fish scales close up

Fish ageing: A powerful tool for fisheries!

Have you ever wondered how old your last catch was? Or perhaps how long different species can survive for, given the right conditions? This month, we have a special guest blog post from Environment Agency staff member Conor McCormick, to explain more on the fascinating topic of fish ageing.

Fish ageing is one of the most important tools for understanding the performance of our fish populations. Often seen as a dark art, a quick look at the scale of a fish reveals a fascinating insight into its biology, behaviour and life history, and helps fishery managers understand exactly what’s going on beneath the surface.

Scale ageing is a rod licence funded service provided by the National Fisheries Laboratory (NFL) in Brampton and is a key monitoring tool that helps us understand fish stocks and the health of our fisheries.

A scale sample can tell us not only how old a fish is, but much about its lifestyle and environment!

The scale tells the story

As fish grow, their scales grow with them, laying down marks a bit like rings on a tree. At the NFL, experts in fish ageing can read these marks opening up a whole new world about each and every fish within the population. Taking scales from fish is harmless and lost scales will regrow allowing fish ageing to be applied to fisheries of all types and sizes, and to the great majority of species from salmon to carp and tench to tarpon.

If you removed a scale sample from a fish and observed it under a microscope you would see numerous dark and light bands radiating out from the centre, or focus, of the scale. These dark bands are termed ‘circuli’ and it is the spacing between these circuli that we use to determine the age of a fish.

During winter the spacing between the circuli is less due to slower growth of the fish, whereas in summer, the circuli are spaced further apart as the fish feeds and grows more quickly. By counting the winter bands of reduced growth, we can age the fish. Through further analysis of the circuli, we can also evaluate the growth of the fish throughout its life and pinpoint landmark events such as when it was stocked, spawning events, and in some cases, even where it has been to feed.

Fish of all ages

Fish naturally lose scales during their life and with them goes the ability to read its life history. However, most scales remain in place and if we look at a scale that has been present on the fish its whole life we can assess how old it is. 

As the years rack up it can become harder to accurately age a fish, but that’s where the skill and experience of the scientist comes in. Interestingly, fish size is not always a reliable indicator of age. We have aged 3lb perch at just 3 years old, and chub of 6lbs to both 11 years and 17 years.

how to age a fish

Differences between wild and stocked fish

Fish in a fish farm are actively fed and often live in a slightly warmer and more stable environment – this enhances growth rates. When they are stocked into a fishery, growth rates tend to slow to a more natural rate and we see that in the scale as a clear change. As fish are moved from the hatching facility into growing-on ponds, the change in environment can lead to a disruption in the circuli pattern, known as a false check.

how long do trout live

By spotting these false checks and changes in growth it is possible to track stocked fish, tell how well they are doing in their natural environment and whether stocking has been successful. It also removes the need for more costly or invasive tracking options like dyes and tags. We have worked on a number of projects to confirm the success of stocking and even aged record-nudging fish that have been both farmed and spawned naturally.

Competition and fishery performance

Fish ageing across populations can reveal competition between species and provide an indicator of how well a fishery is performing. Essentially, the growth rates we’re looking at can tell us how much food is available to fish.

If fish are eating well, in healthy environments, they can grow well too – we have developed growth rate baselines that we use to compare the growth rate of fish populations across the country. If the population is slow growing or stunted it can suggest there isn’t enough food available, or something else is acting as a pressure on the fishery.

how long do carp live fish lifespan
Ageing fish of different types and sizes can tell us a lot about the health of a fishery

This can be an early warning sign that a closer look or management measures are needed. For example, this might suggest that creating more foraging habitat would enhance the fishery, or may suggest that removing some fish would actually help fishery performance.

Knowing this information can help inform good fishery management, particularly in stillwaters – removing some of the silver fish species from a carp water can increase specimen carp growth and the silvers that have been removed can aid another water – we call this cropping or stock manipulation. This can often be cheaper and carries less risk than stocking more fish.

Identifying stressors within fisheries

A stressed fish doesn’t tend to eat or grow as quickly, and if that period is prolonged we can spot it when looking at a scale sample. Extremes in weather both cold and warm will definitely show this – as will disease such as heavy parasite infections or poor water quality. Some natural stressors can also appear on scales, like spawning events.

Knowing the impact of a stressful event can answer other fisheries management questions or back up theories from other investigatory work – we can even see how many times a fish has spawned in its life.

Life histories of migratory species

Some species have evolved the ability to live in multiple environments during their lifespan. Sea trout and salmon are both examples of anadromous fish which start life in freshwater, before migrating to sea to take advantage of the plentiful marine food supply, and finally returning to their natal river to spawn.

Due to the marked difference in the growth of these fish between the freshwater and marine environments, we can identify exactly when the fish migrated to sea by analysing the spacing between the growth rings on their scales. In this way scales can reveal huge amounts of information about these fish; how long they spent in freshwater before migrating, how long they spent at sea before returning, whether they have previously spawned, and how many times.

The scale below is from a sea trout that spent three years in freshwater before smolting, followed by a full year at sea, before returning to spawn. After spawning, it returned to sea for a short period before returning to the river to spawn again. It repeated this journey on two further occasions and was likely returning to spawn a fourth time when it was caught. Fish ecologists record this information in note form, so this fish would been known as a 3.1+3SM+. As well as being very interesting, this information is extremely important for understanding the structure of wild salmonid populations and factors such as marine survival.

sea trout age migration studies

Notable catches – a scale can tell a huge amount

We are regularly sent scales from anglers and fisheries from notable and record fish, unusual species or to support catch data. We also work with groups to improve understanding of particular species, either in specific rivers or nationally.

