A fish cannot tell us its birthday, but its body keeps a surprisingly detailed record of the years it has lived.
Much like the rings inside a tree trunk, certain fish structures develop visible growth patterns over time. Fisheries biologists can examine these patterns to estimate a fish’s age (like the oldest fish ever recorded in the great lakes), reconstruct parts of its growth history and learn how an entire population is changing.
That information is especially valuable in a vast ecosystem like Lake Superior, where lake trout, lake whitefish, ciscoes, kiyi and other species may live for many years beneath the surface.
Why Does the Age of a Fish Matter?
Knowing the ages of fish within a population helps fisheries managers answer several important questions:
- How quickly are the fish growing?
- At what age do they begin reproducing?
- How long do they normally live?
- Are enough young fish surviving?
- Is the population dominated by young fish, older fish or a healthy mixture?
- Are fishing pressure, food availability or changing habitat conditions affecting growth?
According to Michigan State University Extension’s guide, Determining the Age of Fish, age information helps scientists understand fish growth, spawning, migration and changes in habitat use.
For Lake Superior, these questions are directly connected to decisions involving fishing regulations, stocking programs, harvest limits and the long-term recovery of native species.
The Great Lakes Fishery Commission’s Lake Superior Fish Age Assessment Workshop report explains that inaccurate age estimates can distort measurements of fish growth, maturity, survival and recruitment. Those errors can ultimately contribute to excessive harvest limits or unnecessary restrictions on fishing.
Fish Scales Can Work Like Tree Rings
Fish do not have birth certificates, but their scales, bones and otoliths (earstones) preserve growth rings that help scientists determine their age and monitor Lake Superior fish populations.
For many species, scientists can estimate age by examining the scales.
The original Michigan State University fish age guide (PDF) uses a bluegill as an example. Biologists typically remove several scales from a consistent location on the fish, often near the front edge of the dorsal fin.
Taking more than one scale is important because some may be replacement scales that formed after an earlier scale was lost or damaged. Replacement scales do not contain the fish’s complete growth history and may therefore produce an inaccurate age estimate.
The collected scales are cleaned, moistened and enlarged with specialized imaging equipment. Older methods included microfiche readers and slide projectors, while modern laboratories may use microscopes, digital cameras and image-analysis software.
Under magnification, the scale reveals a pattern of growth rings.
What Are Annuli?
The prominent annual growth marks found on a fish scale, bone or otolith are called annuli. The singular form is annulus.
During favorable growing conditions, a fish generally grows more quickly. The markings on its scales or bones are spread farther apart. Growth usually slows during winter or other stressful periods, producing closely spaced marks.
One cycle of faster and slower growth can create an identifiable annual band.
Scientists count the annuli to estimate the fish’s age. The process resembles counting rings in a tree, although fish-age interpretation can be more complicated. False rings, irregular growth, injuries and environmental stress may all affect the pattern.
The Lake Superior Fish Age Assessment Workshop report distinguishes true annuli from other marks that may form within a fish’s calcified structures. These irregular marks, sometimes called checks or split annuli, can make accurate interpretation difficult.
Not Every Scale Should Be Used
The location from which a scale is collected matters.
Scales near the lateral line may contain a tube-like canal associated with the fish’s sensory system. That feature can obscure the growth pattern and make the scale difficult to interpret.
Biologists therefore use standardized sampling locations for each species. Consistency allows them to compare fish collected in different years, different areas of a lake or by different agencies.
Each sample must also be carefully documented. Records may include:
- Species
- Length and weight
- Capture location
- Date and time
- Collection method
- Sample identification number
- Name of the person collecting the sample
Without those details, an age estimate has far less scientific value.
Standardized collection and interpretation are particularly important on Lake Superior, where state, federal, provincial and tribal agencies all participate in fisheries monitoring. The Great Lakes Fishery Commission report describes efforts to create consistent, species-specific aging methods that can be used across the lake.
Scientists Can Reconstruct Earlier Growth
A scale may reveal more than a fish’s present age.
Because the scale grows along with the fish, scientists can measure the distance between annual growth marks and estimate how large the fish was during previous years of its life.
This process is known as back-calculation.
A fish’s growth history can offer clues about changing conditions in the lake. A year of unusually slow growth might correspond with limited food, intense competition, disease, unusual water temperatures or another environmental stressor.
However, size by itself is not a reliable way to determine age.
Two fish of the same species and length may be very different ages. One may have grown quickly in productive habitat, while another may have grown slowly under less favorable conditions.
What About Trout and Fish With Tiny Scales?
Scales are not always the best aging structure.
Some fish have no useful scales, while others—including trout—may have scales that become increasingly difficult to interpret as the fish grows older. In these situations, scientists may examine harder body structures such as:
- Otoliths
- Fin rays
- Fin spines
- Vertebrae
- Maxillae, or upper-jaw bones
- Other calcified structures
These structures also develop recurring growth bands.
An otolith is a small calcified structure inside a fish’s head. Otoliths help fish maintain balance and detect sound, but they also preserve a remarkably useful record of growth.
Researchers may remove an otolith, cut it into a thin section, polish it and examine the internal bands under magnification. This procedure is more involved than reading a scale, but it can produce more dependable estimates for older fish.
