Predator stress decreases standard metabolic rate and growth in juvenile crucian carp under changing food availability.

Animals adapt to the challenges of fluctuations in predator risk and food availability in their natural habitats. Phenotypic plasticity allows animals to handle environmental changes. However, the patterns of flexibility in metabolic rates and its ecological consequences under different predator stress and food availability conditions are poorly understood. Here, we used crucial carp (Carassius auratus) as a prey species and northern snakehead (Channa argus) as a predator to test whether predator stress influences metabolism and growth, and alters the link between flexibility in metabolic rate and its ecological consequences (e.g., growth) in crucial carp. The experiment was carried out under the conditions of predator stress (with or without a predator) and three food availabilities (satiation feeding 1 time per day, low food availability; 2 times per day, intermediate food availability; and 3 times per day, high food availability) for 3 weeks. After 21 days of feeding, the final body mass and body length in the two treatments increased compared to the initial values in all three food availabilities. The feeding intake (FI) and specific growth rate (SGR) of the two treatments increased with increasing food availability. The control treatment had a higher FI and SGR than the predator stress treatment in all three food availabilities. The feeding efficiency (FE) of the two treatments was higher at the high and intermediate food availabilities than at the low food availability. However, no effect of predator stress on FE was detected. The final values of original or standardized SMR were higher in the control treatment than the predator stress treatment at the intermediate and high food availabilities. The changes in SMR (ΔSMR) were higher in the control treatment than in the predator stress treatment. The positive correlation between the ΔSMR and SGR was found in the intermediate food availability in the predator stress treatment, suggesting that individuals with a higher flexibility in SMR had a larger growth rate and vice versa, but this relationship was dependent on food availability. Our results suggest that predator stress decreased maintenance metabolism, feeding and growth of juvenile crucial carp irrespective of food availability. Predator stress does not alter the growth advantages conferred by the metabolic plasticity of the fish under changing food availability.