LO: EGG PRODUCTION OF NORTHERN ANCHOVY 



production estimate (Po ) in 1975 was caused by the 

 high standing stock of eggs {mti = 30.06/0.05 m^ 

 per m depth) which was more than 10 times that of 

 other years, and the high egg IMR {a = 0.36) 

 (Table 3). The high daily egg production in 1975 

 reflects either a high fecundity (high spawning 

 frequency) or a high spawning biomass or some 

 combination of these effects. The present level of 

 egg production is the same as that in the middle 

 1960's. Both egg IMR (a) and larval IMR coeffi- 

 cient )8, 2(0 = pit, vary from year to year (Fig. 5). 

 In addition to providing a 24-yr time series of 

 HEP for the northern anchovy, two important 

 conclusions can be drawn from this analysis: 



1. The form of IMR of eggs (and yolk-sac larvae) is 

 different from that of older larvae (6-20 d). 



2. Egg production is a better index of stock 

 abundance than is the standing stock of larvae. 



Little doubt exists that mortality rates change 

 sometime between the hatching of the eggs and 

 the onset of feeding. Analysis of the daily egg and 

 larval production by age for 1979-81 (Fig. 3) 

 suggested a constant IMR for eggs (or eggs and 

 yolk-sac larvae) and an age-dependent IMR of 

 Pareto form for older larvae {z{t) = fB/t for tc < t < 

 20 d) (Table 2). The age tc in Equation (3) could be 



2.5 r 



2.0 



- 1.5 



I 

 U 





A 



V 



1 / 



I; 



H 

 Larval IMR coefficient (/3) 



1950 



1960 



1970 



1980 



YEAR 



Figure 5. — Estimated egg instantaneous mortality rate (EMR) 

 (a) from series 2 method of estimating egg production and the 

 larval mortality coefficient (y3) of the central stock of northern 

 anchovy, 1951-82. 



considered to mark the end of the critical period 

 after which mortality decreases (Ahlstrom 1954; 

 Marr 1956; Farris 1960; Saville 1964). Series 1 

 assumed tc = incubation time and series 2 as- 

 sumed tc - average age of yolk-sac larvae. From 

 the existing data, I could not ascertain which 

 assumption was the more likely, but it was evi- 

 dent that larvae at hatching or near first-feeding 

 (yolk absorption) suffer higher mortality than do 

 older larvae. 



The HEP (Po) is certainly preferable to larval 

 standing stock (larval census estimate - LCE) for 

 use as an index of spawning biomass. Egg produc- 

 tion is related to the spawning biomass through 

 Equation (2), i.e., Po = Ba'C, where the propor- 

 tionality C is the reproductive output (R-F-EIW). 

 If the reproductive output remains constant be- 

 tween years, as shown by 1980-82 anchovy data 

 (Picquelle^), the HEP will be an unbiased index of 

 the spawning biomass. The LEG assumes Ba - 

 K-La where La is the larval abundance and K is 

 a constant proportionality (Smith 1972; Stauffer 

 and Charter 1982) (Table 3, Fig. 4). Thus to 

 provide an unbiased index of biomass, the method 

 requires that not only the reproductive output be 

 constant from year to year but also the egg and 

 larval mortality must remain constant as well. 

 Using Equation (8), the larval abundance (age 

 < 30 d old) can be written as 



30 



La=J Ptdt 



'ti 



I PoSit;z(t))dt 

 •>ti 



'ti 



= Ba 



, - atl tl 



/3 



-k w\ 



where g{a, /3, ti) —' 



for /3 7^ 1 



.e-«'^(ln30- \nti) 



a is the egg IMR and /3 is the larval mortality 

 coefficient. 



The larval abundance (La) is proportional 

 to the spawning biomass (Ba) with constant 

 proportionality only if the reproductive output 



*S. J. Picquelle, Statistician, Northwest and Alaska Fisheries 

 Center, National Marine Fisheries Service, NOAA, 2725 Mont- 

 lake Boulevard E, Seattle, WA 98112, pers. commun. July 1983. 



147 



