Fish have an internal "compass" to help get them home, study finds

Fish have an internal compass.

We’ve learned from Pixar films that fish like Nemo and Dory need to use the power of friendship to find their way home, but many creatures in the wild, from birds to marine mammals and sharks, are capable of doing the job solo, thanks a range of homing abilities including the sensing of magnetic fields.

Now, for the first time, scientists have discovered that tiny reef fish the size of a fingernail have a similar internal magnetic “compass” that allows them to find their way home at night.

The new study was jointly led by scientists at the University of Oldenburg in Germany and James Cook University in Townsville, Australia, who found that Cardinal fish larvae collected from One Tree Island on the Great Barrier Reef have a build-in sense of direction that helps with nighttime navigation.

The story goes that early on in their development, coral reef fish like the Cardinal fish have limited swimming power and often get pulled out to see by ocean currents, sometimes ending up tens of kilometres away from their settling grounds in the coral reefs.

Previous research had shown that once they’ve grown up a bit, these little go-getters use the sun during daytime to guide their way home. But how do they do it at night?

To test the hypothesis that reef fish larvae might also have an internal magnetic sense, the research team put collected Cardinal fish in lab-induced total darkness, with a similar magnetic field in play as found at the Great Barrier Reef. Then they changed the magnetic field and – voila! The fish changed direction.

“Normally, fish orientated to the south east, but when we altered the magnetic field clockwise by 120 degrees, there was a significant change in the direction the fish swam,” says study co-author Mike Kingsford of the ARC Centre of Excellence for Coral Reef Studies at James Cook. “They all turned further west, thinking they were still on track to their destination. This study is the first clear demonstration that reef fish larvae possess magnetic senses to orient them at night,” says Kingsford.

Once the fish get closer to home, they have been shown to rely on other senses like smell, sight and hearing to figure out where in the reef is their chosen abode. But the new research shows that even at an early stage, reef fish have well developed neural capabilities.

“The study tells us these baby fish actually have brains. They know where they are going and are strong swimmers,” says Kingsford. “As a result they have some control over the reef they end up on. It’s not just about being led by the currents.”

Kingsford says that scientists can use the new information to help develop more accurate models of where the larvae go, so as to better protect and maintain these fish stocks.

The tracking of fish stocks is said to be of increasing importance to commercial fisheries, as environmental changes brought on by global warming cause fish to alter their feeding and breeding grounds.

A recent study by scientists at Fisheries and Oceans Canada in BC estimates that warming of the oceans of the North Pacific will actually give albacore tuna much more foraging habitat in the region, thus likely expanding its stock in the area. Researchers determined the “thermal niche” for the tuna to be between 14 and 19 degrees Celsius and then, using climate model simulations to chart the expected rise in ocean temperatures, estimated that climate change will likely open up over a half a million square kilometres of the North East Pacific as new foraging ground for the tuna.