Small fish biomass in the North Sea is far greater than previously estimated

Small fish, including species with small adult sizes and juveniles of larger species, play a central role in marine food webs as prey for top predators, such as seabirds, marine mammals, and piscivorous fish. However, reliable data on small fish are lacking as conventional fisheries surveys primarily focus on larger, commercially important species and underestimate small fish. Consequently, little is known about absolute biomasses, and fine-scale distribution patterns of this important trophic group. Based on 1307 quantitative Triple-D samples from the Dutch Exclusive Economic Zone and the UK sector of the Dogger Bank, biomass densities were estimated for pooled small demersal fish and for the most abundant species individually. Our estimates suggest that small demersal fish biomass is at least twice as high as reported in trawl-based studies. Uniformly distributed species such as dab (Limanda limanda) and plaice (Pleuronectes platessa) contributed most to the overall small fish biomass, while sandeels (Ammodytidae) showed particularly high local densities. These new prey biomass estimates might change our understanding of the North Sea ecosystem's carrying capacity, and establishes a baseline for monitoring changes in small fish communities driven by climate change and human impacts.