Assimilation efficiency and nitrogen excretion of a filter- feeding planktivore the Atlantic menhaden Brevoortia tyrannus (Pisces: Clupeidae).

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Elimination of a meal was exponential (mean = 0.366/hr) and thus meals of different size reached the same stage of digestion at the same time. Assimilation, indicated by the patterns of N-excretion, and elimination of a meal were rapid: 50% of exogenous N excretion occurred within 1 or 2 hr after 50% of the meal was ingested; 50% of the feces were eliminated with a mean of 5.7 hr after the midpoint of feeding. Mean carbon, N and caloric assimilation efficiences for Ditylum brightwelli were 86.4, 92.4, and 89.5%; for zooplankton dominated by Acartia tonsa these were 86.7, 93.3, and 87.7%, respectively. Assimilation efficiencies decreased when fecal elimination rates declined below 0.3 mg dry weight per hour. This resulted in lower assimilation at the beginning and end of each experiment, and a slight decrease in the overall assimilation efficiency in low ration experiments, where the fecal elimination rates were also low. Oxygen:nitrogen ratios indicated that Atlantic menhaden use protein as a metabolic fuel at all times.-from Authors

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Fishery Bulletin