From “Scientists Solve Trout Navigation Riddle” (SKY News, July 9, 2012), we learn, “German scientists claim to have worked out how trout – and possibly other animals – use magnetic cells to help them navigate”:
Trout have cells in their noses that act like miniature compasses to help them navigate, a study has shown.
The cells, which are believed to exist in other animals, contain iron-rich deposits of a magnetic material called magnetite.
Tests suggested the cells were capable of detecting magnetic north as well as small changes in magnetic fields.
They were about 100 times more sensitive to magnetic fields than had been expected.
It remains to be seen whether this is what, say, other migrating fish, or birds and insects use. It’s reasonable, but not yet demonstrated.