The seasonal cycle of reproduction in Temora longicornis was investigated in the Bornholm Basin, Baltic Sea, from March 2002 to May 2003. Variations in egg production of the population (EPR) and spawning females (sfEPR, ~clutch size), proportion of spawning females (%FS), egg hatching success (HS), female prosome length (PL) and weight-specific egg production (spEPR) were compared with the seasonal variations in temperature, salinity, and food concentration and composition. Females reproduced year round with maxima of 9.8 to 12.3 eggs female −1 d −1 in spring and low to moderate egg production during the remaining seasons. PL was maximal during spring, and %FS, sfEPR and spEPR paralleled egg production. HS was low during winter and increased in spring. The statistical analyses showed that mean egg production correlated with both sfEPR and %FS. While %FS was significantly related to food concentration, sfEPR was dependent on both food availability and PL, which in turn was inversely related to temperature. Salinity had no effect on the seasonal variation in egg production because females maintained their vertical position in water with low seasonal amplitudes in salinity and temperature, presumably to avoid high energetic costs due to osmoregulation under fluctuating salinity. Nevertheless, the costs due to osmoregulation during development likely resulted in small female PL, and thus indirectly affected reproduction. Using empirical non-linear regression, 80% of the seasonal variation in egg production of T. longicornis was explained by female length and food concen tration. However, despite the pronounced seasonal variation in egg production, the recruitment of nauplii was continuously high except throughout the productive season, indicating that a low reproductive success was offset by female abundance.KEY WORDS: Fecundity · Hatching success · Temora longicornis · Baltic Sea · Food · Temperature · Body mass Resale or republication not permitted without written consent of the publisher1991, Kiørboe & Nielsen 1994), food quality in terms of mineral or biochemical composition (Jónasdóttir et al. 1995, Peters et al. 2007 or presence of noxious species (Van Rijswijk et al. 1989, Devreker et al. 2005 and suspended sediments (Castellani & Altunbaş 2006). In contrast, the variation in egg viability and egg hatching success is largely related to maternal effects such as female nutrition or resting egg production (Castellani & Lucas 2003, Paffenhöfer et al. 2005.While the variables determining egg production and egg viability are generally well understood, our knowledge on the control of reproduction and recruitment of a species in specific environments is incomplete. Temperate copepod species often display a wide geographical distribution, and the relative importance of the variables controlling reproductive success and recruitment likely differ over the distribution range, depending on regional conditions. This information is pivotal for understanding the effect of climate variability and change on copepod r...