Relative batch fecundity, spawning frequency, size at 50% maturity, sex ratio, and egg production were estimated for the Bonaerense and Patagonian Engraulis anchoita stocks. Samples were collected during 19 research surveys carried out over the main spawning seasons and areas of both groups from 1993 to 2008. Reproductive potential between stocks was compared by analyzing the length-class structure and considering environmental variations. Daily egg production varied between 0.2 × 10 7 and 7.3 × 10 7 oocytes and the mean annual value was higher for the Bonaerense than the Patagonian stock (2.9 × 10 7 and 0.54 × 10 7 oocytes, respectively; P 0.788), and mean relative batch fecundity values were 508 and 493 oocytes g -1 (P > 0.680), respectively. Mean annual relative batch fecundity values for the Bonaerense stock showed a negative trend (P < 0.016) in the period 1993-2008, coinciding with a 4 oC increase in mean sea surface temperature in the area during the same period.

The Daily Egg Production Method (DEPM) was used to compute spawning biomass of Engraulis anchoita off Argentina. Estimates of the daily egg production (P0) for the northern stock ranged from 594 to 936 eggs m−2, whereas the annual means of the Patagonian stock ranged from 185 to 605 eggs m−2. The mean values estimated for the Argentine anchovy DEPM parameters were characterized by inter-annual differences greater than inter-regional differences with the exception of P0. For the northern population, the estimates of mean weight of mature females ranged from 15 to 26 g, the relative batch fecundity from 414 to 600 eggs g−1, the spawning frequency (S) from 0.078 to 0.179, and the females ratio from 0.519 to 0.622 of the spawning stock. The estimates corresponding to the Patagonian stock were similar being 15 to 24 g, 418 to 583 eggs g−1, 0.079 to 0.244, and 0.394 to 0.590, respectively. The annual estimates of the spawning biomass for the northern and Patagonian populations ranged between 1.6–3.5 and 0.3–1.5 million metric tons, respectively. A weakness in the application of DEPM was the low precision in the estimation of the daily egg production and the spawning fraction. Alternative methodologies to increase the precision of P0 and S are discussed. In spite of the high variance of the spawning biomass estimates, the use of DEPM offers valuable information to adjust acoustic estimates and provides time series of anchovy population size and biological parameters for basic research.