The factors driving the spatiotemporal population dynamics of sandeel in the North Sea

Trophic levels in “wasp-waist” systems are characterized by an intermediate level that are occupied by a few highly abundant schooling planktivorous species, also termed forage fish. The flow of energy in the food webs are channelled from a species-rich lower trophic level of plankton through forage fish to higher trophic levels of a diverse predator community.

The abundance of forage fish fluctuates and thus, ecosystems can be highly sensitive to changes in populations dynamics of these species. While the impact on the ecosystem have been well documented, the underlying processes and mechanisms behind are still vaguely understood. In the North Sea, the majority of forage fish are small pelagic fish that migrate between areas of foraging and spawning.

Yet, sandeel (Ammodytes marinus) represent a forage fish with a completely different life history, which include a specialized burying strategy that links this species to a specific habitat making them highly site-attached and patchy in distribution. As a consequence of these life history traits, sandeels are vulnerable to changes in environment, demographic processes, and exploitation.

They play a major role in the diet of seabirds, marine mammals and predatory fish, as well as being one of the largest single species fisheries in the North Sea. Hence, knowledge about what drives the population dynamics are essential for conservation and sustainable exploitation of this species, which support important ecosystem services, including biodiversity and human livelihoods.

This thesis consist of four individual studies ( Paper I to IV ) and a literature review, constituting the synthesis. The review was conducted in order to provide an overview of current knowledge about important factors that drive the population dynamics of sandeel in the North Sea. Presented in this thesis, are sections on growth, density dependence, habitat, food availability, predation, fisheries, temperature, oceanography and broad-scale climate.

Additional explorative analyses were made, when applicable, providing supporting information to aid interpretations and facilitate discussion. Findings from the four studies, representing three years of research, are highlighted in boxes (see Box overviews) throughout the thesis, where they serve to fill in knowledge gaps on the effect of oceanography ( Paper I ), density dependence ( Paper II ) and temperature ( Paper III and Paper IV ).