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Background

The ecosystem in the Norwegian Sea has been under constant change during the last decades. Ecologically and commercially important pelagic fish stocks; Norwegian Spring Spawning herring, North East Atlantic mackerel, and blue whiting, have all had their turn in being the most abundant pelagic species, however, the exact nature of the interspecific interactions between these species is still poorly understood.

In the last decade the summer feeding area of the NEA mackerel stock has been expanding drastically and in 2014 mackerel was observed all over the Nordic Seas. While mackerel stock is thriving, Norwegian spring spawning herring stock is currently at its lowest level in 20 years. Some hypothesize that the absence of strong year classes of Norwegian spring spawning herring is a direct result of expansion of mackerel as they prey upon herring larvae. In addition to the suggestion that mackerel is competing for food with herring, it has also been hypothesized that by eating herring larvae, the mackerel would worsen the already dire conditions of many sea bird populations along the Norwegian coast.

In the spirit of ecosystem based fisheries management the supposedly detrimental effect of mackerel on other fish stocks and sea birds should be taken into account in the management of the Norwegian Sea Ecosystem. In this study we aim at improving our understanding about the species interactions and the dynamics of the Norwegian Sea ecosystem by developing and using modern methods for identifying and quantifying the diet of these ecologically and economically important pelagic fish populations. We will use modern statistical methods to scrutinize these data, and end-to-end ecosystem models to test the hypotheses regarding ecosystem dynamics from plankton, via fish, to sea birds under the current climate change. This project is an investment in understanding and long-term monitoring and management of the Norwegian Sea and other ecosystems.