A structure that grows with the fish
Inside the heads of bony fish are calcium-carbonate structures called otoliths. They contribute to hearing and balance and accumulate material as the fish grows. NOAA Fisheries uses their growth layers to estimate age. Seasonal changes can produce paired opaque and translucent zones that form an annual increment.
The tree-ring comparison is helpful, but it is not a licence to count every visible line as a birthday. Methods must suit the species and the structure being examined.
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Agreement is one check; known age is another
NOAA distinguishes accuracy checks against known-age reference material from precision checks in which a reader or a second reader examines samples again. If two readers agree, the estimate is repeatable. The reference material asks whether that agreement is also correct. Both matter when ages become inputs to population models.
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The wider pattern can help
NOAA's otolith-chronology work also applies crossdating: matching conspicuously wide or narrow increments among fish. A shared sequence helps detect missed increments or extra checks and place growth into calendar years. A regional chronology can then support questions about growth and environmental variation.
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What to retain with an age claim
An age record is easier to reuse when it retains the species, collection place and date, preparation method, reading protocol, reader agreement and validation source. An image labelled 'old fish' loses most of that value. The same discipline applies to fish remains recovered from a settlement: identify what was measured before using it to infer fishing pressure, climate or everyday diet.