Recovery in eastern Baltic cod: is increased recruitment caused by decreased predation on early life stages?

Abstract

Cod (Gadus morhua) recruitment in the eastern Baltic Sea is influenced by predation on early life stages by sprat (Sprattus sprattus) and herring (Clupea harengus), which is considered as one of the mechanisms preventing cod recovery in the 1990s. In the light of improved cod recruitment in the second half of the 2000s, new analyses of stomach content of sprat and herring were conducted, to elucidate the contribution of changes in predation pressure on cod recruitment. Comparison of stomach contents of sprat and herring in 2004–2008 with data from the 1990s showed a similar diet composition in the two periods; however, changes were found in the ichthyoplankton abundance and composition in the diet, indicating reduced predation pressure on cod eggs in the most recent period. The abundance of cod eggs in the field, availability of other prey, and horizontal and vertical overlap between predator and prey were investigated as potential factors influencing cod egg predation.

Fish egg predation by Baltic sprat and herring: do species characteristics and development stage matter?

“…This corroborates earlier findings, which explained this by a limited vertical overlap between the preferred medium-sized to larger larvae inhabiting the upper water layers above the halocline and the clupeids concentrating in deeper layers during the daily feeding period (Köster and Schnack 1994). Along these overall patterns, a high variability in both composition and quantity of ichthyoplankton in sprat and herring diet is evident (Neumann et al 2014), both within a cruise as well as between seasons and years.…”

“…Cod eggs were generally positively and sprat eggs negatively selected/ avoided in spring, which could partly be related to the vertical overlap of predator and prey (Neumann et al 2014) rather than direct prey selectivity from a common prey field. In our data, the catching depth of the clupeids matches to a large extent the vertical distribution of cod and sprat eggs (Fig.…”

“…The details on sampling gears, catching methods, and processing of samples are described in Neumann et al (2014). The data and calculation procedures involved in deriving fish egg consumption and production rates are described in Neumann et al (2017).…”

“…After fractioning stomach contents into major taxonomic groups and assessing their relative contribution, the ichthyoplankton components of the stomach contents were analysed in further detail, including determination of taxa and development stage. Stomach handling followed the procedures described by Köster (1994) and Köster and Schnack (1994) and data processing to obtain diet compositions is described in Neumann et al (2014).…”

“…Horizontal small-scale variability in egg abundance can as well cause a mis-match between predator and prey, however, there is no indication from the standard ichthyoplankton surveys that such variability is high, i.e. there are hardly extreme outliers encountered (Neumann et al 2014). …”