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MARINFORSKHAV-Marine ressurser og miljø - havmiljø

The fundamental role of mesopelagic fishes for the structure and change of Northeast Atlantic marine ecosystems

Alternative title: Mesopelagiske fiskers fundamentale rolle i å strukturere økosystemene i Nord-Atlanteren

Awarded: NOK 10.2 mill.

As you move from the coast and towards the open ocean, there is no bottom or kelp forests to hide in. The small fishes and other prey organisms that occupy the pelagic live a game of hide and seek, where their only safe option is to swim down to the darker waters to hide. The darkness, however, also makes it difficult to find food, which has resulted in the world?s largest animal migrations as fish swim down to hide during day and swim up again in the evening to forage at night. The largest stocks in terms of biomass are the mesopelagic fishes, which are a couple centimeters long and thrive in the ocean?s twilight zone, down to about 1000 meters in clear water on a sunny day. North of the Arctic Circle, primary production and grazing zooplankton thrive 24/7 during the midnight sun, but the mesopelagic fish are trapped deep below, in the dark waters they prefer. During a typical summer day, there is no safe period for them to swim towards the surface to forage, with the result that they starve in the deep. Few mesopelagic fish are therefore found at northern latitudes, which reduces the grazing pressure on copepod zooplankton. This has opened a niche for migrating fish species that visit northern oceans during summer, such as herring and mackerel. And among zooplankton, the absence of mesopelagic fishes in in the north may allow for multi-year life cycles because zooplankton can expect to live longer before they are eventually eaten. This project will use computer modelling to put together what we know about these species and their ecosystem to identify the niches and adaptations of key players. The project will also focus on effects of climate change. In particular, periods of starvation can become harder if temperatures rise. Mesopelagic fishes may thus run into trouble during summer and herring during winter, with the like consequence that climate warming may push the distribution of these fishes southwards southwards, not northwards as is generally expected.

We aim to show that mesopelagic fishes are a key player that structures fundamental characteristics of the Norwegian Sea ecosystem. Mesopelagic fishes are only a few cm long, live at depths down to ca thousand meters and migrate vertically towards the surface to feed on zooplankton during night. They are extremely abundant and found everywhere in the world’s oceans but with declining biomass towards the poles. We have preliminary models showing that the northward distribution of mesopelagic fishes is constrained by seasonality: in summer because the midnight sun means there is no night during which they can migrate safely to the surface to feed, and in winter because especially juveniles may starve during the polar night with its low productivity. This is of significance because what is elsewhere a major predator on zooplankton from below leaves a planktivore niche vacant at high latitudes. This niche has become occupied by schooling and horizontally migrating fish species such as herring, mackerel, and capelin, which are among the largest fished stocks in the world and of significant commercial value. Furthermore, we hypothesize that the lack of predation from mesopelagic fish on overwintering stages of zooplankton enable multi-year life cycles for Arctic copepods such as Calanus glacialis and C. hyperboreus. These form large visible targets and are an attractive resource for planktivorous fish. Finally, the models suggest that ocean warming will affect the northern range limits of mesopelagic fish opposite to what is expected from conventional climate envelope models: increased metabolic rates will make mesopelagic fish struggle more with both midnight sun and polar night. Consequently, we expect equatorward shifts of the northern range boundaries. Any alteration of mesopelagic biogeography has the potential to cause large-scale changes in competitive relationships in the Norwegian Sea, with consequences for herring, mackerel, and the rest of the food chain.

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MARINFORSKHAV-Marine ressurser og miljø - havmiljø