How to tell the age of a fish
Fish ageing. It's off the scale! This article is brought to you by Gareth Davies from the Environment Agency and covers one of the many technical skills that a fisheries scientist might need to accomplish in their day job.
How old are you?
Did you know that fish can be aged, and that the way in which they grow can provide a valuable insight into their life history? This provides us with an important fisheries management tool, helping us to understand how well a fishery is performing and what measures we may want to take to sustain or improve this.
Which part of a fish can be used to work out their age?
The calcified (or hard) structures of a fish record seasonal growth patterns in the form of annuli (or rings) which can be counted to determine the age of the fish. The most commonly used structures are scales, otoliths (ear bones) and hard fin rays. From just a single scale you can not only determine the age of the fish, but also assess its growth, spawning events and other life-history traits.
How does the Environment Agency age fish?
The Environment Agency prefer to use the fish’s scales for ageing. This is because this is a non-destructive method, meaning that the fish isn’t killed in the process. Also, unlike the other structures, scales do not need any special preparation prior to being aged.
However, for some species, other methods have to be used. Examples include scaleless fish, such as the non-native Wels catfish, or those with micro-scales, such as the European eel. In these cases, we have to use other bony structures, such as their otoliths or fin rays. (Figure 2.)
These need to be carefully prepared by cutting very thin sections with specialised tools. These sections can then be viewed under a microscope and the annular bands (annuli) are counted, similar to the way you would age a tree.
Age analysis provides valuable information for fisheries, supporting management decisions and helping us to identify changes in fish populations over time. We also share material with universities and other institutes to further our understanding of both native and non-native fish species.
How can fish ageing aid fishery performance?
The Environment Agency’s National Fisheries Services (NFS) provides a bespoke and flexible fish ageing service to help improve the way fisheries are managed. Using information from reading scales, combined with background information on the fishery, NFS can offer advice on how to maximise the growth potential of fish. Fish ageing is the quickest and easiest way to assess fish growth and mortality rates. It also allows us to determine when fish mature and how well species are recruiting within a population. This is key information, essential for underpinning fishery management practices such as stocking or cropping. For example, fish ageing allows us to understand if fish are stunted (very small for their age) due to over competition for food and other resources, suggesting a water could benefit from removing some of the fish.
Fish ageing is also an effective way to show the success of mitigation measures, such as habitat improvements. We are able to do this by sampling before and after the improvement works have taken place. If there is an improvement in growth and recruitment after the work has taken place this can be attributed to the improvement in habitat.
Case study: An over-stocked fishery
A great example that demonstrated the effectiveness of fish ageing was a large stillwater fishery suffering from poor common bream catch returns. Using a seine net, the water was surveyed by the Environment Agency to see what species of fish and how many were present within the fishery. This revealed that despite poor fishery performance and low capture rates, the fishery had a high stock density.
At this stage scales were collected from multiple fish species and sent to NFS for analysis. Our work revealed that the common bream were stunted, most likely as a result of the limited resources available from overstocking, leading to the fish being several years older than you might expect for fish of their size.
This is shown through the two scales in Figure 3, which are from two common bream both 102mm long.
The one on the right is the correct or expected age of one year old, and the one of the left is from the overstocked fishery and is three years old. As a result of Environment Agency investigations, this particular fishery was cropped and the fishery’s performance began to improve shortly after. This shows the importance of good fishery management and assessing your fishery before taking action. In this case it would have been easy to assume that more bream were required in the fishery due to poor catch returns. However, as can be seen from this example, this isn’t always the best course of action and could even cause fishery performance to reduce even further.
For any fish ageing queries, email the National Fisheries Services’ Fish Ageing Team. For all others, please contact the Environment Agency on 0800 80 70 60.
Last date edited: 5 January 2021