RICHARDSON; SPAWNING BIOMASS AND EARLY LIFE OF NORTHERN ANCHOVY 



the area represented by the survey grid during 

 each cruise in 1975 and 1976) were determined by 

 two methods, the Sette and Ahlstrom Census and 

 the Smith Census. The Sette and Ahlstrom Census 

 was the polygon method of Sette and Ahlstrom 

 ( 1948) in which the number of individuals under 10 

 m^ sea surface at each station was weighted by the 

 area represented by the station. These areas are 

 polygons formed "by constructing perpendicular 

 bisectors of lines drawn from the station to each of 

 all surrounding stations" (Sette and Ahlstrom 

 1948). The census estimate is then: 



10 



n 

 V 



(a, b, c,d,) 



(2) 



where Cf, = estimate of abundance of eggs or lar- 

 vae during cruise k 

 A, = area of a polygon constructed of per- 

 pendicular bisectors of lines be- 

 tween station / and all adjacent 

 stations 

 n = number of stations. 



Polygon areas, determined by planimeter, ranged 

 from 0.38 to 2.89 x 10^ m^ but were >2.00 x lO^ m^ 

 for all stations 46 km or more from the coast. 



The Smith Census was the "regional census es- 

 timate" of Smith (1972): 



Ckr = 10 A,n' 1 (a;'b;\,d,) (3) 



1=1 



where C^^ = estimate of abundance of eggs or lar- 

 vae in region r during cruise k 

 Ar - area of region r in numbers of 10 m^ 

 areas ( in my study, 



10 A, = 2 A, = 148.81 X lO^m^). 

 1=1 



Spawning Biomass Estimates 



Four methods were used to estimate spawning 

 biomass, three based on egg census estimates 

 (Sette and Ahlstrom Egg Method, Simpson Egg 

 Method, Saville Egg Method) and one based on 

 larva census estimates (Smith Larva Method). 

 The use of three egg methods generally follows the 

 approach of Houde (1977) in estimating spavniing 

 biomass of round herring, Etrumeus teres. Be- 

 cause the shape of the egg production curve 

 throughout the spawning season is unknown for 

 northern anchovy, the use of these three egg 

 methods takes into account the range of pos- 

 sibilities (discussed below) for comparative pur- 

 poses. 



All three egg methods ultimately use the same 

 formula (Saville 1964) to estimate spawning 

 biomass: 



Es 

 ^ - KF 



(4) 



where B = biomass of spawning adults 



Eg = total number of eggs spawned during 

 a season, i.e., seasonal egg produc- 

 tion. (This estimate varies accord- 

 ing to the egg method used as de- 

 scribed below.) 

 K = the proportion of spawning adults 

 that are females. [In this paper the 

 overall sex ratio of E. mordax is 

 assumed to be 1:1 following Smith 

 (1972) and Klingbiel (1978).] 

 F = mean fecundity, i.e., the number of 

 eggs produced per gram of female 

 per spawning season. [The mean 

 fecundity of E. mordax in the 

 northern subpopulation off Ore- 

 gon is estimated to be 720 ova/g 

 total weight female (Laroche and 

 Richardson 1981).] 



The Smith Census method can be less tedious than 

 the Sette and Ahlstrom Census method if the area 

 represented by the survey can be determined in a 

 gross manner, i.e., without planimetry. Values of 

 the Smith Census in my study were always lower 

 than those of the Sette and Ahlstrom Census. The 

 Smith Census was computed here primarily to 

 allow for comparison with CalCOFI data (Smith 

 1972; Pacific Fisheries Management Council foot- 

 note 2). 



The number of eggs spavined in a season {E^) was 

 estimated by three methods, each using the two 

 egg census estimates described above. 



The Sette and Ahlstrom Egg Method of estimat- 

 ing seasonal egg production (£■>,) follows the ap- 

 proach of Sette and Ahlstrom (1948). 



