A fish's ear bone, or otolith, lets them orient themselves (relative to their surroundings) in water. Think of the otolith as somewhat of a compass for them. Within these ear bones, growth rings form, much like those of trees.
These growth rings allow scientists to determine a fish's age and their growth rates. Worldwide, literally millions of otoliths have been preserved, dating as far back as to the 1800's.
Through analysis of these rings, researchers can compare current fishes growth rates against those of the historical archives. By linking these growth rates to historical climate data, researchers can determine how climate change and environmental conditions impact commercially-important species.