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Understanding the resilience of aquatic ectothermic animals to climate warming has been hindered by the absence of experimental systems experiencing warming across relevant timescales (for example, decades). Here, we examine European perch (Perca fluviatilis, L.) from the Biotest enclosure, a unique coastal ecosystem that maintains natural thermal fluctuations but has been warmed by 5-10 degrees C by a nuclear power plant for over three decades. We show that Biotest perch grow faster and display thermally compensated resting cardiorespiratory functions compared with reference perch living at natural temperatures in adjacent waters. However, maximum cardiorespiratory capacities and heat tolerance limits exhibit limited or no thermal compensation when compared with acutely heated reference perch. We propose that while basal energy requirements and resting cardiorespiratory functions (floors) are thermally plastic, maximum capacities and upper critical heat limits (ceilings) are much less flexible and thus will limit the adaptive capacity of fishes in a warming climate.

Environmental warming and acute stress increase cardiorespiratory activity in ectothermic animals like fish. While thermal acclimation can buffer the direct thermal effects on basal cardiorespiratory function during chronic warming, little is known about how acclimation affects stress-induced cardiorespiratory responses. We compared cardiovascular and haematological responses to chasing stress in cannulated wild European perch (Perca fluviatilis) from a reference area at natural temperature (16 A degrees C) with perch from the 'Biotest enclosure'; an experimental system chronically warmed (22 A degrees C) by effluents from a nuclear power plant. Routine blood pressure was similar, but Biotest perch had slightly higher resting heart rate (59.9 +/- 2.8 vs 51.3 +/- 2.9 beats min(-1)), although the Q (10) for heart rate was 1.3, indicating pronounced thermal compensation. Chasing stress caused hypertension and a delayed tachycardia in both groups, but the maximum heart rate increase was 2.5-fold greater in Biotest fish (43.3 +/- 4.3 vs 16.9 +/- 2.7 beats min(-1)). Moreover, the pulse pressure response after stress was greater in reference fish, possibly due to the less pronounced tachycardia or a greater ventricular pressure generating capacity and thermally mediated differences in aortic compliance. Baseline haematological status was also similar, but after chasing stress, the haematocrit was higher in Biotest fish due to exacerbated red blood cell swelling. This study highlights that while eurythermal fishes can greatly compensate routine cardiorespiratory functions through acclimation processes, stress-induced responses may still differ markedly. This knowledge is essential when utilising cardiorespiratory variables to quantify and compare stress responses across environmental temperatures, and to forecast energetic costs and physiological constraints in ectothermic animals under global warming.

Thermal plasticity of cardiorespiratory function allows ectotherms like fish to cope with seasonal temperature changes and is critical for resilience to climate change. Yet, the chronic thermal effects on cardiovascular homeostatic reflexes in fish are little understood although this may have important implications for physiological performance and overall resilience to climate warming. We compared cardiac autonomic control and baroreflex regulation of heart rate in perch (Perca fluviatilis L.) from a reference area in the Baltic Sea at 18-19 degrees C with conspecifics from the Biotest enclosure, a chronically heated ecosystem receiving warmed effluent water (24-25 degrees C) from a nuclear power plant. Resting heart rate of Biotest fish displayed clear thermal compensation and was 58.3+/-2.3 beats min(-1) compared with 52.4+/-2.6 beats min(-1) in reference fish at their respective environmental temperatures (Q(10) = 1.2). The thermally compensated heart rate of Biotest fish was a combined effect of elevated inhibitory cholinergic tone (105% in Biotest fish versus 70% in reference fish) and reduced intrinsic cardiac pacemaker rate. A barostatic response was evident in both groups, as pharmacologically induced increases and decreases in blood pressure resulted in atropine-sensitive bradycardia and tachycardia, respectively. Yet, the tachycardia in Biotest fish was significantly greater, presumably due to the larger scope for vagal release. Acclimation of Biotest fish to 18 degrees C for 3 weeks abolished differences in intrinsic heart rate and autonomic tone, suggesting considerable short-term thermal plasticity of cardiovascular control in this species. The heightened hypotensive tachycardia in Biotest perch may represent an important mechanism of ectothermic vertebrates that safeguards tissue perfusion pressure when tissue oxygen demand is elevated by environmental warming.

