Nitrogen excretion and absorption efficiencies of sardine Sardinops sagax fed phytoplankton and zooplankton diets

Laboratory experiments to measure nitrogen excretion rates and estimate absorption efficiencies were conducted on adult sardine Sardinops sagax fed phytoplankton and zooplankton diets. Sardine are predominantly ammoniotelic, with an endogenous total nitrogen excretion rate of 28.11 ± 1.90 μg g-1 DBM (dry body mass) h-1. Neither fish size nor temperature had a significant effect on endogenous excretion rates. Peak post-feeding exogenous nitrogen excretion rates were 3.5 to 10 times higher than endogenous rates, were linearly related to nitrogen ration, and occurred within 3 h of the ingestion of food. Absorption of a meal was rapid; 50% of the exogenous nitrogen was excreted by 2.2 ± 1.0 h and 90% by 7.4 ± 3.6 h. Faeces elimination was exponential and dependent on food type; phytoplankton was eliminated faster than zooplankton. Elimination of the meal was much slower than the excretion of dissolved nitrogen, requiring 13.0 ± 4.1 h for 50% elimination and 50.8 ± 8.3 h for 90% elimination. Faeces C:N ratios decreased over time for fish fed phytoplankton, but increased when zooplankton was the food. A constant portion of the ingested and absorbed nitrogen was excreted (69.8 ± 11.1 and 74.4 ± 12.1% respectively), and it is estimated that adult sardine require an absorbed daily maintenance ration of 2.64 mg N g-1 DBM d-1. Sardine are omnivorous, but have higher absorption efficiencies when fed zooplankton (77.6, 88.2 and 93.3% for dry mass, carbon and nitrogen respectively) than when fed phytoplankton (42.6, 62.6 and 78.7%). That Sardinops sagax is able to better utilize zooplankton than phytoplankton suggests that this species derives more of its nutritional requirements from carnivory than from herbivory.