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As the world gets hotter and the oceans warm, many marine species are shifting their ranges to higher latitudes. This has some researchers worried that new fish populations in polar waters might wreak havoc on food webs. But there’s one thing that could stop some of these polar migrators: light.

Tiny crustaceans called copepods are at the foundation of many marine food webs around the world. In the Arctic, these creatures tend to be munched in summer by fish that feed close to the surface, like herring, which then go on to feed larger fish, seabirds, and marine mammals. But the copepods still thrive overall because they overwinter in the deep waters of the mesopelagic zone, where they are relatively safe from predatory fish. With a warming climate, that might change.

Mesopelagic waters, between 100 and 200 meters deep, are also called the twilight zone because only about one percent of light from above penetrates to these depths. Fish that live in this zone at lower latitudes, such as the glacier lanternfish, have eyes evolved to hunt in low light. The dimness of mesopelagic waters also helps hide them from larger predators. If it’s too bright, it becomes dangerous for these fish; if it’s too dark, they can’t spot their prey. About half of mesopelagic fish species tend to commute to the surface in the dark of night to feed, retreating to depth during the bright day.

Today, mesopelagic fish are sparse in Arctic waters. The big question, says ecologist Stein Kaartvedt from the University of Oslo in Norway, is whether temperature or light is more important to them. If cold temperatures are currently all that’s keeping these fish out of the Arctic, then as waters warm they might move in. And that, says Kaartvedt, may be a catastrophe for the big, lipid-rich copepods living at high latitudes that could find themselves with new predators to contend with. But if light is more important to the fish, he adds, then the Arctic’s summer midnight sun, and round-the-clock nights in winter, might keep them away. That would be good news for the copepods.

So far, it’s not clear what will happen. But Kaartvedt has an as-yet-unpublished computer model of mesopelagic fish behavior suggesting that light will be a barrier to at least some species. “Basically, the conclusion is that these fishes actually are constrained by the light,” he says. Some previous work supports that conclusion. A 2016 study tracked some mesopelagic species’ daily commute from the depths to the surface. The distance decreased with increasing latitude until eventually the summer nighttime water was too light for the fish to dare travel to the surface at all. This suggests that light might be important enough to keep at least some mesopelagic fish away from the Arctic’s winter copepod buffet.

The light won’t stop all fish, however. Ecologist Anna Ólafsdóttir of the Marine and Freshwater Research Institute in Iceland says that northeast Atlantic mackerel, which live in shallower waters, have moved farther north for their summer feeding in recent years, though she credits population pressures rather than warmer water for the shift. For them, the Arctic’s midnight sun has made it possible to feed around the clock, she says.

Many factors other than temperature, from ice cover to salinity, will affect future marine life distributions, notes Kaartvedt. Light is an interesting variable as it isn’t affected by climate change, but working out the net effect is challenging. “The distribution shift of fish species is a complex ecological process,” says ecologist Lifei Wang of the University of Toronto in Ontario, who studies changes in the distribution of fish species. “It is difficult to simply say that a species is driven northward by warming temperatures and shifting prey or other factors,” he adds.

Kaartvedt and his colleagues are now working on other ways to test whether the extreme length of Arctic days and nights will keep mesopelagic fish away, or whether these fish will become new, fierce predators in the north.