Energetics of arctic Alaskan fishes: carbon isotope evidence

Abstract

The natural abundance of carbon isotopes were used to investigate the energy requirements of arctic aquatic consumer organisms. A mathematical model was developed that describes the relationship between the rate of consumer isotope turnover rates of growth and metabolism, and the utility of the model for calculating energy requirements from seasonal changes in consumer isotopic composition was demonstrated in laboratory experiments and with field data. The energy requirements of anadromous fishes, which were the major consumers studied, could not be determined using the isotopic data. Instead, the requirements were calculated using biochemical data and rates of growth and oxygen consumption, and were about 2.6-6.0 kcal day⁻¹ in the summer and 0.4-1.2 kcal day⁻¹ in the winter. Seasonal changes in lipid and protein contents indicate that anadromous fishes cannot find enough food during the winter to supply their energy requirements, and that about 50% of the energy needed comes from the metabolism of tissues accumulated during the summer. The relative importances of marine and freshwater food webs in supplying the energy requirements were determined by comparing the seasonal isotopic components of anadromous fishes to the isotopic compositions of fish that are permanent residents in each environment. It appears that anadromous fishes that overwinter in the Coleville River (the largest river on the Alaska North Slope) depend almost entirely on the marine environment for their energy, whereas those that overwinter in Canada's Mackenzie River rely on both marine and freshwater habitats.