For example, we receive annual submissions of scales from specialist angling groups and societies through which we analyse scales from various species captured by anglers from around England. We have also received scales from particularly large and record breaking fish.

Through the years we have received scales of a 26lb 6oz brown trout from Eyebrook Reservoir which was over 11 years old and a record-breaking 100+ lb carp from France which was only 16 years old!

carp scale how to tell age of fish
brown trout scale sample

Stillwater ageing surveys: helping inform best management

Good fishery management is key to the performance of stillwater fisheries and the health of the fish they contain. However good management decisions depend on an understanding of the conditions in a fishery and the influence these have on fish. Stocking is often seen as a quick-fix solution to an underperforming fishery. However in the case of an overpopulated water, the opposite is often true and cropping a water may be the best way to increase performance.

An ageing survey can reveal valuable insights about a fishery, providing information on growth, recruitment and population structure; all vital when making informed management decisions. At the NFL, we provide stillwater fish ageing kits for fisheries and angling clubs to gain a better understanding of how fish are performing within their waters.

fish scale ageing kit environment agency

The kit is free and contains everything needed to sample the fish, including a measuring tape, tweezers, scale packets, a step by step guide and even a prepaid envelope to return the scales to us. Once we receive the scales, we will then age the samples and provide a report of our findings. This service is there to help identify problems before they escalate and inform best management practise based on sound scientific information.

So much more than just an age:

In addition to ageing, scales can also provide a wealth of additional information on fish and fisheries using specialised analytical techniques. At the NFL, we always archive scales from across the country and have used these to support many different studies.

What type of food fish have been eating…

Using a technique called stable isotope analysis we can determine the diet of fish and even reveal their trophic position i.e. where they sit in the food chain. In this way we can tell what food they are eating and even if they’ve been munching on anglers baits like pellets, or targeting natural items like invertebrates or vegetation.

We can use this to understand the interactions and competition between different species – called trophic interaction. This information can also be used to assess the impact that invasive fish species have on our native species and ecosystems.

… and where they’ve been

Stable isotope analysis can also reveal where fish are feeding. For example, scales of salmon have been used to identify feeding grounds in the marine environment, and by using microchemistry analysis it’s even possible to distinguish between wild salmon and those of a farmed origin.

Fish identification and genetics

Scales can also be used to identify the genetic characteristics of fish, including their identity and variation in strains. Although scales may often look the same, an experienced fish ager can identify the species of fish simply from the scale. Scales can also be analysed using molecular techniques to confirm exotic or invasive species. This can be a powerful tool to help protect our fisheries from harmful species.

Frequently asked questions

How are scales taken from fish? Scales are taken from a specific area on the flank of fish using a tweezers. Multiple scales are taken to ensure that at least one scale has been on the fish for its entire life – replacement scales grow a lot quicker than original scales and do not hold the growth records required to age fish. The process is quick and straightforward, however when taking scales from live fish, it is always important to prioritise the welfare of the animal. Care should always be taken and the time the fish spends out of the water should be minimised to reduce stress. If in doubt when attempting to remove scales, don’t do it and seek advice from someone with experience.

Do we age all species – We can age any species of fish, but some species are used more than others for assessing the health of our fisheries. Generally, we don’t tend to age very short lived species like gudgeon and bleak which only live 3 to 4 years. Many others are routinely aged however, and some longer living species like barbel and chub provide lots of information.

Can we age fish in other ways? Not all fish have scales and in some species (e.g. eels) they are incredibly small making scale ageing very difficult. Other hard structures can also be used to age fish such as otoliths (ear bones; pictured above) and fin rays. Sampling these structures requires significantly more preparatory work than scales as they need to be cut into very thin sections using specialised tools. Additionally, sampling these structures is destructive meaning that the animal will not survive the process, and therefore such techniques are usually only used on fish that have already died.

What fish are the oldest? Barbel and carp tend to be the oldest fish and can live beyond 25 years and 50 years respectively. Eels can live even longer although we rarely age really old eels.

How long do fish live eel
Eels are one of the longest lived fish

Can I tell the age of a fish with my naked eye? You can sometimes distinguish original scales from those that fish have replaced with the naked eye, but detailed ageing requires observations of microscopic changes and patterns.We use specific specialised microscopes to look at scales usually at resolutions of x10 magnification to x30 magnification.

Where do you take the scales from? – The best area to take scales can vary between species however it is usually on the upper flank (above the lateral line), in the area just behind the dorsal fin.Scales from this area tend to be clearer and better specimens for reading, and this is also an area where they are least likely to have suffered damage or lost scales previously.

Do scales easily grow back? Yes, if a scale is lost or removed from a fish, they can rapidly regenerate a replacement. Mucus naturally produced by a fish will help prevent against infection until a new scale is formed.

Is the stillwater ageing service available to me or my fishery? Our fish ageing service is rod licence funded and free to fisheries needing help. If you are interested in knowing more about fish ageing, or know a fishery that needs help, management advice or assistance, please get in touch with your local fisheries officer. Click here for our “contact us” page.

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