How To Know The Age Of A Fish? Using The Science Of Calcified Structures

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I have often wondered how old the fish I have caught is, and it is a fascinating thing to know. 

But how can you tell? Is it size or length? Can you work it out on the water while you are fishing? 

These are all questions we anglers would like to have the answer to. However, it is, unfortunately, a lot more complicated than that.

There are a lot of different factors that determine the length and weight of a fish, which makes it very hard to use these metrics to judge a fish’s age. But there is a tried and trusted way. 

Join me as I run through how the age of a fish is worked out and provide you with resources so you can try to find the age of the fish you catch.

The Fish Ageing Process

When I ran a bonefishing lodge in The Bahamas, I always spent part of my day on the flats chasing them, and I ended up catching a trophy 10 lb bonefish on one of the days. 

I was curious to know how old it was. Luckily I worked alongside the Bonefish & Tarpon Trust, who have been studying the bonefish of The Bahamas for years, and when I asked them, they said the fish was about 30 years old. 

Considering I was only 28 years old then, I was blown away that I had caught a fish far older than me. It’s pretty cool! But I had more questions to ask. Are all 10 lb bonefish 30 years old? The answer was no. 

Bonefish in The Bahamas grow slower than bonefish in the Florida Keys, so a 10 lb Keys bonefish would be younger than a 10 lb Bahamas bonefish. So, how did they work this all out? 

Which Parts Of Fish Are Used To Find Its Age?

The hard parts of a fish, called calcified structures, are used to establish its age. These are the scales, bones, and hard rays of fins. The patterns found in these structures record the seasonal growth, which can then be measured to determine the age of the fish.

It Is All In The Scales 

The most common way of figuring out how old a fish is is by taking samples of their scales. The scales are collected and analyzed with length, weight, date of capture, and location. 

The fish scales are viewed under a microfiche reader, revealing rings, just like the ones you would see when you cut a tree and determine its age. 

These rings are called circulus and are made whenever the scale grows, which is when the fish grows. During warmer periods, new circuli form with wider gaps as fish grow much faster. 

But, in cold periods, new circuli grow with smaller gaps between them since the growth rate of fish is much slower when it is colder. When a few circuli are very close together, they form a thick ring called annuli. 

The annuli is a mark of the annual time of year when the fish’s growth rate is slow. This makes it a year marker (hence the name), and you can work out how old the fish is by counting the annuli on a scale. 

However, it is not a foolproof method as many things apart from seasonality can affect growth rate. For example, a fish could get sick or be in shock, creating the same annuli rings in the scales. 

Taking scales is an excellent way to age ID a fish, as it doesn’t require killing the fish, and they can be released safely back into the wild. But, not all fish scales are big enough for this process to work, and in these cases, bones are used. 

Read more information on fish scales and slime and understand why they are crucial for fish.

Bones Can Also Show A Fish’s Age 

Some fish, such as catfish, don’t have scales, and some are too small for the scale aging process, like those found on eels and trout. So, how do you work out how old these fish are? The answer is by using their bones! 

Generally speaking, the otoliths (ear bones) are the best fish bones for aging, but vertebrae and spines are also often used. 

The bones also form similar rings to the circuli and annuli ring as the scales, but a lighter ring forms in the warmer seasons, and a darker ring is apparent in the colder seasons. 

The bones are first softened, then a very thin slice is taken and soaked in chemicals to make the rings easier to see. The sample is then put under a microfiche reader, and the dark rings are counted to determine the fish’s age, as one dark ring should equal one cold season.

Is Counting The Annuli An Accurate Method? 

As I already mentioned, annuli are created during periods of slow growth rate, which are assumed to be formed by a cold season. But, multiple other things can cause a reduced growth rate, such as disease, shock, and other environmental changes. 

Also, some fish have harder Annuli to read; for example, bass will have incomplete Circuli that form an Annuli rather than Circuli being close together. Plus, as fish get older, their growth rate slows down, and this can also make it increasingly difficult to read the Annuli correctly. 

Scales & Back Calculation

If you recorded the length of a fish when a scale sample was taken, you could calculate the length of the fish at every Annuli mark in the scale. For example, if you know the length and scales of 4-year-old bluegill, you can work out how long it was at 1, 2, and 3 years old! 

The reason this is possible is that the length of a fish is directionally proportional to the size of its scale. So by using a particular equation, scientists can work out the fish length by year and calculate the growth rate by distance. 

As An Angler, How Old Is The Fish I Caught? 

Now, you will not find anglers taking scale samples of their fish to determine their age under a special microscope at home. Nor will they kill them, remove their ear bones and analyze them! 

Luckily, fisheries scientists have been collecting data on the length vs. age of most fish species for many years. This means that if you have the total length of a fish, then through the power of the internet, chances are you can find out the fish’s age. 

But it does depend on where you caught your fish. 

For example, a brown trout caught in a river in Vermont with a length of 9 – 14 inches is about four years old. In comparison, a brown trout caught in a lake in Vermont with a length of 13 – 21 inches is the same age. Therefore, brown trout grow faster in lakes, as you can see. 

If you want to know the exact age of a fish you caught, you will have to hope some fishery studies are available in the area you caught it in. Resources such as the Alaska fisheries science center allow you to use a drop-down menu to select the species and provide a detailed otolith survey.  

If unavailable, you can utilize locations nearby with similar climates and habitats to calculate a reasonably accurate age. 

Ensure that you measure them carefully from the front of their mouth to the fork of their tail. Check out this Quik measure pro ruler to measure the length of your catch. It’s a stick-on ruler attached to your fishing rod, so it’s easy to use and won’t go missing from your tackle box.

Final Thoughts

Calculating the age of a fish is easier than it sounds. Utilizing scientific data from internet sources for a specific fish and its habitat can give an accurate age for any fish. For the angler, you will only need to measure its overall length. 

If you are a bass angler, check out my article about what is considered a good size largemouth bass, which explains in detail all you need to know about the weight and length of big bass in states across the US.  

Steve Fitzjohn