Fishing and jellyfish eradicate fish 180 years ago

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Sustainable fish populations require both healthy ecosystems in which they can live and grow, and protection from overfishing. A rare historical example from the first half of the 19th century allowed us to describe the chronology of how the vulnerability of a herring (Clupea harengus) population to ecosystem variability was increased by effects of fishing on the population and food web interactions within the ecosystem. Both the population and fishery collapsed when several years of increasing exploitation were followed by an extreme climatic-hydrographic perturbation in 1825 that affected herring survival and changed food web structure (jellyfish bloom). Estimated levels of fishing mortality in years leading up to the collapse of Limfjord herring were 3-5 fold higher than natural mortality rates and similar to those which later led to collapses of 3 major herring populations in the north Atlantic (North Sea Norwegian spring-spawning herring, Georges Bank). These high exploitation rates suggest that fishing was a key factor which increased the vulnerability of the Limfjord herring to collapse. Changes in trophic interactions in the Limfjord after 1825 were therefore facilitated by fishing, which had reduced the abundance of jellyfish food competitors (herring) to low levels, and consequently expanded a niche in the ecosystem for other zooplanktivores with oppportunistic lifehistories. Lowering the risks of future collapses of fish populations and of trophic re-organisations to less desirable configurations, including gelatinous dominance of foodwebs, requires low exploitation rates that buffer against environmental variability and measures to support well-functioning and structured ecosystems.