Whether it’s in ponds, lakes or oceans, the management of fish species in those bodies of water depends on aging. Not “aging” as in describing senior citizens, but determining how old the fish are, and whether their growth rates are good — or not so good — for their age.
And exactly how does one determine a fish’s age? It’s a lot like counting the rings in the trunk of a tree. But in most cases, the rings that biologists are counting are on something smaller than a thumbnail. They’re called otoliths, and they must be viewed under a microscope.
Most bony fish have them but not sharks, rays or skates, said Don Swatzel, fisheries biologist for the Ohio Division of Wildlife’s District 4, headquartered in Athens, Ohio. Those bony fish actually have three pairs of them located behind or below their brains. They’re made of calcium carbonate, which is basically limestone.
“The otolith is like our inner ear,” Swatzel said. “It allows the fish to hear and sense vibrations. It also helps them balance and stay upright in the water.”
The three pairs of otoliths vary in shape and size but they all work together. The largest pair, the one that converts sound into electrical signals, is the one used to age walleye, largemouth bass, sauger and crappie, Swatzel said.
Counting rings
The rings on the otoliths form because of differences in growth rates during the year. When it’s summer and the fish are feeding well, a sort of translucent area forms on the otolith.
“When it’s colder and their metabolism slows, the material is not laid down as quickly so the rings look opaque,” he said. “Each area of translucence, then opaque, we consider a year.”
It sounds pretty straightforward, but it isn’t. Depending on when the fish was caught, biologists like Swatzel have to decide whether to count the outer edge of the otolith as a ring.
For fish sampled in the spring, when the water temperature is between 59 and 69 degrees, he assumes they haven’t started to grow their next rings yet. So for yellow perch, largemouth and smallmouth bass, bluegill and redear sunfish, he counts the outer edge of the otolith as another year.
On the other hand, sauger, walleye and crappie are among the fish sampled in the fall. They’ve been growing all summer and could still be growing in November because those fish are more coldwater species, he said. They may not have completed the translucent-to-opaque cycle, so he doesn’t count the outer edge of the otolith as an additional year.
Taking measurements
When sampled fish are brought back to the lab, Swatzel measures and weighs them, then takes out the largest pair of otoliths. He uses tweezers to remove the membranes that grow over the otoliths, then rinses them. He takes one of the pair, places it on a hotplate and toasts it till it’s a dark, amber brown. Again using tweezers, he snaps the otolith in half and plants it cracked end up in black Play-Doh on a slide.
Then it goes under the microscope. Well, actually a stereoscope, which doesn’t magnify quite as much. Swatzel counts the rings, records and analyzes the data, then enters it into Ohio’s fisheries information system.
Bones, scales and the rays of a fish’s fin can also be used to age fish, “but we use the otolith because it’s the most accurate,” he said. “If you have a largemouth bass that’s 10 years old, you can have 10 people read the scale and get 10 different answers.”
Management implications
And accuracy is important when it comes to management. For instance, the largemouth bass should be 11.3 inches long by the time it’s three years old. If Swatzel and others do a random sampling of a lake, they can come up with an average length for each age group.
“We call it a growth curve,” he said. “If the growth is below average for their age, that would make us consider changing the regulations in that lake… If a lake’s population of largemouth bass are all older and small, less than 12 inches, and if few individuals are larger, then we could consider placing a protected slot and say anglers can’t keep them if they’re between 12 and 15 inches.”
There are seven lakes in Ohio with those protected slot limits. They include Burr Oak and Dow lakes, which are popular with anglers.
“There’s still a daily bag limit of five fish, but you can keep five that are less than 12 inches, or five that are more than 15 inches, or a combination,” Swatzel said. “If we get more folks keeping the smaller bass, the regulations could be adjusted.”
Size matters
And therein lies the biggest challenge of his 30 years as a fisheries biologist: Convincing anglers to keep fish under a foot long.
“There’s a belief that it’s not OK to harvest bass till they’re 12 inches. Maybe that comes from articles about different environments like Florida, Alabama or Texas. They grow better there because the growing season is longer,” he said. “In Ohio, it’s a short growing season. They have to grow a lot in a short amount of time. If there’s not enough food, or cover, or good habitat, then you end up with a lot of 10-inch bass.”
Aging also helps biologists to determine mortality rates, both natural mortality and fishing mortality.
“If there are very few older, larger fish, that fishery could be over-exploited; anglers are taking too many fish home,” Swatzel said. “That could trigger a 15-inch length limit to allow them to get both older and larger.”
If the opposite is true and there are few young fish in a lake, “it could signal a lack of older, spawning fish,” he said. “They aren’t producing enough offspring to support the fishery.”
In lakes like Piedmont, Salt Fork and Seneca, there’s a 12-inch harvest limit for all three species of black bass: Largemouth, smallmouth and spotted.
“It was determined that both largemouth and smallmouth would benefit,” Swatzel said. “Plus, some folks can’t tell the difference between a largemouth and a spotted bass, so the regulations apply to all three.”
By the beginning of summer, Swatzel had sampled four lakes for perch, two lakes for sunfish and eight lakes for bass. Then began at least a month of meticulous work with the microscope, after which he’ll go out and sample some more. And for Swatzel and other biologists, it will be “rinse and repeat” for many years to come.
“We can’t go to a lake in just one year and expect to know what’s going on,” he said. “We have to go over a period of years, sampling multiple times, before making a regulation change. Those decisions have to be based on the best science available to us.”
