Seasonal and sex-based variations in energy use in wild plaice: Differences in energetic costs of spawning and feeding behaviours outweigh effects of temperature on field metabolism

Wild animals frequently show strong variations in allocations of energy to growth, reproduction and locomotion, typically corresponding to seasonal variations in temperature or resource availability. Seasonal variations in energy allocation to growth are commonly expressed in hard tissues as growth bands, but energy allocation to growth may be decoupled from absolute energy use especially in reproductively active individuals. Consequently, determining variations in absolute energy use in wild animals through annual cycles is challenging, particularly for aquatic animals such as fishes. The stable isotopic composition of carbon in the otolith of marine teleost fishes co-varies with the rate of oxidation of respiratory carbon and therefore serves as a proxy for energy use. Stable isotope analyses of otolith aragonite allow reconstruction of seasonal variations in total energy use independent from sclerochronological assessments of growth. Here we describe how total energy use varies seasonally within a sample of 272 free-roaming European plaice from the North Sea. Estimated energy (field metabolic rate) use peaked in early summer months prior to spawning, co-incident with formation of otolith opaque zones and increases in body condition, but decoupled from seasonal variations in experienced temperature. Maximum observed FMR estimates in years characatreised by high metabolc rates, approach laboratory inferred maximum metabolic rates, implying maximum feeding rates are limited by respiratory capacity rather than prey availability. The strong co-incidence between opaque zone formation and high energy use supports suggestions that otolith transparency is directly related to the rate of production of respiratory CO 2 (and therefore metabolic rate), and indirectly associated with somatic growth rates. Seasonal patterns of energy use differed between sexes. Males showed reduced energy use through summer months compared to females, but higher apparent energy use outside of the summer feeding season. Female-biased sexual size dimorphism in plaice may emerge via both greater energy allocation to reproduction and reduced energy acquisition in summer in males. Observed seasonal, sex-based and among-individual variations in energy use far exceed predicted thermal effects on enzyme reaction rates, implying that individual variations in realized field metabolic rates are constrained more by physiological and behavioural factors than by external ambient temperature. Our observations provide context for considering the implications of temperature on energy partitioning and physiological performance in wild fish populations across seasons and demonstrate how isotopic proxies for energy use compliment energy budget models and sclerochronological growth analysis.