By Patricia Smith
Fish Eye News
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.
But then a red drum is called a drum because it makes a noise, and it needs to be able to hear the noises other red drum make.
There are two methods of removing otoliths from a fish, said Aging Lab Technician Kevin Aman. The easier method is to slice the fish’s head off, but at tournaments and fish markets, it’s better to go in through the gills so that the fish remains intact, he said.
MORE THAN LINES: Aging Lab Coordinator Randy Gregory reads the growth pattern on fish scales
Most of the otoliths that come to the Aging Lab, come from various division sampling programs with a form that lists the species, length, weight and sex of the fish.
Aging Lab technicians use a rock cutting saw with a diamond saw blade to grind to the middle otolith, called the focus. They use UV glue to mount the grinded side to a glass slide.
“Then you grind in a different direction to the other side of the focus until it is paper thin and can read through it,” said Aging Lab Technician Jacob Boyd.
The division has been aging fish since the late 1980s, but the Aging Lab did not form until 1995. By centralizing the operation, the division was able to concentrate all its aging equipment into one place and hire staff who specialize in this area.
“The Aging Lab does the bulk of the aging for the division,” Gregory said.
Funded by a federal Sport Fish Restoration Fund grant, Aging Lab staff age 14 different species, all recreational fish. From 2006 to 2010, the Aging Lab aged 27,572 fish of all ages up to a 43-year-old red drum.
“Red drum are the oldest ones that we age,” Gregory said.
The Aging Lab also has a Marine Fisheries Initiative (MARFIN) grant from NOAA Fisheries Service to conduct length and age sampling of the commercial snapper/grouper fishery.
Aging Lab Technician Garry Wright is using a computer program to compare shapes of otoliths to identify different sub-stocks of vermillion snapper.