Understanding the resilience of aquatic ectothermic animals to climate warming has been hindered by the absence of experimental systems experiencing warming across relevant timescales (for example, decades). Here, we examine European perch (Perca fluviatilis, L.) from the Biotest enclosure, a unique coastal ecosystem that maintains natural thermal fluctuations but has been warmed by 5-10 degrees C by a nuclear power plant for over three decades. We show that Biotest perch grow faster and display thermally compensated resting cardiorespiratory functions compared with reference perch living at natural temperatures in adjacent waters. However, maximum cardiorespiratory capacities and heat tolerance limits exhibit limited or no thermal compensation when compared with acutely heated reference perch. We propose that while basal energy requirements and resting cardiorespiratory functions (floors) are thermally plastic, maximum capacities and upper critical heat limits (ceilings) are much less flexible and thus will limit the adaptive capacity of fishes in a warming climate.

Understanding the resilience of aquatic ectothermic animals to climate warming has been hindered by the absence of experimental systems experiencing warming across relevant timescales (for example, decades). Here, we examine European perch (Perca fluviatilis, L.) from the Biotest enclosure, a unique coastal ecosystem that maintains natural thermal fluctuations but has been warmed by 5-10 degrees C by a nuclear power plant for over three decades. We show that Biotest perch grow faster and display thermally compensated resting cardiorespiratory functions compared with reference perch living at natural temperatures in adjacent waters. However, maximum cardiorespiratory capacities and heat tolerance limits exhibit limited or no thermal compensation when compared with acutely heated reference perch. We propose that while basal energy requirements and resting cardiorespiratory functions (floors) are thermally plastic, maximum capacities and upper critical heat limits (ceilings) are much less flexible and thus will limit the adaptive capacity of fishes in a warming climate.

Understanding the resilience of aquatic ectothermic animals to climate warming has been hindered by the absence of experimental systems experiencing warming across relevant timescales (for example, decades). Here, we examine European perch (Perca fluviatilis, L.) from the Biotest enclosure, a unique coastal ecosystem that maintains natural thermal fluctuations but has been warmed by 5-10 degrees C by a nuclear power plant for over three decades. We show that Biotest perch grow faster and display thermally compensated resting cardiorespiratory functions compared with reference perch living at natural temperatures in adjacent waters. However, maximum cardiorespiratory capacities and heat tolerance limits exhibit limited or no thermal compensation when compared with acutely heated reference perch. We propose that while basal energy requirements and resting cardiorespiratory functions (floors) are thermally plastic, maximum capacities and upper critical heat limits (ceilings) are much less flexible and thus will limit the adaptive capacity of fishes in a warming climate.

Understanding the resilience of aquatic ectothermic animals to climate warming has been hindered by the absence of experimental systems experiencing warming across relevant timescales (for example, decades). Here, we examine European perch (Perca fluviatilis, L.) from the Biotest enclosure, a unique coastal ecosystem that maintains natural thermal fluctuations but has been warmed by 5-10 degrees C by a nuclear power plant for over three decades. We show that Biotest perch grow faster and display thermally compensated resting cardiorespiratory functions compared with reference perch living at natural temperatures in adjacent waters. However, maximum cardiorespiratory capacities and heat tolerance limits exhibit limited or no thermal compensation when compared with acutely heated reference perch. We propose that while basal energy requirements and resting cardiorespiratory functions (floors) are thermally plastic, maximum capacities and upper critical heat limits (ceilings) are much less flexible and thus will limit the adaptive capacity of fishes in a warming climate.