How Can You Tell How Old a Fish Is?

Fish have various methods for tracking growth patterns on their scales, fin spines and ear bones (known as otoliths). One way of telling how old a fish is is by counting its annuli (dark lines on scales).

Examining one of a fish’s scales and counting the annuli (dark rings that space out over time). Another way is examining bones or fin rays after it has died; unfortunately this method only works when fish have passed on.

Table of Contents


Scales on a fish’s body are an integral component of its anatomy and can help determine its age. Scientists count annuli, or annual growth zones in its scales, to estimate its age; these rings act similar to tree rings in showing when growth rate has accelerated or decelerated than usual compared with other years. According to Northeast Fisheries Science Center research, fish grow faster during summer than winter months as evidenced in its scales’ patterns; wider summer zones indicate young fish while narrower winter zones indicate older ones.

There are various methods available to scientists when it comes to estimating a fish’s age, including counting fin spines and opercula, but scientists prefer scales as they require no special preparation before being observed. Furthermore, this method works across many fish species unlike structures like otoliths or fin rays which may vary according to type.

Scales provide an effective means of measuring aging because they record growth increments that decrease with age, providing biologists with a precise way of ascertaining an age from these increments. A biologist may then compare their numbers against known-age reference collections to assess an individual fish.

Scale-ringing methods tend to be accurate; however, they do have their limitations. Otoliths (ear bones) in certain species of fish may be difficult to see due to being translucent or possessing complex microstructures; furthermore, their shapes and colors may change as the fish matures.

Other structures may also be used to estimate a fish’s age, but for accurate assessment, boney structures like otoliths and bony fin rays provide the most reliable methods. Furthermore, collection can occur without harming the fish, with results quickly available for analysis.

Knowing the age of fishes is vitally important for various reasons: from assessing population sizes to understanding mortality rates caused by fishing and natural causes. Furthermore, knowledge of an individual fish’s age allows researchers to gauge growth rates and spawning frequencies more effectively.


Similar to arborists who count tree rings for age estimation, fisheries scientists utilize the hard structures found within a fish’s body – such as scales, fin rays, bones or an otolith (or “ear stones”) – to read its growth patterns and estimate its age. Scales, fin rays and bones may all serve this purpose but an otolith (or “ear stones”) remains the go-to structure.

Otoliths, calcium carbonate structures found within the heads of bony fish (except sharks and rays which do not possess them ) that serve to assist with balance and hearing, aid balance perception, as well as provide annual growth increment indicators, known as annuli, that help estimate age estimation. They typically form patterns of opaque and translucent areas known as annuli that correspond with annual growth increment. By measuring spacing between annuli otoliths can provide insight into age estimation.

Otoliths of most fish contain rings that are very close together, necessitating close examination under a microscope at 6-40 times magnification in order to see them clearly. Some may be examined whole while others must be sectioned for better examination of the rings. Furthermore, keeping it moist ensures that its rings do not dry out prematurely.

Otolith rings can be validated by comparing their size and pattern with that of an established population over time. In one such study involving snapper, fish were all tagged, measured, injected with oxytetracycline injections which leave an easily visible layer on sectioned otoliths – recaptured fish showed identical zones as those from initial sample, showing that the otoliths accurately recorded annual growth rings.

This method of ageing fishes with variable otoliths is non-destructive and allows for measurement of growth, spawning events, and other life history traits that would otherwise be impossible to assess. With appropriate equipment it may even be possible to obtain an approximate age estimate with just one scale scale; though this requires practice.


As with counting tree rings to ascertain an age of an organism, fish scales and bones can tell an equally compelling tale. That’s what the six staff members at WDFW’s Fish Aging Lab do each year when collecting data on salmon, rockfish, ling cod and other marine fishes at our fish ageing lab for salmonids such as rockfish or rock cod for stock assessments or future fisheries management strategies. Each year our lab processes 60-80,000 Pacific salmon scales and 10-20K Otoliths from Pacific fish to maintain data on age/life histories which helps guide future fisheries planning/assessments/assessments/stock assessments/guide future fisheries/stock assessments/assessments/stock assessments/guide future fisheries management decisions/stock assessments/stock assessments/fisheries management strategies/assessments/stock assessments/stock assessments/guide future fisheries management strategies/guide future fisheries management decisions as well as life history/age /life histories/stock assessments. This lab processes 60-80,000 Pacific salmon scales annually as part of this task in this task of collecting data to maintain data on age/life history so we can maintain data to help inform future fisheries/stock assessments/incorpusa/ / assessments when necessary//fishery assessments etc.

Under a microscope, compact bone may seem solid all the way through, yet its surface hides tiny passages for blood vessels and nerves arranged in rings around channels called canals. The canals are lined with special cells called osteocytes that form osteons – thick tubes with blood vessels and nerves running along one direction like bundles of straws containing blood vessels and nerves – which form its structure.

By drilling up to 60 microscopic holes into an otolith and chemically analyzing its powder, scientists can uncover chemical clues that reveal a timeline of fish life. When combined with mineral profiles from locations, this timeline shows where and when each fish lived – like playing detective.

Fin rays

There are various methods of estimating fish age, but one of the easiest and most accurate ways is through examination of bones, scales and otoliths. These structures contain annual growth rings similar to tree rings that can be counted to estimate an approximate fish’s age. Fin rays may also be useful when removed easily for examination and processing; their count can provide insight into an individual fish’s age.

However, some calcified structures are more challenging than others to interpret; lake whitefish otolith rings can be particularly challenging even for highly experienced age readers to interpret. Furthermore, in order to age fish without killing them it’s essential that nonlethal structures such as scales or fin rays be assessed regularly to ensure they provide accurate age estimations and remain reliable over time.

Researchers performed experiments to address these challenges by comparing precision, bias and reader uncertainty of otoliths, fin rays and pectoral/dorsal fin spines from Cycleptus elongatus (formerly known as Blue Sucker). Otoliths produced more accurate age estimates with lower CV values compared to both scales and fin rays; additionally they had the lowest median reading error at only 0.5 years for age determination.

Scales and fin rays have long been used to estimate fish age; this study marked the first attempt to see whether these nonlethal structures could reliably estimate it. Results revealed that accuracy and precision for aging these structures was comparable with whole otoliths. Furthermore, less experienced readers estimated age with higher bias and precision; suggesting experience levels have an impactful role to play when reading age estimates of this kind.

Ageing fish is integral to both population structure and stock health management goals, as individual growth rates vary between individuals. Knowing the age of individual fish allows harvests to be adjusted appropriately; plus it helps managers keep an eye on fisheries, including protecting endangered species.