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Fish Age Determination

How Are Fish Aged?

Sectioned otolith from a 5-year-old haddock

See our PDF flyer on Fish Age and Growth (200 kb)

The scales, otoliths (earbones), and other bony parts of fish often contain a record of seasonal growth patterns. This is because fish grow faster in the summer than in winter, due to higher temperatures and increased food consumption. Patterns in the scales and otoliths reflect this change: summer zones are wide, and winter zones are narrow. Fish age, in years, can therefore be determined by counting the number of paired summer/winter growth zones. One pair of such zones is called an annulus (plural, annuli). Thus, the process of ageing fish is similar to finding the age of a tree by counting rings in the wood.

For different species of fish, different structures are used to determine ages. In general, annuli from these various structures are visible under a low-power microscope. Scales are used for some species. On these structures, winter zones appear as regions where the ridges on the scale are closer together. Scales are easy to sample and can be impressed in plastic for long-term storage. If scales canít be used, otoliths (located inside the skull) may be used instead. In otoliths, the color of the zones differs, resulting in alternating opaque and translucent bands. Some otoliths are clear enough for these zones to be seen from the surface of the otolith and may be aged whole; otherwise, the otolith must be thinly sliced in order to see the zones. Other structures used for ageing include vertebrae or fin rays.

In 1988, the Fishery Biology Program developed and published a manual with detailed descriptions of the age determination methods we routinely use (Penttila & Dery 1988). Some of these methods have changed over the years, and current descriptions of all of our methods (with high-quality images) are reprinted in our Ageing Manual. These descriptions are written for use by fishery biologists and professional age readers and students, but they may be of general interest as well. See also this list of the species we age, along with which structures and preparation methods are used.

The choice of age structure for a species depends on which structure most accurately reflects the true age of the fish, has the clearest zones, and has the most cost-effective preparation method. For each species, a validation study is conducted to demonstrate that the chosen structure yields accurate ages. This entails determining the position of the first annulus and proving that the zones are laid down exactly once per year. Common validation techniques include tag/recapture studies, including marking with oxytetracycline or other chemicals, back-calculation, marginal increment analysis, and radiometric/isotope analysis.

A final aspect of fish ageing is the need to maintain consistency. Within the Fishery Biology Program, each age readerís level of precision (that is, how often they will assign the same age twice for a given fish) is regularly tested. A reader will be assigned about 100 fish to re-examine, and the test ages will be compared against the original ages. Accuracy (an age readerís frequency of assigning a fish its true age) is tested for the species for which we have samples for which the ages are reliably known. Regular exchanges with other ageing laboratories also ensure that our ages remain consistent from year to year. The results of all these tests for each species are posted, as well as explanations of the statistics used.

Why Age Fish?

Once the ages are known for a sample of fish, scientists can measure the rates of various processes affecting these fish. For instance, data on fish size can be combined with age information to provide growth rates. Also, the decrease in abundance from one year (age) to the next gives a measure of mortality rates (due to the combination of fishing and natural causes). Finally, age data can be used to determine how long it takes individuals of a species to mature. Any of these vital rates may change over time, so it is important to examine age samples regularly.

Knowledge of fish age also allows scientists to learn more from capturing and measuring fewer fish. It is impossible to catch all the fish in the ocean. However, if a small portion of the fish are captured and aged, the relative abundance of fish at each age can be determined. These age data, with data from other sources, can then be expanded to estimate the total number of fish in the wild. Population models, using such data, enable scientists to monitor trends in the size of fish populations and to predict potential effects of fishing on those populations. The most detailed models include age-specific estimates of weight, mortality, and growth; this requires that larger numbers of fish be aged.

How Are the Data Used?

Our data are used in modeling the populations of various marine species off the northeast coast of the United States. These models estimate the biomass and numbers at each age, and allow assessment scientists to examine the potential effects of continuing removals from those populations. Within the NEFSC, the Resource Evaluation and Assessment Division conducts these population models for many fish species, incorporating data from the Fishery Biology Program. Results of these stock assessemnts can be found on NOAA's FishWatch website.

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(File Modified May. 24 2017)