Protein synthesis, nitrogen excretion and long-term growth of juvenile Pleuronectes flesus

This study aimed to measure protein synthesis using a stable isotope method, investigate protein-nitrogen flux in a flatfish Pleuronectes flesus, and use the data to test the hypothesis that individual differences in growth efficiency were related to individual differences in protein-nitrogen flux mediated through differences in protein synthesis and degradation. Three measurements of protein-nitrogen flux via consumption, protein synthesis and nitrogenous excretion were made for individual flounder during a 212-day period and fractional rates of protein-nitrogen flux were scaled for a 50-g flounder to provide mean values for protein consumption (2-11 ± 0.21% day-1), protein synthesis (2.08 ± 0.23% day-1), protein growth (0.71 ± 0.06% day-1) and protein degradation (1.37 ± 0.24% day-1). Mean rates of nitrogenous excretion were 0.142 mg N g-1 day-1 and 0.047 mg N g-1 day-1 for ammonia and urea, respectively. Individual flounder had different protein growth efficiencies and this was correlated negatively and significantly with mean rates of protein synthesis (r= - 0.70; P<0.05) and degradation (r= - 0.67; P<0.05) and correlated positively and significantly with the efficiency of retaining synthesized protein (r+0.63, P<0.05). This supported the proposed hypothesis that flounder which grow more efficiently achieve this through adopting a low protein turnover strategy.