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.

Spatial overlap between 0-group cod and grey gurnard can explain recruitment success of cod

The spatial interaction between 0-group cod and grey gurnard in the last two decades (1991–2008) in the North Sea was analysed by quantifying changes in the patchiness of 0-group cod and grey gurnard distributions with the help of regression kriging and assessing the dynamics of the related spatial predator-prey overlap of these two species at different spatial scales. Results were used to hypothesise mechanisms driving the distribution of the two species and the respective impact for their recruitment dynamics.

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Grey gurnard was found mainly in waters with high salinity (>33), whereas cod was caught mainly in the low saline waters. This can give guidance on sensible areas for cod recruitment.

The coherence between low salinity and cod abundance is unlikely to express the preferences for certain salinity ranges, but rather a consequence of other processes and factors. Munk et al.1 suggested that larval and juvenile cod aggregate in highly productive haline fronts in German Bight and in Skagerrak. Such fronts are formed there due to the river runoff and fresh water outflow from the Baltic Sea and therefore coincide with the areas of low salinity. Our study confirms that the Skagerrak is a most important nursery area for cod <15cm. On average 67% of the total 3rd quarter IBTS 0-group cod catches stem from this relatively small area.

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Predator-prey interactions may play an important role in shaping the spatial distribution and recruitment success of cod.

Our analysis demonstrated that grey gurnard expanded its high density areas in the central North Sea and shifted the centre of gravity of its modelled distribution northward during 1991 to 2008. The strong increase in abundance of grey gurnard during the 1990s may have caused a density-dependent habitat selection, contributing to the observed increase in the occupied area. The spatial expansion and increase in abundance during the 1990’s of the grey gurnard stock has most likely contributed to low survival rates of larval and juvenile (0-group) North Sea cod. During the years with high year class strength of cod, large numbers of 0-group cod were caught in the North Sea and such years were associated with larger differences in average latitude between grey gurnard and small cod. Whether this is the result of a top-down effect or just shows that larger year classes expand their area of distribution to the north and towards the North Sea remains unclear since it is impossible to deduce causal relationships from this analysis alone. However, as further argument for a top-down control the principal average overlap estimated as Morans I index between juvenile cod and medium sized grey gurnard at the spatial scale of 650km could explain ca. 57% of 3rd quarter cod recruitment success between 1991 and 2008.

References

Lead Author:

Alexander Kempf
(alexander.kempfnospam@ti.bund.de)
vTI-Institute of Sea Fisheries (vTI-SF)
Date of research: September 2012

Related articles:

Cod, recruitment, temperature and zooplankton

Early life stage survival of Baltic cod

Impact of environmental changes on North Sea herring

Sexual size dimorphism in three North Sea gadoids 

The drivers of a common sole population 

Changes in herring larvae and environment 1957-2010

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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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