Claire Corlett

Fish Food, Fish Tanks, and More

Why Atlantic fish are invading the Arctic

As many as half of Earth’s species may be already migrating to escape warming temperatures. Plants are inching northward and so are many animals. The Arctic ecosystem is particularly vulnerable to invasion. Scientists aboard this research ship are hunting the most important fish in the Arctic: polar cod. Mixed in with the Arctic fish are invaders including Atlantic cod and capelin. The southern species compete for polar cod’s food. So here is a very typical Arctic food web or food chain. This food web is changing now. These fish are among many species migrating into the warming Arctic. Relatively few species have evolved to survive its frigid darkness. Polar cod have some of the most impressive adaptations to survive. They use a trick that’s common among arctic animals, they store lots of fat in their bodies. Polar cod eat tiny copepods, which also store fat to survive the long winter. And a high energy meal is crucial for surviving the harsh Arctic. One trawl after another reveals southern interlopers. Out of a thousand fish caught in this trawl, roughly 70% by weight are southern species. It’s hard for the scientists to pinpoint precisely when we cross the boundary between Atlantic and Arctic waters, but that boundary has been slowly creeping northward. Scientists call this process the ‘Atlantification’ of the Arctic. Atlantic waters are saltier and they mostly lack ice. The biggest difference? Atlantic water is as much as seven degrees warmer. The polar Cod likes the colder water, but it’s losing territory. Polar Cod flourish in the shallow waters of the Barents Sea, but further north
it’s too deep for them. Meanwhile, the warmer waters bring with them new species that are shaking up the food web. Along with new fish, there are Atlantic varieties of copepods. They store much less fat than copepods native to the Arctic, making them a less nutritious food source. That means polar cod may have to spend more energy hunting. For the same amount of animals that are gonna be eaten, you don’t transfer the same amount of energy. Capelin, as well as Atlantic Cod, were among the invaders we just caught in the trawl. Capelin will also feed on the same prey as the polar cod, so we start adding competition between these two species. The Atlantic Cod goes one step further: it eats the polar cod itself. Capelin’s not as rich as polar cods, so that goes or diminishes, then these levels will also be impacted. It’s easy to vilify the southern species threatening the ecological balance here. But take the Atlantic cod, in their natural habitat, overfishing and now rising temperatures have decimated its population. So here in the Arctic, Atlantic Cod seems like an invader, but it’s also a climate refugee from the south and its main predator is another key species advancing northward. Historically, most humans who have come to the Arctic have taken a lot more than data and a few samples. For at least 45,000 years, people have been coming to this paradoxical region. Punishing, desolate but rich with resources. Humans are the Arctic’s ultimate apex predator and yet we’re only getting started. With countries like Norway extracting
fossil fuels offshore, other sectors are also expanding northward. From tourism, to shipping, to military operations. So the stressers in the Arctic are probably going to intensify. As the climate here changes so will the meaning of the word Arctic. For the animals who, for now, call it home.

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