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Alaska Fish & Wildlife News
October 2013

How Old is That Fish?
Fish Aging: The Art of Science

By Amy Carroll
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Age readers April Rebert, Rob Dinneford and Kevin McNeel at the Age Determination Unit in Juneau.


—Kara Hilwig


As it turns out, Leibman’s rockfish was only 64 years old. How did we know?

How are rockfish aged?

Leibman’s rockfish – like thousands of other fish caught in Alaska waters – was aged by a special laboratory within the Alaska Department of Fish and Game known as the Age Determination Unit (ADU) in Juneau, Alaska.

Fish ages are revealed by analyzing their otoliths, or “fish ear bones,” a structure found in their inner ear. Like the inner ear mechanism in humans, the otolith in fish is used for balance, direction, and hearing. Because an aquatic environment demands a lot of feedback in the areas of balance and direction, fish otoliths are enormous compared to human otoliths, which are about the size of a grain of rice. Among bottom dwellers, rockfish otoliths are disproportionally large relative to their body size. Depending on the fish and its age, otoliths may be the size of a nickel or a dime, sometimes larger.

Often it is assumed that the fish size correlates with age, a guess that proved incorrect with Liebman’s rockfish. Genetics, nutrition, environment, and other factors affect how big a fish can get. With some freshwater fish, the availability of calcium in the fish’s diet (critical for the skeleton) may limit how big it can grow.

Otoliths are made of calcium carbonate and protein that grow in annual patterns like tree rings. In the early years of a fish’s life, body and otolith growth is rapid. This can be seen as widely spaced annual increments on a fish’s otolith. During ‘adolescence’, body and otolith growth decline as more energy is focused on reproductive demands. This is seen as a gradual decline in increment spacing on the otolith. At adulthood, growth slows and the increments are spaced very close together. New growth may even obscure older bands or rings, but these can be seen if the otolith is broken in half. Currently, the oldest yelloweye rockfish the lab has aged was 121 years old. It was caught this year in the Southeast Alaska commercial fishery.

Age reading fish in Juneau

The ADU uses the break and burn technique in which otoliths are broken in half at the center and toasted over an alcohol flame, making the bands easier to see and count. These specimens are then examined under a microscope. For many years, scientists just counted the rings on the surface of the otolith to estimate age without breaking it.

“It wasn’t until recently that we realized rockfish were way older than we thought,” said Kara Hilwig, current director of the Age Determination Unit. “There are rings inside the otolith that might add 100 years to a fish’s age.”

Age reading requires skill in pattern interpretation. A strong age reader often has an inherent ability to see familiar shapes in clouds or repeating patterns in nature. Age reading is not as easy as counting slats in a fence, venetian blinds, or even tree rings. For the more difficult to read species, like black cod (also known as sablefish), it could take 18 months to train a new reader. One of the most difficult things about age reading is that you rarely know the true age of a fish, and because you don’t know when it was born, you must estimate age based on perceivable patterns that are assumed to be annual. “It’s like the art of science,” said Hilwig.

Alaska’s black cod fishery is worth hundreds of millions of dollars, yet “worldwide, there are probably less than 20 people who know how to age sablefish, and three of them work here,” said Hilwig. “It’s the most difficult species to age---sexually dimorphic growth patterns, lots of pattern interpretation. It’s quite noisy.”

Age readers at the lab are Rob Dinneford, Kevin McNeel, and April Rebert. Kevin and April spend nearly three-quarters of their work day age reading otoliths at a microscope, while Rob divides his time between age reading and keeping the wheels of the data production process rolling.

Rob Dinneford, a fish and wildlife technician, has been at the lab for three and a half years, but just started age reading this summer, when his job duties expanded to include lingcod age reading. Unlike rockfish, lingcod only live to be about 35 years of age. “They are a lower-priority species with less complex age patterns,” Dinneford said. “It’s an incredibly intriguing biological and spatial challenge,” he said of trying to discern three-dimensional accretion patterns. “It’s just a fantastic puzzle.”

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Kevin McNeel prepares an otolith for reading by heating it over an alcohol flame.

Lingcod otolith complexity varies by location. “Region 1 (Southeast Alaska) lingcod are hard to age, Region 2 (Central Alaska) ling cod are easier.” According to Hilwig, this could be attributed to differences in latitudes with northerly fish otoliths expressing clear slower growth patterns and southerly otoliths showing fast growth noisy patterns because of differences in the growth environment.

Kevin McNeel, a fishery biologist, has been at the lab since February of 2011 and started training as an age reader right away. He received his undergraduate degree from UAS in marine biology mostly involved crab physiology.

“It’s a completely new science to me,” he said of fish aging. “It’s a long training process and it can be difficult at times. It’s a really different skill set.”

Currently he’s aging black cod, but has aged several types of rockfish, ling cod, and geoduck clams. “Keeping track of 50 to 100 little lines can be kind of difficult,” he said. “I’ll age a specimen a couple of times; there’s a high chance of losing count.” I asked if he’d aged the 121-year-old rockfish. “Oh, we were all in on that,” he said.

April Rebert found her way to the lab through Workplace Alaska, and was hired in February of this year. “I didn’t know what fish aging was, so I did some research and it looked pretty fascinating.” Originally from Michigan, her background is in animal physiology and plant research. “I had a job in canine toxicology and it was really depressing.”

Rebert started her age training on geoducks. After six months, she was skilled enough to produce geoduck age data used by state biologists to manage geoduck. “Geoducks don’t have complex age patterns,” she added.

From the start she was made aware that age training would be rigorous. “It’s not as easy as it might seem, counting lines,” she said. Actually, I responded, it sounds exhausting. What if you lose count and have to keep starting over? “Younger fish are actually more challenging. If you’re off by a year it’s a much bigger deal.”

