"We could get them to flip their poles from north to south when we changed the field around them - you can actually get the crystals to almost twinkle under the scope for you."
Researchers in New Zealand say they have discovered crystals in the noses of rainbow trout that act as direction finders for the fish.
The tiny crystals are made of magnetite, an oxide of iron that has magnetic properties.
The crystals are linked to individual receptor cells in the brain, allowing the trout to sense changes in magnetic fields.
Dr Diebel says the fish definitely respond to changing magnetic field lines but they are not overpowered by it.
"It is not a huge sense, the magnetic sense," she told BBC News Online. "We think of it more as being supportive. It helps the other senses, and it helps the animal go in the right direction.
"But it doesn't hit them over the head, it just gives them a little nudge."
Flipping Poles
In previous research, she traced a nerve from the brain to a cell in the trout's nose. She was convinced that magnetite crystals existed within the cell. Now, using a powerful laser-scanning microscope, she has found the crystals arranged in a one-micron (millionth of a metre) chain formation.
"What we did was use the laser to focus into the cell, and the beam flares off the crystal surface - it appears larger than it really is and we can see it," Dr Diebel said.
Using a different type of microscope, she then proved that the crystals had magnetic properties.
"We could get them to flip their poles from north to south when we changed the field around them - you can actually get the crystals to almost twinkle under the scope for you."
So could this knowledge about the trout's magnetic nose be of use to a wily angler? Dr Diebel doubts it.
"We're not saying that rainbow trout are necessarily attracted to magnetic fields, we're just saying that they can use it. So if I was a fisherman, I probably wouldn't invest a large amount of money in a big magnet."
She believes that scientists are on the verge of discovering a general magnetic receptor system in several different animal species.
Work is going on with pigeons and turtles. Magnetic direction finding is also being studied in relation to whale strandings.