Getting to the bottom of demersal fish - Distribution, diurnal behaviour, and energetic characteristics of sandeel and other prey fishes in the Dutch North Sea

Small demersal fish are crucial prey for marine mammals, seabirds, and predatory fish, yet their distribution, abundance, energetic value, and behaviour remain poorly understood. This thesis investigates these aspects in the southern North Sea, with a focus on the sandeel species Ammodytes marinus and A. tobianus. Using fine-meshed bottom dredges, we found higher prey biomass than previously reported, and species-specific distributions maps were created. Energy densities were determined, revealing seasonal differences among species, and used to transform biomass maps into energy landscapes. DNA-based methods were developed to improve sandeel species identification. The two Ammodytes species showed offshore–nearshore separation, although some overlap occurred. Both species were negatively associated with shellfish biomass, potentially indicating constraints on burrowing into the sediment. They also displayed similar diurnal patterns, with lowest seabed densities around midday and highest densities in the evening. These results provide a baseline for assessing future changes in small demersal fish and improving our understanding of prey-predator interactions in the North Sea.

Bram Parmentier

The candidate of the PhD defence "Getting to the bottom of demersal fish - Distribution, diurnal behaviour, and energetic characteristics of sandeel and other prey fishes in the Dutch North Sea".