FISHERY BULLETIN: VOL. 79, N0.4 



secreted by the gut, resulting in lower apparent 

 assimilation efficiencies; and 2) a possible lag in 

 the secretion of digestive enzymes after a period of 

 fasting, which would cause assimilation to be ini- 

 tially low. The latter effect was observed by D. J. W. 

 Moriarty and C. M. Moriarty (1973), who found 

 that in Tilapia a period of about 4 h was required 

 for the secretion of stomach enzymes, and hence 

 assimilation efficiency, to reach high values. We 

 have no measures of temporal changes in stomach 

 enzymes for the Atlantic menhaden. While expla- 

 nation 2 may have contributed to the initial low 

 assimilation efficiency at the beginning of feeding, 

 it would not explain the decline in assimilation 

 towards the end of feces elimination, since this 

 food was presumably digested at the end of the 

 feeding period when stomach enzymes should 

 have been maximal. Visual observations of the 

 organic material surrounding the feces at low 

 fecal production rates provided support for the 

 first explanation, and would be consistent with the 

 reduced assimilation efficiency observed at both 

 the beginning and end of feces production. 



The reason for the reduced assimilation effi- 

 ciency at the highest ration level is unclear. If the 

 maximum capacity of the digestive tract was 

 reached, the assimilatory processes may have be- 

 come saturated, causing a reduction in assimila- 

 tion efficiency. 



However, the supply rate of plankton in the 

 highest ration experiment exceeded the concen- 

 tration of diatoms which Atlantic menhaden 

 would normally be expected to encounter on its 

 summer feeding grounds in Narragansett Bay 

 (Durbin and Durbin 1981). Thus Atlantic menha- 

 den probably does not ordinarily feed at such high 

 rates for prolonged periods in nature. Therefore, 

 the slight decline in assimilation efficiency at the 

 high feeding rate of Experiment 6 may not have 

 much ecological significance. We conclude that 

 overall, the effect of meal size on the assimilation 

 efficiency of Atlantic menhaden is small. Because 

 of the very rapid digestion rates and high assimi- 

 lation efficiencies of Atlantic menhaden, this 

 planktivore appears to be adapted to process effi- 

 ciently large amounts of food continuously. 



Nitrogen Excretion 



trimethylamine oxide (Watts and Watts 1974). In 

 Atlantic menhaden the percent of total N excreted 

 as ammonia (69.6'7f ) appears to be similar to that 

 observed in other species (Smith 1929; Atherton 

 and Aitken 1970; McCarthy and Whitledge 1972). 



Nitrogen excretion by Atlantic menhaden 

 changed according to whether or not the fish were 

 feeding, the rate at which they fed, and the 

 time since the last meal. Previous studies have 

 also found that N excretion increases as a result of 

 feeding ( Brett and Zala 1975; Elliott 1976; Savitz 

 et al. 1977). These studies differed from the pres- 

 ent study, however, in the timing of the peak of N 

 excretion and the subsequent return to endoge- 

 nous rates. Fingerlingsockeye salmon, Oncorhyn- 

 chus nerka, at 15° C showed a peak ammonia 

 excretion rate about 4 h after a meal and did not 

 return to a basal rate until about 16 h after feeding 

 ( Brett and Zala 1975). Similarly, N excretion rates 

 of brown trout at 17° C did not return to baseline 

 until about 12-20 h after feeding ( Elliott 1976) and 

 largemouth bass, Micropterus salmoides, at 21°- 

 23° C took 1 or 2 d (Savitz et al. 1977). In contrast, 

 when Atlantic menhaden fed continuously for 

 7 h, the excretion rate remained high 

 throughout the feeding period and lagged only 1 or 

 2 h behind ingestion of the food. The return to 

 baseline was also rapid, with 90^7^ of the exogenous 

 N excretion occurring within a mean of 2.4 h fol- 

 lowing the end of feeding. 



The Atlantic menhaden excreted a constant 

 proportion of N in its ration (61.67r of the ingested 

 and 65.5% of the assimilated ration). Savitz et al. 

 (1977) also found a linear relationship between 

 ingestion and N excretion in largemouth bass, al- 

 though in that case only 40% of the ingested N was 

 excreted. Gerking (1971) reported that in bluegill 

 there was a linear relationship between the 

 amount of N consumed and the amount retained 

 for growth, which implied that the relationship 

 between the amount of N ingested and that ex- 

 creted was also linear. Additional studies are 

 needed to determine the extent to which the pro- 

 portion of N excreted by different species varies. 

 Factors which may be expected to affect this pro- 

 portion are the nutritional requirements of the 

 fish, which may change seasonally, relative to the 

 chemical composition of the food. 



In most teleosts ammonia is the principal end 

 product of protein catabolism, and is the major 

 component of N excretion; other N compounds ex- 

 creted include urea, creatine, creatinine, and 



Oxygen: Nitrogen Ratios 



The chemical composition of Atlantic menhaden 

 compared with that of plankton indicates that the 



614 



