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.
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The composition and the content of lipids, fatty acids and sterols during spawning (spring) in different tissues of herring Clupea harengus membras were compared with quantities of lipid compounds in abdomen muscle of brown shrimp Crangon crangon. The largest quantity of fatty acids in lipids (93.7%), was observed in fish muscle. The liver was characterized by a higher content and variety of sterols (about 4.6 times more than in fish muscle), and in fish sperm, additional fatty acids were identified and the highest amount of EPA and DHA (22% and 34% of fatty acids, respectively) was recorded. The brown shrimp, despite its small size, contained significantly more lipids than the Baltic herring per g of tissues. Fatty acids were at the same level (83% of the total lipids in shrimp muscle and 93.7% in fish muscle), but the amount of sterols was significantly higher in the muscle of shrimp (5.50 š0.31 mg g-1, 17% of total sterols, n=10) than in fish muscle (1.33 š0.04 mg g-1, 6.3% of total lipids, n=6). And thus, shrimp is a good source of food for higher trophic levels, and in consequence – a good source of PUFAs for humans. With these results we prove that shrimp and herring play an important role in the supply of EFAs, which has great pharmaceutical and medical benefits.
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