The number of eggs found in egg sacs of Eurytemora affinis in the Schlei, a mesohaline fjord in the Western Baltic, was determined between January and August, and female prosome length measured. Prosome length at 25°C was only half that at 0°C and was significantly correlated with mean temperature during development. Clutch size increased from lower winter values to a maximum in April and thereafter sharply decreased to a minimum at the end of July. The correlation between clutch size and body size was stronger than that between clutch size and temperature at collection. From calculations using regressions of body size and clutch size with temperature, a curve was derived for female fecundity at satiating food levels with a minimum at 12°C and increased values at both lower and higher temperatures. Depending on the length-weight conversion applied, P/B for egg production was 0·01 to 0·02 day-1 in winter and 0·43 to 0·51 day-1 at the temperature maximum in summer. Reproductive production is similar to somatic production of larval stages at low temperatures, but increases faster with increasing temperatures. The considerably smaller fecundity and weight specific egg production rate of E. affinis may be the reason why it is outnumbered by Acartia tonsa in the summer in many locations. Seasonal partitioning of the biotope by the two species is maintained by k-strategy at lower temperatures with E. affinis carrying few large eggs, and r-strategy in summer with A. tonsa depositing many small eggs. © 1992 Academic Press Limited.