The trigeminal cranial nerve of rainbow trout helps them detect magnetic fields by containing magnetosensitive nerve fibers.

References

"In 1997, the first known magnetoreceptors -- directly linking magnetite to neural connections and activity -- were found in vertebrates. A team of zoologists from Auckland University, led by Dr. Michael Walker, had been studying this mysterious sense in trout, and knew that a region of its skull contained magnetite.

"Recording neural activity from that region, they discovered that a specific subgroup of nerve fibers within a branch of the trigeminal cranial nerve called the ros V nerve fired in response to changes in the surrounding magnetic field. They also found magnetite in a tissue layer directly beneath the trout's olfactory (smell) organs. When they injected a colored dye into the ros V nerve's newly exposed magnetosensitive fibers, the dye revealed that the fibers terminated and ramified all around the magnetite-containing cells within the trout's olfactory tissue." (Shuker 2001:46)

Book
The Hidden Powers of Animals: Uncovering the Secrets of NatureJanuary 1, 1970
Dr. Karl P. N. Shuker

Journal article
MICHAEL WALKER: Seeking Nature's Inner CompassScienceSeptember 11, 2007
J. Bohannon

Organism
Rainbow Trout And SteelheadOncorhynchus mykissSpecies