FEEDING SELECTIVITY OF SCHOOLS OF NORTHERN ANCHOVY, 

 ENGRAULIS MORDAX, IN THE SOUTHERN CALIFORNIA BIGHT 



J. Anthony Koslow^ 



ABSTRACT 



Direct field measurements of the feeding of five schools of northern anchovy over four sets of 

 conditions indicate consistent size-selective feeding on the dominant zooplankton taxa. At low-to- 

 moderate prey concentrations (10-40 mg carbon per cubic meter), the schools consumed 35-50% of the 

 total zooplankton biomass and >90% of the largest zooplankters present. The schools' feeding was a 

 positive function of prey size primarily. The density of particular prey items did not significantly 

 affect feeding selectivity. The northern anchovy fed preferentially upon a particular species in only 

 one instance. No significant difference was found in the selectivity of two northern anchovy schools 

 composed primarily of late 0-group and Il-group fish, respectively, that were feeding under similar 

 feeding conditions. At prey concentrations of 10-40 mg carbon per cubic meter, the degree of 

 selectivity was inversely related to the size of the largest prey available. The prey size at which 

 consumption is predicted to be 100% was proportional to the size of the largest prey. 



Field studies have demonstrated that planktiv- 

 orous fish can control zooplankton community 

 structure in oligotrophic lakes and stocked fish 

 ponds by selectively feeding upon the larger, 

 more visible prey organisms (see Gliwicz and 

 Prejs 1977 and Dodson 1979 for a critical discus- 

 sion of this work). However, while highly produc- 

 tive regions in the world's oceans typically sup- 

 port large populations of schooling, planktivorous 

 fish, the impact of these fish populations upon 

 marine zooplankton communities is not known. 



Taking as an example the estimated consump- 

 tion of zooplankton by the northern anchovy, 

 Engraulis mordax, in the Southern California 

 Bight, it becomes clear that marine fish popula- 

 tions may have considerable impact on the zoo- 

 plankton in the system. The prey consumption of 

 the northern anchovy may be calculated based 

 upon data for the biomass of the population, its 

 annual reproduction and growth, and assump- 

 tions concerning its metabolic efficiency. The 

 results of this calculation can then be compared 

 with estimates of zooplankton production in the 

 region. 



In the Southern California Bight, the spawning 

 biomass of the northern anchovy in the mid- 

 1960's to early 1970's averaged between 1.32 and 

 2.35 X 10^ t over a 40 x 10^ km^ area or 1.34-2.25 



g C/m^ (calculated from Smith 1972). Engraulis 

 mordax spawns approximately 20 times annually 

 and produces 389 eggs/g wet weight at each 

 spawning (Hunter and Goldberg 1980). Averaged 

 over the year, this is equivalent to a daily produc- 

 tion rate of 0.43%, based upon a dry weight per 

 egg of 0.030 mg (Hunter and Leong^) or 0.20 mg 

 wet weight (assuming a 15% wet weight:dry 

 weight conversion): [(389 x 20 x 0.2 x 10"^)/ 

 365] x 100 = 0.43. 



The growth rate of the northern anchovy past 

 the first year of life is negligible, approximately 

 0.08% /d from the end of the first year to the end 

 of the third year (calculated from Sakagawa and 

 Kimura 1976). The total daily production of the 

 adult northern anchovy is thus approximately 

 0.43% + 0.08% = 0.51%. Assuming a 10-30% 

 efficiency of food conversion (Paloheimo and 

 Dickie 1966 and references therein; Jones and 

 Hislop 1978; Lane et al. 1979), mature northern 

 anchovy consmne 1.7-5.1% of body weight daily: 

 0.51 X 1/0.30 = 1.7; 0.51 x 1/0.10 = 5.1. [The food 

 consumption rate for 0-group northern anchovy 

 is considerably greater since the daily growth 

 rate during the first year is about 6.1% (calcu- 

 lated from Sakagawa and Kimura 1976).] The ma- 

 ture northern anchovy stock therefore consumes 



'Scripps Institution of Oceanography A-008, University of 

 California, San Diego, La Jolla, CA 92093; present address: 

 Department of Oceanography, Dalhousie University, Halifax, 

 Nova Scotia B3H 4J1, Canada. 



^Hunter, J. R., and R. Leong. The spawning energetics of 

 female northern anchovy, Engraulis mordax. Unpubl. 

 manuscr. Southwest Fisheries Center La Jolla Laboratory, 

 National Marine Fisheries Service, NOAA, PO. Box 271, La 

 Jolla, CA 92038. 



Manuscript accepted August 1980. 

 FISHERY BULLETIN: VOL. 79, NO. 1, 1981. 



131 



