimation temperatures, but strains responded in a similar way to temperature acclimation. To explore potential physiological mechanisms underlying intraspecific differences in thermal tolerance, we quantified inducible and constitutive heat shock proteins (HSP70 and HSC70, respectively). HSPs were associated with variation in thermal tolerance among strains and acclimation temperatures; HSP70 in cardiac and white muscle tissues exhibited similar patterns, whereas expression in hepatic tissue varied among acclimation temperatures but not strains. Taken together, these results suggest that populations of brook trout will vary in their ability to cope with a changing climate.

Environmental variability in aquatic ecosystems makes the study of ectotherms complex and challenging. Physiologists have historically overcome this hurdle in the laboratory by using 'average' conditions, representative of the natural environment for any given animal. Temperature, in particular, has widespread impact on the physiology of animals, and it is becoming increasingly important to understand these effects as we face future climate challenges. The majority of research to date has focused on the expected global average increase in temperature; however, increases in climate variability are predicted to affect animals as much or more than climate warming. Physiological responses associated with the acclimation to a new stable temperature are distinct from those in thermally variable environments. Our goal is to highlight these physiological differences as they relate to both thermal acclimation and the 'fallacy of the average' or Jensen's inequality using theoretical models and novel empirical data. We encourage the use of more realistic thermal environments in experimental design to advance our understanding of these physiological responses such that we can better predict how aquatic animals will respond to future changes in our climate.

The retention of a nucleus in the mature state of fish red blood cells (RBCs) and the ability to easily collect and manipulate blood in nonterminal experiments make blood an ideal tissue on which to study the cellular stress response in fish. Through the use of the cGRASP 16K salmonid microarray, we investigated differences in RBC global gene transcription in fish held under control conditions (11 degrees C) and exposed to heat stress (1 h at 25 degrees C followed by recovery at 11 degrees C). Repeated blood sampling (via a dorsal aorta cannula) enables us to examine the individual stress response over time. Samples were taken preheat stress (representing individual control) and at 4 and 24 h postheat stress (representing early and late transcriptional regulation). Approximately 3,000 microarray features had signal above threshold when hybridized with RBC RNA-derived targets, and cannulation did not have a detectable effect on RBC mRNA expression at the investigated time points. Genes involved in the stress response, immune response, and apoptosis were among those showing the highest dysregulation during both early and late transcriptional regulation. Additionally, genes related to the differentiation and development of blood cells were transcriptionally upregulated at the 24 h time point. This study provides a broader understanding of the mechanisms underpinning the stress response in fish and the discovery of novel genes that are regulated in a stress specific manner. Moreover, salmonid transcripts that are consistently dysregulated in blood in response to heat stress are potential candidates of nonlethal biomarkers of exposure to this particular stressor.