Marketers have long known the power of color to manipulate consumers. Red and yellow supposedly make us hungry, while blue can calm us down. Apparently, we’re not alone in our subconscious response to color; scientists say salmon kept in tanks with black backgrounds were four times less aggressive than those in light blue tanks. The findings were published last week in the journal PLOS ONE.

Fish consumption is on the rise, and a lot of that demand is met by a growing aquaculture (fish farming) industry. But even as consumers begin to demand beef and eggs from humanely raised cows and chickens, few have given fish welfare much thought.

Fortunately, scientists have. A large body of research indicates that fish are incredibly visual animals and are very sensitive to the appearance of their environments. Previous studies have shown that even the colors of gravel and light filtering through the water can affect fish health, fertility, and aggression. But nobody had considered the potential influence of background color until researchers from the University of British Columbia’s Animal Welfare Program decided to take a look.

The scientists brought in 100 Coho salmon (Oncorhynchus kisutch) and divided them into 10 tanks of 10 fish. Every day, each tank was lined with a piece of laminated paper divided into two different colors: either black with white, light gray, dark gray, or a pattern; and light blue with white, light gray, dark gray, or black. This way, the fish could swim to the side of the tank backed with the color they liked better. Each tank was equipped with a camera that filmed the salmon from above. Afterward, the researchers watched each tape and recorded how the fish behaved, as well as how many fish chose each color.

The salmon had very clear preferences. Time and time again, they opted to stay in front of the darkest background available. When they could have black, they chose black; in the absence of black, they settled for dark gray. And even when they weren’t on the darker side of the tank, just having a black or gray section in the tank seemed to soothe the fish.

The reverse was also true. Like Morticia Addams, the salmon seemed almost offended by pastels. When the fish were forced to endure light blue backgrounds they became four times more aggressive than they were in black-backed tanks, chasing, charging, and even biting their tank-mates.

Why does this matter? Well, guess what color is currently the default for salmon fishery tank backgrounds. Did you guess light blue? You’re right.

The researchers note that their study had some limitations. The fish they studied were all the same age, and a salmon’s aggressive tendencies do vary over its lifespan. They also didn’t test the effects of water temperature and season. Still, this is progress towards a better understanding.

“Fish both preferred and were dramatically less aggressive in darker backgrounds,” co-author Marina Keyserlink said in a press statement. “Thus, we found a relatively simple environmental modification that has the potential to improve animal welfare."