Applying the daily egg production method (DEPM) to small stocks in highly heterogeneous seas

Abstract

In the present study, the first daily egg production method (DEPM) parameters were estimated for European anchovy ( Engraulis encrasicolus (Linnaeus, 1758)) stocks of the central Aegean and Ionian seas. This exploratory application of DEPM, which was carried out in May–June 1999, presented particular interest and difficulties due to the peculiar topography of the survey area (consisting of many small-sized, semi-enclosed gulfs), the genetic segregation of two stocks and the small size of the anchovy populations. Because of the high subsurface temperatures characteristic to the eastern Mediterranean, the egg data set consisted mainly of eggs younger than 2 days. To increase the number of age categories for constructing mortality curves and estimating daily egg production, we assumed that the mortality rate was the same for eggs and yolk-sac larvae, and we included both in single embryonic mortality curves. Compared to the existing values for other European anchovy stocks, the estimates of spawning frequency were considerably lower, especially in the Ionian Sea. In contrast, batch fecundity in the Ionian Sea was exceptionally high. The total spawning biomass estimate was 11 861 MT with a coefficient of variation CV=0.278.