FISHERY BULLETIN: VOL. 83, NO. 2 



estimating biomass using conventional ichthyo- 

 plankton data rather than the extensive sets of 

 specialized information required by the EPM 

 method. This alternative method provides esti- 

 mates of the daily egg production (Pq) and is 

 referred to as the historical egg production (HEP) 

 to distinguish it from the current EPM. This 

 model for HEP requires only the standing stock of 

 unstaged eggs, and the numbers of larvae in 

 various length classes subsequently transformed 

 into age classes using Gompertz growth curve 

 (Zweifel and Hunter"; Methot and Hewitt^; Lo 

 1983). Daily egg production varies proportionally 

 with the stock size if the reproduction effort of the 

 population remains constant. The production of 

 eggs by a stock is certainly a better index of stock 

 size than the standing stock of larvae (Smith 1972 ) 

 because both egg and larval mortality rates are 

 considered in the former case. 



In addition to development of the model, I 

 provide a time series of northern anchovy HEP for 

 1951-82. This historical record of daily egg produc- 

 tion rather than the EPM i Equation (2) ) was used 

 to estimate anchovy biomass for these past years 

 because data were not available for all the female 

 reproductive parameters until 1980 and none of 

 the eggs have been staged. It would be unpractical 

 and take years to do all the staging of eggs that 

 would be required for all the years. The HEP is an 

 unbiased index for the spawning biomass (Ba ) of 

 the anchovy population for those years if the 

 annual reproductive output per fish weight has 

 remained constant. I do not have sufficient data 

 to validate the assumption of constant reproduc- 

 tive output although 1981-82 data do indicate so. 



peak spawning season of northern anchovy was 

 usually February- April, daily egg production for 

 the central stock northern anchovy was computed 

 from egg and larval data (CalCOFI'^) collected in 

 January-April within these eight regions. The 

 CalCOFI survey was conducted each year until 

 1966 after which the survey was conducted every 

 3 yr. Owing to various improvements in the design 

 of the plankton nets over the past 20 yr (Smith and 

 Richardson 1977; Stauffer and Picquelle 1980*^), 

 different calibration factors were necessary to 

 standardize the catch of eggs and larvae taken in 

 different nets: Aim ring net with 0.55 mm silk 

 mesh was used until 1969 when it was replaced by 

 aim ring net with 0.505 mm nylon mesh; this net 

 was used until 1978 when it was replaced by the 

 bongo net of 0.505 mm nylon mesh. Beginning in 

 1979, a vertical tow of the 0.333 mm mesh, 25.23 

 cm diameter CalVET net (CalCOFI vertical egg 

 net) (Hewitt 1983) was used along with the 0.505 

 mm mesh bongo net to collect egg and larval 

 samples in order to estimate the northern anchovy 

 spawning biomass using the egg production meth- 

 od (EPM) (Parker 1980). In addition to the bias in 

 catch caused by the different mesh sizes, biases 

 also existed due to avoidance of the net, water 

 volume filtered through the net (measured by 

 water flowmeter readings), growth rate of larvae, 

 temperature dependent incubation time (in days), 

 and proportion of larvae from each plankton 

 sample sorted (Zweifel and Smith 1981; Lo 1983). 

 All data (counts of eggs and larvae) were adjusted 

 for the above biases, when it was appropriate, 

 following the procedures outlined by Zweifel and 

 Smith (1981). 



ASSEMBLY AND BIAS CORRECTION OF 

 EGG AND LARVAL DATA 



The northern anchovy spawning area lies off 

 central and southern California and Baja Califor- 

 nia. The sampling area was divided into 23 regions 

 covering 17.556 x 10" m" (Fig. 1). The central 

 anchovy stock is enclosed by eight regions (4, 5, 7, 

 8, 9, 11, 13, and 14) with a total of 5.703 x 10" m^ 

 (Duke 1976^ Huppert et al. 1980). Because the 



Egg Data 



The counts of unstaged eggs from each tow were 

 adjusted to a standardized volume of water filtered 

 per unit depth (0.05 m^/1 m depth = 0.05 m^ sea 

 surface area = area sampled by the CalVET net). 

 The adjusted egg counts per 0.05 m^ sea surface 

 area were then stratified by CalCOFI regions. A 

 weighted mean egg count per 0.05 m^ was com- 

 puted as 



^Zweifel, J. R., and J. R. Hunter. Unpubl-. manuscr. Tem- 

 perature specific equations for growth and development of an- 

 chovy, Ertf^rciiilis mordax, during embryonic and larval stages. 



^Methot, R. D., and R. R Hewitt." 1980. A generalized 

 growth curve for young anchovy larvae, derivation and tabular 

 example. Natl. Mar Fish. Serv, Southwe.st Fish. Cent. Admin. 

 Rep. LJ-80-17, 8 p. 



•'Duke, S. 1976. CalCOFI station and region specifica- 

 tions. Natl. Mar. Fish. Serv., Southwest Fish. Cent. Admin. 

 Rep, LJ-76-3, 37 p. 



^CalCOFI. California Cooperative Ocean Fisheries Investi- 

 gation, a program sponsored by the State of California. The 

 cooperating agencies in the program are California Department 

 of Fish and Game, National Marine Fisheries Service, and 

 Scripps Institution of Oceanography, University of California. 



"Stauffer, G.D., and S.J. Picquelle. 1980. E.stimates of the 

 1980 spawning biomass of central subpopulation of northern 

 anchovy. Natl. Mar. Fish, Serv, Southwest Fish. Cent. Admin. 

 Rep. LJ-80-09, 



138 



