FISHERY BULLETIN: VOL. 70, NO. 2 



bacore {Thunniis alalunga)] is estimated at 

 20-25 X 10" metric tons wet weight (Dr. P. E. 

 Smith, personal communication). Using an 

 area of 70 x 10^°m-, a depth of 140 m, an average 

 excretion rate from Table 1 (E. mordax, exper- 

 iments 1 and 2, and T. symmetricus) , and a 

 conversion factor (dry wt = 28 9r wet wt) ap- 

 proximate production rates of 0.0075 /x.g at 

 ammonia-N liter/day and 0.0015 /xg at urea-N/ 

 liter day can be calculated. These rates would 

 account for 10 '"f of the ammonia and 2% of the 

 urea utilized by the phytoplankton. 



Other investigators— Harris, 1959; Dugdale 

 and Goering, 1967 and 1970; and Martin, 1968 

 — have attempted to balance ammonia utiliza- 

 tion by phytoplankton and excretion by zoo- 

 plankton in Long Island Sound, the Bermuda re- 

 gion, the Peru Current and Narragansett Bay 

 respectively. Further calculations can be made 

 for the area off southern California to compare 

 the significance of fish ammonia and urea excre- 

 tion to that of zooplankton. A zooplankton stand- 

 ing crop estimate of 0.125 mg dry wt/liter was 

 calculated from 10 years of data collected in the 

 California Current as part of the CalCOFI pro- 

 gram by multiplying the mean catch by a factor 

 of 3 to compensate for the biomass of the smaller 

 zooplankton lost through the 0.505 mm mesh (Dr. 

 P. E. Smith, personal communication). Using 

 average excretion rates for recently fed zoo- 

 plankton {Calaniis helgolandims, Calanus chil- 

 enis, and Clausocalanus sp.) of 0.73 ixg at ammo- 

 nia-N/mg dry wt/day and 0.36 fj-g at urea-N/mg 

 dry wt/day (McCarthy, 1971) average regener- 

 ation rates of 0.090 (xg at ammonia/liter/day 

 and 0.045 /jLg at urea-N/liter/day can be cal- 

 culated. These rates would account for 123% 

 of the ammonia and 68 Sf of the urea uti- 

 lized per day. If on the other hand, micro- 

 zooplankton and zooplankton biomass data col- 

 lected from April through September 1967 in the 

 same approximate area as two of the three sta- 

 tions used for the phytoplankton utilization cal- 

 culations are applied (Beers and Stewart, 1970; 

 Mullin and Brooks, 1970) the calculated regen- 

 eration would be 0.020 ixg at ammonia-N/liter/ 

 day and 0.010 /xg at urea-N/liter/day. These 

 rates account for 27*"/ of the ammonia and 15% 

 of the urea utilized by the phytoplankton. Hence, 



the fish contribution would amount to 8-27% of 

 the ammonia and 3-13 ^r of the urea released by 

 both groups of organisms. There undoubtedly 

 are, however, situations in which large fish or 

 schools of fish (the mean density of an E. mordax 

 school is estimated as the equivalent of 1,300 

 10-cm fish/m^ Dr. P. E. Smith, personal com- 

 munication) are more important than zooplank- 

 ton in supplying ammonia and urea to a partic- 

 ular parcel of water. 



Obviously these calculations are based on 

 many simplified assumptions. Other than the 

 fact that the zooplankton and fish biomass esti- 

 mates are averages for larger areas and longer 

 periods of time than can be represented by the 

 phytoplankton utilization rates, perhaps the most 

 poorly based assumption is the application of 

 mean excretion rates for three zooplankton spe- 

 cies and two fish species to the entire zooplankton 

 and fish populations. More reliable estimates of 

 excretion rates are needed for smaller species of 

 zooplankton and larger species of fish. 



ACKNOWLEDGMENTS 



We are grateful to Dr. 0. Holm-Hansen for 

 the Trachurus symmetriais specimens, to Dr. 

 Reuben Lasker for the Engraulis mordax spec- 

 imens, laboratory facilities, and helpful advice 

 and to Drs. Paul E. Smith and John A. McGowan 

 for advice and encouragement. This work was 

 supported by Federal Water Quality Administra- 

 tion Grant 16010 EHC to Dr. R. W. Eppley, and 

 the National Science Foundation under grants 

 GB-8648 and GB-18568 to the University of 

 Washington and GB-24816 to Scripps Institution 

 of Oceanography for operation of the Alpha 

 Helix Research Program. 



LITERATURE CITED 



Armstrong, F. A. J., P. M. Williams, and J. D. H. 

 Strickland. 



1966. Photo-oxidation of organic matter in sea 

 water by ultra-violet radiation, analytical and 

 other applications. Nature (Lond.) 211:481-483. 

 Baldwin, E. 



1964. An introduction to comparative biochemistry. 

 4th ed. Cambridge Univ. Press, Lond., 179 p. 



400 



