Aging Fish

Did you know that bony fish have three pairs of otoliths, also known as ear stones, within their skull? Together, otoliths are involved in the detection of both sound for hearing and gravity for balance. Otoliths also form annuli or rings as fish experience periods of fast growth in the summer and slow growth in the winter, much like tree rings. Sagitta (photo 1 - an example from a 16 inch walleye) are the largest of the three otolith pairs and are often used by fisheries biologists to estimate growth rates, maximum age, year-class strength, and mortality rates for fish populations.

Otoliths must be prepared for analysis by first sectioning the otolith through its center. Annuli of many fish species become more visible by passing the sectioned otolith over a candle to slightly burn the otolith (photo 2 - a Sagittal otolith from a 5 year old walleye) and are then viewed under a microscope.