Age analysis of fish

By analyzing the age of fish in a given area, we can estimate the size of the stock and understand the distribution of fish across different size classes. The age composition provides valuable insights into the stock's health and its changes over time.

Determining fish age helps us gain knowledge about:

• Recruitment – The number of fish born each year.
• Size and growth – The size and growth rates of fish at different ages across various areas.
• Survival – Whether fish tend to die at a young or old age.
• Impact of climate and environmental changes – How the fish community is influenced by these factors.
• Effects of fishing methods – How changes in fishing techniques impact the fish community.
• Fish Environments and Growth Factors

Fish live in environments that vary in salinity, temperature, and food availability, all of which influence their growth and age. Some species live in both brackish and marine waters, or in both freshwater and brackish environments. For example, brackish water is found in the Baltic Sea. This variability means that the difficulty of age analysis can differ even for the same species, depending on the environment.

How We Determine the Age of Fish

To estimate the age of a fish, we examine its otoliths (ear bones), operculum (gill cover), scales, and other hard tissues, which contain annual rings similar to those in trees. We primarily use otoliths to determine the age of most species.

When analyzing otoliths, we first need to prepare them, and the method of preparation varies by species. Depending on the fish, we may analyze the otoliths whole (as with plaice, herring, trout, and char), burn and divide them (as with perch and whitefish), or embed them in plastic and slice them using a precision saw (as with eel, roach, and turbot). Once prepared, we use a microscope, stereomicroscope, or microfiche reader to determine the fish's age.

We can also measure fish growth by examining other tissues. For species like roach, whitefish, pike-perch, and herring, we use scales to assess growth. In perch, we use the gill covers for this purpose.

Our Age Laboratories

We perform age analyses for research purposes and as part of environmental monitoring on behalf of the Swedish Agency for Marine and Water Management, the County Administrative Boards, municipalities and fishing associations. We also perform age determinations and analyses on behalf of clients. Contact our age laboratories for information and pricing.

Age analysis with a magnifying glass

An 8-year-old whitefish otolith.

Eel otoliths	Three pairs of otoliths are found in the fish's head.
Scales are placed on a plastic mat for pressing.	Gill cover from a 3-year-old perch.

 

Perch, placement of otolith (beneath the brain) and gill cover.