Quantifying the cost of living for marine fishes – the stable carbon isotope composition of carbonate biominerals provides new insights into population responses to climate change.
Predicting the responses of animals to climate change is one of the most pressing and challenging tasks faced by ecologists. Species may show population-level responses such as range shifts, local extinctions, and changes in average body size, growth rates and reproductive dynamics. Population dynamics may be altered, and ultimately species may be replaced. All these responses are linked to the energetic status of individuals in response to their thermal and ecological setting. Individual metabolism is therefore a unifying variable in animal ecology, linking the innate adaptive biochemistry of the animal to the local environmental conditions.
Field metabolic rate is recognised as the most ecologically, relevant measure of energy expenditure, but to date we have no practical methods to determine field metabolic rate in aquatic organisms. At present metabolic theory attempting to explain the response of marine animal populations to environmental change draws on respiratory potential - extremes of metabolic performance that can be determined in laboratory conditions. In reality, the energy expended by animals in the wild reflects a complex of energetic trade-offs.
Recently we have shown that field metabolic rate is reflected in carbon supply to biominerals, and that the isotopic composition of carbon in otoliths (aragonitic structures in the fish ear) is directly and predictably related to oxygen consumption rate. The stable isotope composition of oxygen in otolith aragonite also provides an estimate of the body temperature of the fish during the same time period over which field metabolic rate is estimated. Furthermore, otolith growth increment analyses provides a measure of somatic growth. Each otolith therefore provides a lifetime metabolic, thermal and growth history of the individual. Analyses are relatively rapid, cheap and can be performed retrospectively, opening potential of vast historic archives of otoliths.
In this talk I will outline the physiological and biochemical mechanisms underpinning the otolith isotope method for estimating field metabolic rate, and show how measurements of the distribution of field metabolic rates expressed within populations reveals new insights into the thermal biology and ecophysiology of fishes. I will also outline areas in need to new information, and speculate on possible applications in ecology and palaeoecology.