As Arctic ice shrinks, fish will see the region in a whole new light.
With sunlight now permeating previously darkened waters, predatory fish that hunt by sight are set to invade in increasing numbers, scientists predict in a new study.
Models of past and future climate and environmental scenarios forecast a dramatic rise in light-penetrating polar waters from 2040 onward. (Related: "Extreme Research Shows How Arctic Ice Is Dwindling.")
“We know that sea ice extent and thickness is declining and is predicted to continue to do so,” says study leader Øystein Varpe of the University Centre in Svalbard, Norway.
“With that, there comes a dramatic increase in light entering the water column because ice, and particularly snow-covered ice, are good at blocking light.”
As a result, northward-spreading species like Atlantic cod, salmon, mackerel, and herring are expected to find it much easier to see and catch prey.
“You can search a large area much more efficiently if you can use your eyes than if you have to be very close up on your prey,” says Stein Kaartvedt, a marine zoologist at Norway’s University of Oslo who wasn't involved in the research.
“If a visual predator can exploit the Arctic in summertime, it has 24 hours of feeding.”
Published in the ICES Journal of Marine Science, the findings add a previously unrecognized dimension to the potential impact of climate change on Arctic marine life.
Such studies in the past focused on changes in water temperature and food availability, Varpe notes. (See a graphic of our changing Arctic.)
Migratory fish that can cover large distances should have most to gain from the underwater light boost.
“Good swimmers like salmon .... can move out during the winter, which will be dark regardless,” whether there’s ice cover or not, Varpe says.
The finned invaders would target larger zooplankton such as copepods and krill. But these fish could be on the menu themselves, since increased visibility would put them at risk from larger predators, including seals and other marine mammals. (See "7 Species Hit Hard by Climate Change—Including One That's Already Extinct.")
“That makes things complex,” he says.
Kaartvedt, who wasn't involved in the new research, agrees that the new arrivals could also be more vulnerable, though schooling fish such as herring and capelin can seek safety in numbers.
Arrivals Already Underway
While the new study looks to the future, the Arctic is already experiencing an influx of new fish, according to Jørgen Schou Christiansen, a marine biologist at Norway’s University of Tromsø.
“Incoming species from the south have increased, there’s no doubt about that,” Christiansen says. “This summer we had the northernmost observation of Atlantic cod, in northeast Greenland.”
What’s behind this trend isn’t yet clear, he says, but summertime forays by voracious sight-feeders may have serious consequences for the only true Arctic fish they’ll meet, the polar cod (Boreogadus saida).
Whereas the Arctic Ocean’s other native fish are all deep-sea bottom-dwellers, polar cod are uniquely adapted to living just under the ice. (Read "Shrinking Arctic Ice Prompts Drastic Change in National Geographic Atlas.")
While the fish have large eyes, they don’t have to use them to feed—polar cod can also sniff out food in total darkness, Christiansen says.
A keystone species of the Arctic, it may not be able to stand the new competition. The cod’s larger Atlantic cousin would wolf it down, and probably mackerel, too, he notes.
The challenge for future investigations, study leader Varpe adds, “will be to disentangle the light effect from temperature and food-related changes that can also play a role."