Whole otoliths are thin sectioned down the transverse plane, through the nucleus, and along the primary growth axis that is specific to each species’ otolith or otolith type within a species (region between dotted lines for this example bluegill).
The resulting thin section is viewed under a compound microscope, photographed, and annual bands are quantified (3+ years old for this winter-collected bluegill).
A 25 cm (9.8 inch) total length, 7-year-old black crappie that was collected during early summer.
A 45 cm (18.1 inch) total length, 2.04 kg (4.5 lb), 15-year-old largemouth bass.
A 91.5 cm (36 inch) total length, 10.7 kg (23.6 lbs) 81-year-old (1936-2017) female bigmouth buffalo.
Otoliths (earstones) are unique and extremely valuable for age analysis because they are the only structures that grow throughout a fish’s entire life, and they are acellular and metabolically inert – so whatever is “put down” at any given time is fixed in the otolith microstructure. During slow-growth periods (e.g. winters), otolith growth slows, changes in chemical composition, and an annual ring is formed. Even so, considerable experience is necessary in order to process and correctly interpret otolith annuli, as “checks” or “false annuli” can occur, especially during the fish’s first few years of (rapid) growth.
Other fish structures commonly used in age analysis such as scales, fin rays or spines, opercula, cleithra, vertebrae, or any other bone are highly prone to underestimating the true age of a fish. They are structures that cease growth at relatively young ages and are not metabolically inert (i.e. they are prone to cellular resorption).
Age Validation
One of the best ways to validate age estimates of long-lived species is by bomb radiocarbon dating. This works by detecting the global signal that resulted from the widespread testing of nuclear bombs in the mid-20th Century, and the fallout that has occurred to this day. In short, the of amount radiocarbon (carbon-14) suddenly increased in the mid-late 1950s and early 1960s, and quickly spread around the world while becoming incorporated into the organisms that were alive during that time.
For organisms that “lay down” presumed annual rings as they grow (for example trees, clams, corals, and fishes), the amount of radiocarbon can be measured within a given ring, and thus a presumed year. If the resulting radiocarbon measurements from presumed years (rings) within a given organism are consistent with the already-documented bomb-pulse radiocarbon chronology from that general area, it provides strong corroborative evidence that the rings are indeed annual. Bomb radiocarbon dating is powerful, and is perhaps the best method available for age-validating long-lived organisms because it not only can validate the periodicity of deposited rings, but it also can be used to determine absolute and accurate ages of individual organisms.
An example from the Woods Hole Oceanographic Institution NOSAMS facility in Woods Hole, MA: validating the age of a 150-year-old beech tree.
Otolith Lab 