Many of us wonder, "how old is our favorite Koi"? One way to guess at the answer is to measure how long they are, and then check the Koi Growth Charts.
However, most Koi have scales; thin, overlapping plates of bone that continue to grow throughout life. As they grow they do not increase in number, but rather increase in size. The growth of the scales is proportional to the Koi's growth, and annual marks are formed on the scales at the same time every year, along the outer edges.
So if you look at a Koi scale, it turns out that just like trees in cross-section, they have annular rings. If your Koi has lost a scale, look at it closely, even under a microscope. Then count the number of annular rings, and your Koi is at least that old, but it maybe even older. This is more accurate for younger Koi, i.e., less than 5 years old.
However, if a Koi looses a scale, and then grows it back, the new scale will not have any of the older rings. Also females may not add growth rings when they are reproducing, or grow some more after reproducing, causing 1 growth ring to look like 2.
A more accurate way to judge a Koi's age is to examine the cross-section of a fin spine, which also has annular rings. The second anal fin spine is often used. This can be done without doing permanent damage, since the fin will grow back.
The most accurate way to determine a dead Koi's age is to examine the Otolith (Williams and Bedford 1974). The Otolith is the Koi's "ear bone". Otoliths are part of Koi's vestibular apparatus, and reside in the cranial cavity. They are composed of calcium carbonate and protein. They function as sound receptors and are also used for balance and orientation.
There are 3 pairs of otoliths or ear stones in the inner ear of a Koi. The largest pair of Otoliths, the sagittae, are routinely used for aging. So Otolith is synonymous with sagitta. Again you count the number of annular rings under a microscope. White bands are formed during the spring and summer months, while darker bands are formed during winter. The Koi's age can be approximated by counting the light and dark bands as one year.
This technique has recently (5/29/2000) been successfully used by Canadian fishery researchers in Halifax to determine that ocean perch reach the age of 75 years, instead of 30 years as was previously believed.
How old is your favorite Koi?
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