Marine Research Findings of the VECTORS Project

This website provides access to the research results of the VECTORS project, which can be used to support marine management decisions, policies and governance as well as future research and investment. VECTORS was a large scale project that brought together more than 200 expert researchers from 16 different countries. It examined the significant changes taking place in European seas, their causes, and the impacts they will have on society.

Is the increase in eastern Baltic cod recruitment related to reduced predation?

The Baltic Sea fish community is dominated by cod, sprat and herring and characterised by strong predator–prey interactions and feedback loops. Adult cod prey on sprat and herring, which, in return, feed on early life stages of cod. Predation on cod eggs by clupeids during a period of high sprat abundance in the 1990s is considered to have contributed to cod recruitment failure and prevented cod recovery in the Baltic Sea. In recent years, cod recruitment increased substantially, contributing to a rapid increase in stock size. The aim of this study was to provide insights to the mechanisms behind the improved recruitment and the potential contribution of changes in predation pressure. As potential underlying factors affecting changes in predation pressure, cod egg abundance, availability of alternative prey for sprat and herring, and hydrographic conditions influencing small scale changes in distribution and consequently predator– prey overlap were investigated.

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Comparison of stomach contents of sprat and herring in 2004–2008 with data from the 1990s showed changes in the ichthyoplankton abundance and composition in the diet, indicating reduced predation pressure on cod eggs in the most recent period, which has likely contributed to increased cod recruitment.

We updated the information on diet composition of sprat and herring for 2004–2008, and compared this with the data from the 1990s, when substantial predation on cod eggs has been documented. The recently lower amount of cod eggs encountered in sprat diet in combination with changed distribution of sprat suggests lower predation on cod eggs in 2000s compared with the 1990s. Although the numbers of cod eggs ingested by herring were in some years comparable with that in the 1990s, the reduced importance of summer as a spawning period for cod in combination with lesser abundance of herring in Bornholm Basin can be expected to have reduced predation pressure on cod eggs. It was apparent that inter-annual variability in cod egg predation cannot be explained by single factors. Sampling dates with largest portions of cod eggs in clupeid stomachs were characterized by fulfilment of several conditions leading to high egg predation, i.e. relatively high cod egg abundance, at least an intermediate overlap between predators and prey, and a relatively small proportion of alternative prey in the stomachs.

These results suggest that predation on cod eggs is not directly related to the overall stock size of predators. While the other important factors, such as overlap between predators and prey, are difficult to numerically take into account in management frameworks aiming at considering species interactions. However, egg predation may be important to consider for understanding recruitment dynamics, e.g. to evaluate the effect of management measures, or understand the lack of it.

Lead Author:

Viola Neumann & Margit Eero
(vneu@aqunospama.dtu.dk)
Danmarks Tekniske Universitet (DTU-Aqua)
Date of research: May 2014

Related articles:

Early life stage survival of Baltic cod

Fish distributions and spatial management measures 

Impact of environmental changes on North Sea herring

Population dynamics of sprat in the Baltic Sea 

Processes impacting overwintering larval survival 

The drivers of a common sole population 

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This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 266445
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