Geomagnetic variations and fish year-class strength

For centuries, biologists have tried to explain the pronounced natural population fluctuations of fishes, but without much success. However, to successfully reproduce, fish often perform long migrations to spawn at the same spot as their parents, so-called homing. We do not fully understand how this is possible in a variable environment with few navigational cues, but, like migratory birds, fish species may use the earth’s geomagnetic field to aid navigation. Because the geomagnetic field varies over time, fish migration success, and hence spawning success and year-class strength, may vary accordingly. So the central question is: Can geomagnetic variation explain a significant part of the large unexplained variation in the year-class strength of fish populations?