Research Shows How Artificial Light Affects Arctic Fish Counts

University of Delaware College of Earth, Ocean, and Environment professor Jonathan Cohen has co-written a new study that looks at how artificial light during the polar night disrupts Arctic fish and zooplankton behavior at depths to 200 m. According to the finding, published in Communications Biology, the disruption of species behavior so far down in the water column could introduce biases on stock assessments of commercial and noncommercial fish species.

“You’re trying to measure and understand the position of animals in the water column, a process that's very light sensitive, and you’re completely changing the light environment around you,” Cohen said. “In reality, it’s completely impossible to understand the process from a lit-up research vessel because you’re swamping the organisms and they’re not doing what they would normally do.”

To counteract this process, Cohen and other researchers took acoustic surveys of fish and plankton in the water column directly below the lit-up research vessel, and then took surveys using autonomous surface vehicles without lights that were far away from the research vessel.

According to Cohen’s research team, they found significantly more fish and plankton appearing in the water column underneath the autonomous surface vehicles when they compared the results to the number of fish they saw in the water column directly below the research vessel.

“We made a series of measurements of light around the vessel, trying to get a sense of how far away from the vessel this response occurred,” Cohen said. The ultimate goal of the team was to understand what impact fields of artificial light would have on fisheries during the night.

“In the paper, we demonstrated that there’s a sphere of light around the vessel that causes an influence, and we see it easily through the upper 200 meters of the water column,” Cohen said. “So the effect of artificial light isn’t just limited to the surface. It goes down quite deep in terms of an avoidance of the organisms. They just go away and so you end up predicting the number of organisms to be a lot lower than you might expect.”

The acoustic surveys used in the study show the fish as dots in the water column. When the researchers measured from the research vessel, the dots moved away from the light field, but when the researchers took a surface vehicle or small boat away from the vessel, the natural response came back and more fish appeared in the water column.

Cohen and the other researchers are hoping that these findings will help fishery managers think twice about how they are reporting fish populations in the ocean when they are using artificial light in the Arctic polar night, and elsewhere during the nighttime.

“We’re basically trying to set the parameters to say if you have a vessel and you’re looking at this portion of the water column, the values aren’t going to be as useful,” Cohen said.