ASSIMILATION EFFICIENCY AND NITROGEN EXCRETION OF A 



FILTER-FEEDING PLANKTIVORE, THE ATLANTIC MENHADEN, 



BREVOORTIA TYRANNUS (PISCES: CLUPEIDAE) 



Edward G. Durbin and Ann G. Durbin' 



ABSTRACT 



Experiments were carried out at 20° C with adult Atlantic menhaden, Brevoortia tyranniis. to follow 

 the time course of changes in feces elimination rate, assimilation efficiency, nitrogen excretion rate, 

 and oxygeninitrogen ratios during and after a 7-hour period of feeding on the diatom Ditylum 

 brightwelli. Assimilation efficiency for wild zooplankton dominated by Acartia tonsa is al.so reported. 



The elimination of a meal was exponential (mean = 0.366/hour) and thus meals of different size 

 reached the same stage of digestion at the same time. Changes in stomach contents and evacuation 

 rates with time were calculated using the model of Ell iott and Persson. These corresponded clo.sely with 

 observed changes in feces elimination rates following a meal. Assimilation, as indicated by the patterns 

 of nitrogen excretion, and the elimination of a meal were rapid: 509f of the exogenous nitrogen 

 excretion occurred within 1 or 2 hours after 50'^( of the meal was ingested; 5(Ff of the feces were 

 eliminated within a mean of 5.7 hours after the midpoint of feeding. 



Mean carbon, nitrogen, and caloric assimilation efficiencies for D. brightwelli were 86.4, 92.4, and 

 SS.SCf; for zooplankton these were 86.7, 91.3, and 87.7*7^ , respectively. Assimilation efficiencies de- 

 creased when fecal elimination rates declined below 0.3 mg dry weight/g 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. 



Nitrogen excretion consisted of 69.6"7f ammonia and 30.4'~^ dissolved organic nitrogen. The mean 

 excretion rate offish unfed for 36 hours ( 10.72 /xg nitrogen/g dry weight per hour) increased as much as 

 17-fold when the fish were digesting and assimilating the food. Exogenous nitrogen excretion was a 

 constant proportion of the ingested and the assimilated rations (61.6 and 65.5%. respectively). 



Oxygen:nitrogen ratios indicated that Atlantic menhaden use protein as a metabolic fuel at all 

 times. The mean oxygen: nitrogen (28.2) offish unfed for 36 hours decreased to values as low as 5.0 

 during feeding. 



In the development of an energy budget for fish it METHODS 

 is necessary to determine the proportion of the 



ingested ration which is lost in the feces and the All experiments were carried out on a school of 



excretory products. The remainder represents 12 Atlantic menhaden, with a mean wet weight of 



physiologically useful energy available for growth 302 g, dry weight of 101 g, and fork length of 26 cm. 



and metabolism. Experiments were carried out during 26 July-9 



Here we examine digestion rates, assimilation September 1977 at a temperature of 20.0 ±1.0° C 



efficiency, nitrogen excretion rates, and oxy- and a salinity of 3H,. Details of the procedures 



gen:nitrogen (0:N) ratios of adult Atlantic used for maintenance of the fish and for carrying 



menhaden, Brevoortia tyrannus, a filter-feeding out the experiments are given in Durbin et al. 



planktivore. Since Atlantic menhaden normally (1981). 



feed for a prolonged period each day, the experi- Before an experiment the tank was thoroughly 



ments were designed to permit observations on the cleaned and the inflowing water was filtered 



fish before, during, and after a 7-h feeding period through a GAF polypropylene bag filter of nomi- 



during which food was made available at a con- nal 5 /xm pore size. The fish were deprived of food 



stant rate. This study is part of a larger effort to for 36 h to eliminate the remains of the previous 



determine the energy budget of Atlantic meqha- meal. The experiment began with a measurement 

 den in Narragansett Bay, R.I. , of the excretion rate of the unfed fish between 0600 



'Graduate School of Oceanography, University of Rhode Is- ^Reference to trade names does not imply endorsement by the 



land, Kingston, RI 02881. National Marine Fisheries Service, NOAA. 



Manuscript accepted June 1981. 601 



FISHERY BULLETIN: VOL. 79, NO. 4, 1981. 



