WP 6: Cod: a key species and its role in the Barents Sea food web

Cod is the dominant predator in the Barents Sea food web. It is important both economically and ecologically, constituting 90% of the biomass of commercially exploited demersal fish stocks in the area (Dolgov 2009). It is essential in the benthic pelagic coupling as it feeds in both realms. The Barents cod stock is currently at a historic high level with a very wide spatial distribution. Although cod is one of the most thoroughly studied species in the Barents Sea, the impact of the currently very large stock on the food web is largely unknown. Furthermore, the productivity needed to sustain a large cod stock under different conditions has not been assessed.

Functional response (Holling 1959) is fundamental for predator prey dynamics, and is used in all models that include predator prey interactions such as food web models. Functional response will be studied in this work package to assess the impact of the large cod stock on the food web, and in particular on its main prey capelin. This work will be strengthened through the cooperation with Professor Englund (University of Umeå) who has developed and worked with the scale transition methods that we will apply (Englund and Leonardsen 2008, Englund et al 2011, Hunsicker et al 2011).

The impact of prey availability, food web structure and productivity on cod’s aggregative (spatial distribution) and numerical response (growth and reproduction) will be studied by a combination of statistical analysis of survey data and modelling (Atlantis model see box 1). We will the assess the productivity needed to sustain the large cod stock under different scenarios: e.g. short term cooling (reducing the habitat available to cod), collapse of the main prey capelin (there are indications of poor capelin recruitment in the current year) and change in cod age/length structure ( determined by fishing, growth and recruitment).

The data on Barents Sea cod from standard surveys (the ecosystem and the winter survey) are very rich and underused. These survey data are essential to this WP. There is scope for exploiting the data more fully by using new approaches to data analysis and by combining survey data with modelling. To fully exploit survey data and to expand its use beyond abundance indices used in stock assessments, appropriate statistical methods have to be used and developed to account for spatial and temporal autocorrelation, and changes over different spatial scales. This also includes the statistical analysis of diet data which has problematic statistical properties (skewness, zero-inflation). Increased skills in statistical analyses of survey and diet data are part of the strategic competence building in the work package.

Specific tasks:

- Study how the trophic level and diet of cod vary by year, season and age and the resulting cod growth variability.

- Estimate the functional response of cod by age and length, across space, with season, and under different climatic conditions.

- Assess the productivity needed to sustain a large cod stock under different temperature regimes, different food web scenarios and different cod stock age/length structures.