Inter-annual variation in herring, Clupea harengus , and sprat, Sprattus sprattus , condition in the central Baltic Sea: what gives the tune? Michele Casini, Massimiliano Cardinale and Joakim Hjelm Casini, M., Cardinale, M. and Hjelm, J. 2006. Inter-annual variation in herring, Clupea harengus , and sprat, Sprattus sprattus , condition in the central Baltic Sea: what gives the tune?  / Oikos 112: 638  /650. The Baltic Sea ecosystem has undergone large changes during the last two decades, including a severe reduction in cod and herring biomass but, at the same time, a large increase in sprat abundance. The lower trophic levels of the Baltic Sea also changed due to environmental fluctuations, including variations in salinity and in volume of oxygenated water. In this apparently shifting environment, the conditions of herring and sprat have undergone large inter-annual variations during the past 15  /20 years. In this study, we explore how abiotic factors (i.e. salinity and temperature) and biotic factors (biomass of the copepods Pseudocalanus elongatus , Temora longicornis , Acartia spp. and of cladocerans as well as clupeid abundance) in different seasons (May and August) affect clupeid body condition. Our analyses suggest that data of zooplankton biomass and abiotic factors in August have higher predictive power than May data. Although our analysis suggests that salinity (a bottom-up process) has an effect on sprat condition, total abundance of clupeids (a top-down process) is by far the most significant predictor of both herring and sprat condition. The strong correlation between clupeid abundance and total zooplankton biomass points to food competition and to top-down control by herring and sprat on common food resources. Furthermore, clupeid condition co-varied with the changes in the weight of zooplankton in the stomachs, which further suggest food competition being the main mechanism behind the changes in clupeid condition during the last two decades. Hence, our results are not in agreement with most of the current literature that has suggested that clupeid growth is regulated by environmentally mediated bottom-up processes acting on the abundance of copepods. This is, to our knowledge, the first evidence of food resources mediated density-dependent fish growth in a large marine ecosystem. M. Casini, M. Cardinale and J. Hjelm, Swedish Board of Fisheries, Inst. of Marine Research, Box 4, SE-453 21 Lysekil, Sweden (michele.casini@fiskeriverket.se). The Baltic Sea ecosystem has undergone large changes during the last two decades. Focusing on abiotic factors like hydrology, salinity has decreased since the beginning of the 1980s, due to the lack of inflows of saline and oxygen-rich water from the North Sea (Mattha ¨us and Franck 1992). This situation has resulted in prolonged periods of anoxia in the deeper layers of the Baltic Sea basins (Møller and Hansen 1994). On the other hand, water temperature has been fairly stable and showed no clear trend (Mo ¨ llmann et al. 2000). Considering biotic factors, since the beginning of the 1990s, the biomass of the marine copepod Pseudocalanus elongatus has decreased in parallel to a decrease in salinity, whereas Acartia spp. has increased. Another important copepod, Temora longicornis and cladocerans have also decreased, although not as severely as P. elongatus Accepted 29 July 2005 Copyright # OIKOS 2006 ISSN 0030-1299 OIKOS 112: 638 /650, 2006 638 OIKOS 112:3 (2006)