“I am really enjoying learning about it, and I am so happy to be in Juneau,” she said. “There are lots of opportunities for growth.”

Two skilled fish agers will usually arrive at a similar age for the same otolith. “They will be very close to each other, but you will never know the true age of a fish,” Hilwig said. “Known age fish are so rare.”

The ADF&G ADU has otoliths from nearly 220,000 individual fish representing 39 species. The otoliths are archived on shelves at ADU and the age estimates are saved in an electronic database.

Why do rockfish live so long?

Rockfish are unusual as they give birth to live young. They don’t spawn and fertilization is internal. After internal fertilization, female rockfish give birth every year to millions of live young, most of which do not survive. “Their long-lived life history enables the opportunity for an individual to successfully spawn at least once within its lifetime in the face of a highly variable environment,” said Hilwig. Every now and then, all the environmental stars align and rockfish young survival is high. This high survival, or recruitment pulse, might only happen once every ten or more years, according to Hilwig. Oftentimes, this kind of pulse must sustain harvest for decades until another recruitment event occurs, boosting the population numbers again.

Why is it important to know how old rockfish are?

Fisheries managers need to balance population sustainability with fishery demand. The most effective way to manage fisheries is to use an age structured model. This means fish populations are tracked by their age class, not as a total population number. An age structured model uses age data (like that produced at the ADU) to help managers assess a population’s reproductive potential (sustainability) as individuals of reproductive age are harvested from the population.

Age data also helps fishery managers assess the consequences of harvesting immature fish and older, larger, female fish. Larger females produce more young, thus contributing more to the reproductive potential of a population. From this, harvest strategies are developed (gear, area, season) and limits imposed (quotas) to minimize harvest of fish that contribute most to the sustainability of the population.

Otoliths reveal more than age

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An otolith before and after heating and cracking.

Aging very old fish can establish an aquatic biochronology, where known atmospheric or oceanic events (marker years) can be found reflected in otolith rings of fish estimated to be alive during that event. This can be used not only to get a more accurate age estimate, but to date the lifespan of the fish and to pinpoint environmental and manmade events that affected the fish’s growth and health. One of the most accurate methods of age validation for long-lived fish is bomb radiocarbon, which acted as a large-scale tagging event. In the late 1950s and 60s, nuclear testing raised the atmospheric level of radioactive carbon, which shows up in the otoliths of fish alive during that time.

“In a 200-year-old fish, you can look back over known events and see how they correlate to the otolith growth,” Hilwig said.

Fisheries scientists can also use the composition of trace elements to discover where fish have resided throughout their lives. Tracing a fishes’ travels, and aging them through their otolith annuli, gives fisheries scientists valuable information that can help them more successfully manage stocks. Additionally, since otoliths are somewhat resist digestion, they can be found in the digestive tracts of marine mammals, giving scientists information on the prey composition of marine mammals.

Susceptible to overfishing?

Currently Alaska’s rockfish population is healthy; however, non-pelagic (bottom-dwelling) rockfish are slow to mature, reaching sexual maturity at about 20 years. They are also delicious. Due to their late maturity, rockfish are very vulnerable to overharvest—which would quickly reduce the number of fish available for spawning. With the recent push to limit halibut charter anglers to one fish per day, fisheries scientists wonder if rockfish will become a targeted species, and if their age will become a trophy characteristic with tragic consequences to Alaska’s rockfish populations. Fortunately two easy-to-use deep water release methods were recently developed allowing anglers to safely release rockfish at depth

Catch and release is now an option

As rockfish are brought to the surface, their ascent causes their nonvented swim bladder to expand and compress their internal organs (decompression barotrauma)—as can be seen in the bulging eyes and distended stomach images of rockfish catch. The most damage occurs to the fish in the top 33 feet of water. When rockfish are released on the surface by anglers choosing not to keep them, the barotrauma causes them to float, and they are unable to swim back down to the bottom by themselves and they eventually die.

Since January of this year, Southeast Alaska sport fishing charter operators are required, if they choose to release rockfish, to do so with a deep water release mechanism to 100 feet in depth or to the depth the fish were caught at, whichever is shallower. Additionally, the Alaska Department of Fish and Game’s sportfishing regulations provide tips on how to avoid catching rockfish as bycatch when fishing for halibut.

If a rockfish can be brought back down to depth, even with severe signs of barotrauma, the survival rates are estimated to be up to 98%. One easy-to-make device to assist anglers return rockfish to depth is a nonbarbed jig attached to a fishing weight; a similar device is sold under the name SeaQualizer. An upside down weighted milk crate weighted and attached to a rope will work; if it’s lowered to at least 40 feet the fish will often swim out on its own.

Well, sure, the fish may swim off when released back at depth, but does it really survive? On Sept. 9 of this year, University of Alaska Fairbanks student Brittany Blain defended her masters thesis, “The effects of barotrauma and deepwater-release mechanisms on the reproductive viability of yelloweye rockfish in Prince William Sound, Alaska.” She has shown that yelloweye rockfish caught and released into deep water using a deepwater-release mechanism survive to reproduce, and that embryo quality is not affected by their previous barotrauma event.

So how old are Alaska’s groundfish?

The Alaska Department of Fish and Game has a poster online with current maximum ages of some of Alaska’s groundfish and invertebrate. Learn about the 205-year-old rougheye rockfish, the 146-year-old geoduck clam, and the 35-year-old sea urchin.

Amy Carroll is a publications specialist with the Commercial Fisheries Division in Juneau.


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Riley Woodford

1255 West 8th St.
Juneau, AK 99801
(907) 465-4256

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