McCarthy and whitledge: nitrogen excretion 



gree in the urea release rates; this is clearly 

 reflected in the ammonia-urea ratios. When the 

 period of starvation was the greatest (experi- 

 ment 3a) the ratio was the lowest, and when 

 the starvation was least the ratio was the great- 

 est. The ratio for experiment 3b presumably 

 would have increased to a value comparable to 

 that for experiment 2 when the time since feed- 

 ing had become equal for both. 



Upon thawing, the brine shrimp which were 

 used to feed E. mordax liberated ammonia, urea, 

 and creatine. If these substances were retained 

 in large quantities by the shrimp, the results 

 of experiments 2 and 3b could have been af- 

 fected. Such interference is, however, unlikely 

 since only the ammonia release rate increased 

 with feeding in experiment 3. If this increased 

 rate of release had resulted from ammonia lib- 

 erated from the shrimp after ingestion by the 

 fish, a comparable increase in the rate of urea 

 release would have been expected since the 

 quantities of urea liberated by the shrimp were 

 approximately equal to those of ammonia. 



It is important to note that since experiment 

 3 was conducted at a temperature 2.5 °C lower 

 than the other experiments, a correction of the 

 measured rates should be made in order to com- 

 pare them properly. Since an appropriate Qio 

 value was not available, the correction was not 

 made. Presumably, however, a Qio for an ammo- 

 nia release rate would be similar if not identical 

 to that for urea, and the ammonia-urea ratio 

 would not be changed with a temperature cor- 

 rection. The lack of both a temperature cor- 

 rection and an appropriate relationship between 

 body size and nitrogen excretion also makes the 

 comparison of data between different species dif- 

 ficult. 



The E. ringens experiments produced the high- 

 est release rates (particularly for creatine) and 

 the lowest ammonia-urea ratios. This can pro- 

 bably be attributed to the high level of activity 

 and/or the poor health of the specimens. 



Wood (1958) ran experiments for 24 hr at 

 12°C, and from his Table 2 and an approxima- 

 tion of 28% wet weight = dry weight, one can 

 calculate ammonia and urea release rates. These 

 calculated mean release rates are 0.0141, 0.0179, 

 and 0.0065 fjcg at ammonia-N + urea-N/mg dry 



weight/day, for the sculpin {Leptocottus armcu- 

 tus) , the starry flounder (Platichthys stellatus) , 

 and the blue sea perch {Taeniotoca lateralis) re- 

 spectively. These rates are nearly an order of 

 magnitude lower than those reported here, but 

 a temperature correction (assuming a Qio of 2) 

 would bring them within approximately a factor 

 of five. The calculated ammonia-urea ratios for 

 the sculpin, the starry flounder, and the blue sea 

 perch are 3.09, 7.13, and 1.26 respectively. Since 

 the fish used in Wood's studies were nearly ten 

 times larger than the anchovies, were maintained 

 for some time prior to the experiments on a diet 

 of lingcod muscle, and for the experiments were 

 enclosed in chambers barely larger than the fish, 

 it is difl^cult to compare the results of the dif- 

 ferent sets of data. 



Ammonia and urea are important plant nu- 

 trients and it is of interest to examine the sig- 

 nificance of fish excretion as a source of these 

 substances in the sea. Whitledge and Packard 

 (1971) estimated that nitrogen excretion by the 

 herbivorous E. ringens in the near surface 

 waters of the Peru Current is an order of mag- 

 nitude greater per unit volume of water than 

 zooplankton excretion and they suggested, on 

 the basis of these rates and measured rates of 

 nitrogen uptake by phytoplankton, that fish ex- 

 cretion may be a major source of the ammonia 

 utilized by phytoplankton in this area. 



Oflf the coast of southern California the con- 

 tribution of the fish community in the regenera- 

 tion of ammonia and urea can also be estimated. 

 Integrated values of phytoplankton nitrogen 

 utilization at three stations in the euphotic zone 

 off the coast of San Diego (Stations 1, 4, and 6, 

 McCarthy* averaged 0.073 /xg at ammonia-N/li- 

 ter/day and 0.066 /xg at urea-N/liter/day. For 

 the area included in the California Cooperative 

 Oceanic Fisheries Investigations (CalCOFI) 

 survey, the total biomass of the most common 

 species of near-surface fish [northern anchovy 

 (Engraidis mordax), Pacific hake (Merluccius 

 productus) , jack mackerel (Trachurus symmet- 

 7'icus) , Pacific saury {Cololabis saira), and al- 



* McCarthy, J. J. The uptake of urea by natural pop- 

 ulations of marine phytoplankton. Manuscript in prep- 

 aration. 



399 



