Physiological differentiation among nine-spined stickleback populations: effects of copper exposure

Nine-spined stickleback (Pungitius pungitius) exhibit a very wide geographical distribution and are increasingly used in ecological and evolutionary research. While pronounced morphological and behavioural differentiation among local populations has been shown, physiological differentiation, especially with respect to stress responses, has not been investigated. However, this would be of interest since the increased use of sticklebacks as ecotoxicological sentinel species presumes a uniform response of used populations to stressors. Metabolic rates of nine-spined sticklebacks from five populations residing in similar latitude across Fennoscandia (Baltic Sea, White Sea, two Finnish and one Russian pond) were compared under controlled conditions, and the effects of exposure to increasing copper concentrations (0-7.1micromoll(-1)) on resting metabolic rate, condition factor and survival were tested. While sticklebacks from the two Finnish pond populations consisting of 'giant' fish were the largest, body condition index was highest in the Baltic population. Weight-corrected resting metabolic rates were also significantly different between populations with the highest rate in the Baltic (89.6+/-18.3, n=12) and the lowest in the White Sea and the Russian pond populations (62.2+/-10.3, n=10 and 56.5+/-10.3nmol O(2) min(-1)g(-1), n=12, respectively). Allometric metabolic rate - weight analysis revealed a metabolic scaling exponent of 0.986, significantly higher than a generally accepted exponent for fish (0.88), suggesting an elevated resting metabolic rate for the two 'giant' stickleback populations. Copper exposure caused an overall increase in metabolic rate of 3.1nmol O(2) min(-1)g(-1)/1micromoll(-1) increase in copper. However, the copper-induced changes in metabolic rate differed significantly among populations, with the least increase in the Baltic and the highest increases in the two Finnish pond populations. Survival of fish following copper exposure was significantly lower for Baltic and one Finnish pond population (mean time to death: 32 days), compared to the White Sea and the other Finnish and Russian pond populations (mean time to death: 68 days). The results demonstrate significant physiological differences in metabolic rate and stress response among local populations, and suggest that caution has to be exercised and pilot studies have to be carried out when different wild populations of a single species are used for ecotoxicological monitoring.