Thermal Tolerance and Aerobic Scope

Does cardiac capacity of fish and amphibians determine upper thermal tolerance and how are thermal limits affected by other factors such as hypoxia and digestion.  

Cardio-respiratory capacity and heat tolerance in fish and amphibians

(In collaboration with Tobias Wang, Hans Malte and Mark Bayley)

To survive and reproduce, aquatic animals must have access to sufficient amounts of food and oxygen and the environmental temperature must be within tolerable limits. By virtue of various physiological and biochemical adaptations, species differ enormously in their ability to tolerate different temperatures and this span in tolerance is crucial in determining their niche and population range. Fish have a body temperature that equals the surrounding water, and they rarely have the possibility of seeking colder environments. Aquatic organisms, therefore, are particularly vulnerable to increased temperature, and only a few additional degrees may have devastating effects on population size. Given current rise in global temperatures and the direct link between physiological capacity and geographical distributions, it is therefore imperative to identify and understand the principal physiological mechanisms that determine temperature tolerance.

Current consensus proposes that temperature sensitivity of fish species is tightly coupled to the capacity of their oxygen transport system. In short, it seems that the gills, blood and heart of the fish cannot fulfill the demand for oxygen when temperature rises sufficiently. Although this paradigm has gained tremendous popularity and many supporters, most scientific evidence is based on correlations, while manipulative experiments to verify its validity are lacking. In this project we propose a suite of studies to investigate the thermal sensitivity of the oxygen transport system in a range of commercially and recreationally important fish species.

The conceptual model linking thermal tolerance to oxygen transport capacity

Metabolism of resting, fasting and undisturbed fish - denoted standard metabolic rate (SMR) - represents the energetic costs of maintaining basic life function, such as ion and water homeostasis as well as a constitutive protein synthesis. SMR increases exponentially with increased temperature, typically by a two-three-fold rise when temperature increases by 10oC. When fish perform strenuous exercise, they reach a maximum rate of oxygen uptake (VO2max) that is determined by the physiological capacity to transport oxygen from the water to the mitochondria where metabolism takes place. VO2max also increases with elevated temperature, but declines within the upper portion of the thermal tolerance window. As a result, the aerobic scope, the difference between MMR and SMR, declines at high temperatures (Figure 3). The preferred body temperature, which is also the body temperature with maximal growth, often coincides with the temperature with the highest aerobic scope. The reasons for this overlap are not completely understood, but may simply reflect that maximal scope allows for the animal to invest in necessary life processes, such as foraging, digestion, reproduction, escape from predators. Thus, at high temperatures limit the available potential for exercise, feeding, growth and digestion in fish whichwill directly reduce Darwinian fitness (Figure 3).

The collapse of the oxygen transport cascade at high temperature

The oxygen transport cascade describes how oxygen is conveyed from the water to the mitochondria within the cells where metabolism takes place. The current models indicate that particular the capacity of the heart to generate blood flow decreases at high temperatures causing a decline of VO2max and aerobic scope. This is partially because most fish lack an elaborate coronary perfusion, so that the heart must derive a large part of its O2 from the venous blood that returns from the tissue. The heart therefore becomes oxygen deprived when the oxygen concentration in the venous blood decreases when temperature is increased. Moreover, oxygen solubility of water decreases with increased temperature, which may impair oxygen uptake across the gills.

Specifically, we will try to answer the following questions:

Is there a universal model that can explain the temperature sensitivity of fish species and how is temperature sensitivity linked to oxygen demand and oxygen transport capacity, respectively?

How do different parts of the oxygen transport cascade limit oxygen transport at high temperature?

How is temperature sensitivity of fish population altered by manipulation of the environmental conditions (i.e. oxygenation of the water) and by interactions with other physiological efforts (osmotic challenge, digestion, etc.)?

How is temperature sensitivity of fish population altered by local adaptation and or acclimation?

Involved people:

Johannes Overgaard