The physiological response to abiotic and biotic cues is dependent upon the surrounding water temperature. With the exception of members from the scrombridae (mackerels, tuna and bonitos), lamnidae (mackerel sharks and white sharks) and xiphiidae (swordfish) families, the metabolic rates, bodily temperatures and consumption rates of fish are largely dictated by external thermal conditions. A plethora of temperature gradients can be found throughout the marine environment, and studies have shown that fish indeed have an optimal thermal preferendum. Throughout much of his early career, F.E.J. Fry created a temperature hypothesis regarding these gradients, and it is still valid today. His hypothesis was threefold. First, he noted thermal preferendums are species specific. Secondly, fishes will aggregate in areas where the thermal preferendum is stable. Finally, and perhaps the most important facet of his hypothesis, the final thermal preferendum of fish will coincide with temperatures in which key physiological, biochemical and life history processes can be optimally carried out. Outside the thermal preferendum fish will undergo physiological changes, including changes in gonadal growth, somatic growth and oxygen consumption, to cope with the stress of suboptimal conditions. Increasing temperatures associated with climate change will surpass the thermal preferendum of many fish and motivate such physiological changes.
In water, as temperatures increase the concentration of O2 will decrease. Oxygen limitation is often the biggest facet of thermal intolerance in fish. In 1975, D. M. Rowell and others found a 10°C increase more than doubled the oxidative metabolic rate in winter flounder (Pseudopleuronectes americanus). Similar results regarding the winter flounder were found by J. Cech and others in 1976. However, their data also revealed another interesting physiological change. Cech noted as temperature increased, the difference in O2 between the main artery and the main vein also vastly increased, indicating more oxygen was being delivered to tissues and organs. In winter flounder, and other species, increasing temperatures will also increase heart rate, which could be attributed to the increased temperature of pacemaker cells within the heart. The hemoglobin’s capacity for oxygen appears to be affected at higher temperatures, and in the case of the winter flounder, an elevated heart rate is the physiological response to deliver a higher flow of blood to the body to combat the onset of the decreased capacity of oxygen in hemoglobin. Phenotypic plasticity is another viable option to cope with the lack of oxygen. Portner and others sampled Atlantic cod (Gadus morhua) from various sites in the North and Baltic seas. The variation of thermal regimes at each sampling site was reflected in their data in which differences in the ratio of hemoglobin types were observed. In other words, the specific conditions found at each site would elicit a variety of hemoglobin polymorphs to cope with the local conditions.
In addition to decreased oxygen consumption, the literature is ripe with studies relating growth to temperature. In some thermally stressful instances, the energy typically allocated for growth will be reallocated to cope with increasing maintenance needs. Higher temperatures for short durations have been seen to increase food consumption rates to keep up with metabolic demands. However, if higher temperatures persist or progress towards a species’ critical thermal maximum, decreases in growth usually follow. Within Fry’s thermal preferendum, there lies an optimal temperature in which growth rate is maximized. Events or disturbances decreasing or increasing the temperature above or below this optimum will yield decreased growth rates. When relating growth rate to temperature, cod (Gadus morhua) displays classic trends. In a sub-experiment involving various sizes of cod, Bjornsson and others found an optimum temperature of approximately 8°C yielded the highest growth rate. Interestingly, they also found the optimal temperature decreased as fish size increased. This would suggest one of two ideologies. (1) The younger smaller fish have a wider optimal temperature regime, or (2) the thermal optimum shifts with age and is dependent on size. In either case, higher temperatures would allow for faster development of juveniles to adulthood, and higher temperatures stimulating maximum growth rates would not be necessary at the adult level.
Temperature is also a determining factor in the maturation of fish. Increasing temperatures will delay the various maturation schedules found in the fish community. Although it is unclear exactly how temperature affects this process, it is hypothesized to do so through alteration of maturation regulatory proteins, secretion of gonadotropins, oestrogen in the liver or mechanistically altering oocyte growth rates. This shows changes in temperature regimes carry potential recruitment and fecundity repercussions.
References and Photo Credits
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