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Fisheries’ Aging Lab Has Bones Coming out the Ears

By Patricia Smith
Fish Eye News
Spring 2011 Archive

MOREHEAD CITY — Every fish ear bone tells a story, and Randy Gregory knows how to read it.

He takes a sliver of ear bone, mounts it to a glass slide and places it under a microscope. An image pops up on a computer screen – gone are the days of peering through an eyepiece – and he begins to read . . . or count, to be more accurate.

“One, two, three, four, five,” Gregory said, as he pointed to distinct rings, called annuli, one for each year of the fish’s life. Because the ear bone he is looking at came from a red drum caught after its assigned birthday of Sept. 1, he adds one more year, making it age six.

This is how he reads a fish ear bone, also called an otolith. He does the same thing with fish scales.

“Both put down a growth pattern,” said Gregory, who is the Aging Lab coordinator for the N.C. Division of Marine Fisheries. “All of it is very similar to looking at growth rings on a tree.”

Gregory can tell the years when a fish has its biggest growth spurts (usually its first years of life). With American shad, he can even tell at what age the fish spawned.

Make no bones about it, this is important work.

The age and maturity data collected by the Aging Lab helps stock assessment scientists determine the health of fish stocks in North Carolina.

Otoliths taken from fish at fish houses and fishing tournaments show the ages generally caught by the commercial and recreational fishermen, said Laura Lee, senior stock assessment scientist with the Division of Marine Fisheries.

The Aging Lab also samples otoliths taken from fish caught when biologists sample the waters.

“These samples, together, give us a good picture of the age composition that is out there,” Lee said.

“It also gives us a better understanding of the dynamics of the population,” Lee said. “And the better we understand the dynamics of the population and how it responds to fishing pressure, the better we can manage it.”

Otoliths are a calcium carbonate structure, and they are pretty essential to fish.

“They sit in a membrane at the base of the skull and they act as the fish’s inner ear,” Gregory said.

There are small hairs in the membrane that the fish uses for balance, Gregory said. The fish also uses the inner ear to sense vibrations that alert it to predators.

Otoliths are also pretty important to those who study fish, as well.

Where scales from different fish can be similar, otoliths are very distinct, Gregory said. When biologists look at the stomach contents of a fish to study its diet, they look for the otoliths of the eaten fish, which take longer to digest than other parts of the meal, he said.

“Some of the biggest fish have the smallest otoliths,” Gregory said.

For instance, the otolith of a marlin is about the size of a pin head. The otolith of a red drum is the size of a quarter.

 

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