FISHERY BULLETIN: VOL. 84, NO. 4 



m 3 . The estimates of larval densities are similar to 

 estimates for the Southern California Bight near- 

 shore CalCOFI area in 1978-79 (461 per 1,000 m 3 , 

 calculated from table 4 of Brewer and Smith 1982, 

 assuming average tow depth = 210 m; two-thirds 

 of the stations were >210 m according to their table 

 2). The mean density of eggs in the Bay was much 

 higher than in the Southern California Bight near- 

 shore CalCOFI area (310 per 1,000 m 3 , Brewer and 

 Smith 1982). The seasonal northern anchovies fish- 

 ery in the Bay took approximately 481 tons for 

 frozen and live bait (Smith and Kato 1979). My esti- 

 mate is adequate to permit such a yield. 



Northern anchovy females need a daily ration of 

 4-5% of their body weight of copepods per day to 

 support growth and reproduction (Hunter and 

 Leong 1981). Approximately 5% of caloric intake 

 goes into growth. Using these values, 38.35 tons of 

 copepods per day would be consumed by the July 

 biomass of 767 tons of anchovies. Growth would be 

 about 1.92 tons per day. Doing similar calculations 

 for each month and summing for the 12 mo of this 

 study result in an estimate of 3,260 tons of cope- 

 pods consumed and a net annual production of 158 

 tons of anchovy growth. If the egg estimates based 

 on the area of the Bay, including the shallow areas 

 were used, the consumption of copepods and growth 

 estimates would be approximately doubled. These 

 calculations are a first order estimate of the impact 

 of a carnivorous planktivore on zooplankton in the 

 Bay. The energy converted to anchovy growth 

 would be removed from the Bay, so the estimate of 

 net growth is also a minimum estimate of a sink for 

 Bay production as growth of a transient consumer. 

 In San Francisco Bay where plankton production 

 from a limited area is being consumed by a large, 

 transient anchovy population, grazing by anchovy 

 could conceivably limit zooplankton abundance 

 seasonally. Although it is impossible to distinguish 

 between grazing and interannual differences with- 

 out estimates of zooplankton production, zooplank- 

 ton was more abundant in winter 1978-79 when 

 adult anchovies were absent. 



A large biomass of planktivores could have other 

 effects on the ecology of the Bay. Selective feeding 

 by clupeoids on larger organisms in lakes can affect 

 the zooplankton community structure (Brooks and 

 Dodson 1965). Northern anchovy schools can also 

 have an impact on nutrient cycling. Smith and Epley 

 (1982) calculated that ambient ammonium concen- 

 tration would be nearly doubled behind an anchovy 

 school in the Southern California Bight. McCarthy 

 and Whitledge (1972) estimated that nitrogen excre- 

 tion by the Peruvian anchoveta is an order of mag- 



nitude greater than zooplankton excretion, so fish 

 excretion may be the major source of regenerated 

 nitrogen nutrients for phytoplankton production. 

 These high nitrogen inputs would be patchy (Blax- 

 ter and Hunter 1982) and their importance would 

 depend on whether or not background levels of 

 nutrients were limiting. Nutrients may not be limit- 

 ing in San Francisco Bay where light penetration 

 and residence time control phytoplankton dynamics 

 (Cloern 1979). Laboratory studies of copepod pro- 

 ductivity, anchovy predation, and nutrient regenera- 

 tion are needed to define quantitatively the impact 

 of the northern anchovy on plankton dynamics in 

 the Bay. A complete description of the trophic role 

 of anchovy in the Bay should include estimates of 

 zooplankton consumption by larvae, cannibalism by 

 adults, and predation on adult and larval anchovies. 



CONCLUSION 



San Francisco Bay is a favorable area for north- 

 ern anchovy spawning because it has abundant food 

 for adults, protection from advective loss for eggs, 

 and abundant food for larvae. There is apparent 

 habitat partitioning between spawning adults and 

 larger larvae which could adaptively reduce preda- 

 tion and competition. Recruitment to the Califor- 

 nia Current stocks may be determined more by 

 events in the nursery habitat of larvae and juveniles 

 than by conditions favorable for spawning adults and 

 first-feeding larvae; therefore, further work in estu- 

 aries and nearshore areas is warranted. 



ACKNOWLEDGMENTS 



This study was done under the direction of Mar- 

 garet G. Bradbury as partial fulfillment of the re- 

 quirement for the M.A. in Biology at San Francisco 

 State University. I thank her and the other members 

 of my committee, Robert Berrend and Thomas 

 Niesen, for advice and assistance in completing the 

 work. Michael Hearne supplied the plankton net and 

 assisted in all the field sampling. The figures were 

 drafted by J. Javech. The preparation of the manu- 

 script was supported in part by the National Oceanic 

 and Atmospheric Administration under Cooperative 

 Agreement #NA 84-WC-H-06098. 



LITERATURE CITED 



Aplin, J. A. 



1967. Biological survey of San Francisco Bay, 1963-1966. 

 Calif. Fish Game, Mar. Resour. Oper. Lab. Ref. No. 67-4, 

 131 p. 



892 