The Lake Superior Fish Age Assessment Workshop identified thin-sectioned otoliths examined with transmitted-light microscopy as a preferred method for aging several important Lake Superior fish groups, including coregonines such as ciscoes and whitefish.
Otoliths Are Important in Lake Superior Research
Accurate aging can be particularly challenging in Lake Superior because several native fish species are slow-growing and long-lived.
Lake trout provide a striking example.
A U.S. Geological Survey study of Lake Superior lake trout found that some fish may live as long as 42 years—far longer than many anglers might expect.
Researchers compared ages obtained from scales with ages obtained from sagittal otoliths. The scale readings consistently underestimated the ages of mature lake trout, especially the oldest fish.
In another study of deepwater humper lake trout, researchers found major differences between scale and otolith estimates. Scales placed sampled fish between approximately 5 and 13 years old, while otoliths indicated that some were between 8 and 28 years old. The study is available through the USGS Publications Warehouse.
That difference matters. Underestimating the ages of older fish can distort calculations involving growth, mortality, reproduction and overall population health.
Recognizing this problem, fisheries agencies around Lake Superior have worked together to develop standardized methods, quality-control procedures and reference collections for interpreting lake trout otoliths and other structures.
Aging Methods Differ by Species
There is no single structure that works equally well for every fish.
The preferred method depends on the species, its expected lifespan, the size of the fish and the purpose of the survey.
The Lake Superior Technical Committee’s recommendations include different aging structures for different fish, such as:
- Lake trout: otoliths or maxillae, depending on the fish and survey
- Ciscoes and other coregonines: thin-sectioned otoliths
- Lake whitefish: otoliths
- Yellow perch: spines or otoliths
- Walleye: spines, otoliths or other validated structures
- Lake sturgeon: fin rays
Researchers must validate the chosen structure to make sure it preserves the fish’s complete growth sequence. A method that works well for a young fish may become increasingly unreliable as the fish grows older.
Why Accurate Aging Helps Protect Lake Superior Fish
A fish population needs more than a large number of individuals. It also needs a healthy age structure.
A strong population usually contains multiple year classes, from recently hatched fish to mature adults. If surveys repeatedly find few young fish, reproduction or juvenile survival may be failing. If older adults disappear, excessive mortality may be removing important spawning fish.
Age data can help managers:
- Measure the success of natural reproduction
- Evaluate stocking programs
- Estimate survival and mortality
- Monitor strong and weak year classes
- Set appropriate harvest regulations
- Detect changes in growth over time
- Compare populations across different parts of the lake
- Measure the recovery of native fish populations
For Lake Superior lake trout, population age structure has been central to evaluating the recovery of naturally reproducing stocks following decades of damage from overfishing and invasive sea lamprey.
Accurate ages also improve statistical models used to estimate how many fish are entering a population, how many are surviving and how many can be harvested without jeopardizing future generations.
Different Scientists May Read the Same Fish Differently
Determining a fish’s age is not always as simple as counting a few obvious rings.
Preparation methods, image quality and human interpretation can all affect the final result. A reader may mistake a stress mark for an annual ring or fail to recognize a tightly spaced annulus near the edge of an older fish’s otolith.
Scientists therefore use quality-assurance procedures such as:
- Having more than one reader examine selected samples
- Reading samples without knowing the fish’s length
- Comparing results with established reference collections
- Rechecking samples when readers disagree
- Conducting regular training and calibration exercises
- Recording uncertainty in the final age assignment
The 2023 Lake Superior workshop brought together representatives from 12 state, provincial, federal and tribal governments to improve consistency among agencies.
The workshop recommended shared reference collections and standardized protocols to reduce differences among individual readers and fisheries laboratories.
Can Anglers Determine a Fish’s Age?
An angler can make a rough guess based on species, length and local growth patterns, but appearance alone cannot provide a dependable age.
A large fish is not necessarily extremely old, and a smaller fish is not necessarily young. Water temperature, prey availability, genetics, population density and habitat can all influence growth.
Reliable age determination requires properly collected structures, suitable equipment and experience interpreting the bands. Even trained readers may compare results or examine the same sample more than once.
The best practice for most anglers is to record the fish’s length, weight, location and date, then allow fisheries professionals to collect any biological samples required for an official study.
Anglers who encounter a tagged fish should also carefully record the tag number and follow the reporting instructions printed on the tag. Tagged fish can provide especially valuable information about movement, growth and survival.
Every Ring Tells Part of Lake Superior’s Story
The rings inside a fish scale or otolith are more than a biological curiosity. They are a record of survival through changing seasons, food conditions, migrations and environmental pressures.
When scientists combine age estimates from hundreds or thousands of fish, individual growth rings become a broader history of the lake itself.
That history helps fisheries managers understand what has happened beneath Lake Superior’s surface—and make better-informed decisions about how its remarkable fish populations can be protected for the future.
Sources and Further Reading
- Determining the Age of Fish — Michigan State University Extension
- Lake Superior Fish Age Assessment Workshop Report — Great Lakes Fishery Commission
- Longevity of Lake Superior Lake Trout — U.S. Geological Survey
- Comparison of Scale and Otolith Ages for Deepwater Lake Trout — USGS Publications Warehouse
