What happened to North Sea fish?

Scientists from the Centre for Environment, Fisheries & Aquaculture Science (Cefas) estimate that the total number of fish in the North Sea has decreased from around 26 million tons at the end of the 19th century to around 10 million tons in 1991. To reach these preliminary conclusions they constructed an ecosystem model that quantifies the food web in the North Sea.

Cefas scientists Steve Mackinson and Georgi Daskalov used information on the abundance and ecology of species that have been routinely surveyed by Cefas and other marine institutes for over 30 years, under the guidance of the International Council for the Exploration of the Sea (ICES).

The model includes 71 categories of marine organisms (algae, invertebrates, fish, mammals and birds) representing the state of the ecosystem by 1991. It was during that year that ICES scientists did the most extensive study of the feeding ecology of North Sea fish, which resulted in a comprehensive database quantifying who eats whom and how much. They compared the total fish biomass (weight of living matter) in 1991 to previous estimates for 1880 and 1980.

The biggest change revealed by the model thus far is in exploited target species e.g. gadoids (cod, haddock, saithe), mackerel, herring and flatfish. Gadoids and mackerel/horse mackerel decreased between the 1980s and 1991, but herring and some prey-fish (e.g. dab, flounder) increased in 1991 compared with the1980s. There was also an important decrease in seabirds, and an increase in seals in 1991 compared to the 1880s.

Steve Mackinson says: ‘These preliminary estimates already indicate that important changes in the structure and functioning of the North Sea ecosystem are ongoing. But we do need a better understanding of the linkages and mechanisms within the ecosystem. Such improved knowledge will provide a base for better management, by accounting for major ecosystem elements and evaluating how they might respond to the effects of external forcing factors such as climate change and pollution.

‘Our goal is to further develop the model in both time and space, to run scenarios of possible changes in fisheries management or other effects on the ecosystem. The model will, of course, continue to be rigorously tested and reviewed by peer experts.’