Baltic herring prey selectively on older copepodites of Eurytemora affinis and Limnocalanus macrurus in the Gulf of Riga

Zooplankton availability is a major factor affecting herring body condition that in turn describes its well-being. As herring feeding is known to be selective, it is relevant to access its preferences upon zooplankton species and particular copepod developmental stages to forecast possible intraspecific competition for resources in the species scarce environment of the Gulf of Riga where herring stock size due to successful recruitment has almost doubled since 1989. This study tries to answer whether the small-sized plankters dominated zooplankton community permits herring to be a selective eater. Also how herring body condition has changed in connection to environment driven zooplankton community changes. The time series of zooplankton abundance and herring condition from 1995–2012 were studied; and a detailed study of herring diet was performed monthly by stomach content analysis during the main feeding season in 2011 and 2012. We found that herring selectively prey on Limnocalanus macrurus and older copepodite stages of Eurytemora affinis, and moreover these were species of whose selected copepodite stages explained most of variation in herring condition factor. The found relationship between herring feeding selectivity and long-term variation of herring condition allows applying spring zooplankton abundance of E. affinis and L. macrurus to estimate favourable feeding conditions for herring, and could also require the revision of currently used model for herring recruitment estimations, where only biomass of E. affinis is taken into account. In recent years, the high condition of herring can be associated with a considerable increase of lipid-rich copepod species L. macrurus.