Why do fish look down when they swim? Now we know

Scientsist have uncovered one of nature’s mysteries - why fish look down while they swim.

Studies from Northwestern University confirmed the reason is because it helps to stabilise them when they swim against a current.

By looking down at the riverbed, fish have a better sense of the direction they are swimming in and of their speed. Focusing on other fish, plants or debris around might give them a false sense of direction.

Leader of the study Emma Alexander said: “It’s similar to sitting on a train car that isn’t moving. If the train next to yours starts to pull to away from the station, it can trick you into thinking you are moving too.

“The visual cue from the other train is so strong that it overrides the fact that all of your other senses are telling you that you are sitting still. That’s exactly the same phenomenon that we are studying in fish. There are many misleading motion cues above them, but the most abundant and reliable signals are from the bottom of the river.”

The study focused on zebrafish in their native environment of India, instead of in laboratories.

Ms Alexander, an assistant professor of computer science at Northwestern’s McCormick School of Engineering, said: “It was recently discovered that fish respond to motion below them more strongly than motion above them. We wanted to dig into that mystery and understand why.

“Many zebrafish that we study grow up in lab tanks, but their native habitats shaped the evolution of their brains and behaviours, so we needed to go back to the source to investigate the context for where the organism developed.”

The team of researchers visited seven sites across India with shallow rivers inhabited by zebrafish.

Through the use of a 360-degree camera inside a waterproof diving case, attached to a remotely-controlled robotic arm, they plunged the camera into the water to monitor the fish.

Researchers concluded that the behaviour was an adaptive one that evolved to help fish self-stabilise themselves when swimming against a current.

The study was published in the journal Current Biology.