Farmed salmon like their walls painted black, UBC study finds

Cage-free eggs and massages for dairy cattle represent ways farmers are trying to improve the comfort of the animals we raise for food. But what would comfort and humane treatment look like for farmed salmon?

Scientists at the University of British Columbia say a key factor is the decor of their tanks.

Salmon are traditionally housed in tanks with light blue walls. But Leigh Gaffney, a master's student with the animal welfare program at UBC, wondered if that's really what salmon would prefer if they could choose the colours themselves.

Many fish actually prefer darker colours, scientific reports suggest.

So Gaffney decided to put the question to a school of coho salmon.

What she and her colleagues found was that given the choice between black, white, light blue, light grey, dark grey or dappled walls, coho salmon chose to hang out near the black walls.

The fish were also four times less likely to show aggressive behaviour like chasing or biting each other when they were near black walls, the researchers reported in the journal PLOS ONE this week.

"It was dramatic," said Becca Franks, a post-doctoral researcher at UBC who co-authored the study. "That's a huge difference."

She added that aggression can be a problem for the health of the fish — and their marketability. "If you go into Whole Foods and looking to buy a salmon for brunch, you don't want big bite marks on your salmon."

Low levels of aggression may also signal that the animals are under stress, she said.

But she acknowledged that figuring out whether fish are comfortable or happy isn't easy — or top of mind for people who eat them.

"You can look at a dog and sort of tell if the dog is happy or sad … but you can't do that with fish," she said. "If people realized how smart and interesting they are, they would care about their comfort more."

Tanks manufactured in blue

Don Read, owner and president of West Creek Aquaculture, a B.C.-based company that raises coho salmon in land-based tanks, said that generally the happier and healthier the fish are, the better for production.

He said the study's conclusions are interesting and make a lot of sense.

West Creek raises coho in dark blue tanks because that's one of the colours offered by the tank's manufacturer, along with other shades of blue and green. It's also the available colour that most closely resembles the ocean that salmon normally swim in.

"We want to try and recreate the natural environment as much as possible," he said.

However, some fish farmers may also consider other factors, he said, such as the fact that it's easier to see problems like algae buildup in lighter-coloured tanks.

Read said his company is interested in trying anything that scientific evidence shows is beneficial for the fish. But he's not sure there's quite enough evidence yet to start repainting the walls of his company's fish tanks, given the big difference between controlled lab conditions and those at a working fish farm.

That's something the researchers acknowledged in their paper. They say they're interested in running their experiment in a wider variety of conditions, including higher densities of fish more similar to those in the aquaculture industry and with fish of different ages.