vian anchovy and Pacific sardine eggs were not 

 higher where the eggs occurred in the same 

 sample. 



DISCUSSION 



In this section we will discuss Pacific sardine 

 and Peruvian anchovy egg production and mor- 

 tality off Peru, fundamental statistical problems 

 to be overcome in future studies, some other 

 mechanisms of population control, and egg can- 

 nibaUsm and population control in pelagic spawn- 

 ing, schooling coastal pelagic fishes. 



FISHERY BULLETIN: VOL. 87, NO. 3, 1989 



ovoid shape of the anchovy egg (0.7 x 1.4 mm) as 

 compared with the spherical sardine egg (1.4 

 mm) may reduce the chance of being eaten by 

 filter-feeders. Author H. Santander has ob- 

 served that sardine eggs are more delicate to 

 handle in the laboratory. This may mean that the 

 integument of the sardine egg is more suscep- 

 tible than the anchovy egg to physical damage by 

 predators in the sea. Their susceptibility to dam- 

 age might also explain their apparent low inci- 

 dence in juvenile and adult sardine and anchovy 

 stomachs, because broken eggs might not be 

 detected (Santander et al. 1983). 



Pacific Sardine and Peruvian Anchovy 

 Egg Production and Mortality 



It appears that Pacific sardines and Peruvian 

 anchovies off Peru select the same large-scale 

 sites for egg deposition but select independently 

 of each other at the small scale. While the eggs of 

 both species were concentrated in the inner 30 

 miles of the survey pattern (Fig. 2), the prob- 

 ability of occurrence on a sample-by-sample 

 basis was not demonstrably different from inde- 

 pendent distributions for the two species (Table 

 1). 



Where Pacific sardine and Peruvian anchovy 

 eggs occurred together, Peruvian anchovy egg 

 production rates were equal to those sites where 

 Peruvian anchovy eggs occurred alone, but Pa- 

 cific sardine egg production rates were higher at 

 co-occurring sites than those sites where only 

 Pacific sardine eggs were taken. Mortality rates 

 where both occurred were marginally lower than 

 where each occurred separately: this implies 

 that the proximity of the adults of the other 

 species does not affect the egg mortality rates. 

 The northern region had the maximum rate of 

 mortahty for both Peruvian anchovy and Pacific 

 sardine. The northern region had also the maxi- 

 mum regional rate of production for Pacific sar- 

 dine, virtually equal to the Peruvian anchovy 

 egg production rates of the other two regions 

 (Table 3). 



In all regions, the Pacific sardine mortality 

 rate was higher than the anchovy mortality rate. 

 Of an estimated 2,094 eggs produced/m^ d"^ 

 by Peruvian anchovy, 1,410 (67%) died on the 

 first day. Of an estimated 1,126 eggs produced/ 

 m^ d~' by Pacific sardine, 990 (88%) died on 

 the first day. We suggest two explanations for 

 the cause of higher mortality rate of Pacific sar- 

 dine eggs relative to Peruvian anchovy eggs in 

 the same region: Moser"* pointed out that the 



Fundamental Statistical Problems 



Errors in the estimation of the slope parame- 

 ter induce errors of the same sign in the inter- 

 cept parameter: for this reason there are major 

 problems in comparing adult spawning biomass 

 concentration derived from the intercept param- 

 eter and egg mortality rate derived from the 

 slope parameter. Thus, for the purpose of this 

 paper, the comparison of spawning biomass con- 

 centration and egg mortality is not reported as a 

 result of this research. Instead, Figure 3 and 

 Table 6 are points of discussion. The data come 

 from a plan for stock assessment and as such are 

 not definitive on the question of biomass concen- 

 tration and egg mortality rate. The relationship 

 is suggestive, however, and may be used to de- 

 sign a study of cannibahsm that is not statisti- 

 cally confounded. 



The statistical problem of patchiness (Taft 

 1960), encountered when estimating the abun- 

 dance of fish eggs, becomes more severe when 

 estimating production and mortality of eggs be- 

 cause the intensity of patchiness changes with 

 age (Smith 1973, 1981; Smith and Hewitt 1985a, 

 1985b; McGurk 1987). Since the mortality esti- 

 mate requires the arithmetic means (South wood 

 1978), log transformation to stabilize variance is 

 not used. 



Egg production (P,,) and mortality rate (Z) are 

 determined using the exponential mortality 

 model (Picquelle and Stauffer 1985). It has been 

 determined that the midpoint of spawning is at 

 2200 hours but samples from the spawning inter- 

 val are biased (Smith and Hewitt 1985a). The 

 intercept and mortality rate could be systemati- 

 cally underestimated if the actual mortality rate 



''H. G. Moser, Southwest Fisheries Center La JoUa Labor- 

 atory, NatL Mar. Fish. Serv., La Jolla, CA 92038, pers. 

 commun. May 1983. 



502 



