Age and Growth Program

Commonly Asked Questions About Age Determination of Fish

Questions

Why is it important to age fish?
How do you age a fish?
Is ageing an otolith like counting tree rings?
Is there special equipment used to "read" otoliths?
What are the maximum ages of groundfish aged by the Age and Growth Program?
How many otoliths have been collected by AFSC?
What species are aged by AFSC Age and Growth Program?
How many otoliths are read per year?
What is done for ageing quality control?

Question: Why is it important to age fish? Answer: Knowing the age of fishes allows fisheries managers to understand the dynamics of fish stocks and how fish populations react to environmental stresses like commercial/sport fishing, natural mortality, and predation. Basically, it is a way to monitor and better manage stocks for long-term benefits. What if we catch only young fish? What if we also catch old fish? Back to top

Question: How do you age a fish? Answer: The fishery biologist examines a structure of choice, which could be a scale, otolith, fin ray, dorsal spine, or vertebra. Generally the best structure to use is an otolith. An otolith is a bone located in a fish's skull and is analogous to the human earbone. Vertebrae are often used for sharks and skates which do not have otoliths. Scales are seldom used for ageing groundfish because they tend to underage.

From top: walleye pollock otolith, big skate vertebra, and Pacific cod scale.

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Question:  Is ageing an otolith like counting tree rings?

Answer: It is similar to counting tree rings, because the otolith also forms annual marks. However, the annual marks are not always clearly visible on the entire otolith.  Environmental factors such as water temperature, food supply, salinity, water chemistry, and population density can cause stress on the fish, and marks other than those that are annual can form on the otolith. This can cause a complicated pattern that the biologist must interpret. Try your hand at ageing with the interactive Age Reading Demonstration (ARD).


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Question:  Is there special equipment used to "read" otoliths?

Answer: Yes, the basic equipment needed is as follows: dissecting microscope($1200-$8000), low speed saw($4000), diamond blade($120), alcohol burner($5), lighter($1), tweezers($12), teasing needle($7), petri dish($2), clay($1), mineral oil($1), dish soap($1), and paintbrush($1).






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Question:  What are the maximum ages of groundfish aged by the Age and Growth Program?

Answer:  They can be over 100 years old!

For a list of maximum ages for all species click here


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Question:  How many otoliths have been collected by AFSC?

Answer:  So far the AFSC has collected over 1.1 million fish otoliths for ageing. The AFSC otolith database is a real-time inventory of survey and observer otolith collections.


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Question:  What species are aged by AFSC Age and Growth Program?

Answer:  The Age and Growth Program reads otoliths from groundfish species collected in the Bering Sea, Aleutian Islands, and Gulf of Alaska. Click here for a summary table of the species aged.


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Question:  How many otoliths are read per year?

Answer:  Depending on the requests that are made and the availability of staff, our lab has aged anywhere from 25,000 to 36,000 specimens per year. Our production numbers are available from 1990 to the present time.


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Question:  What is done for ageing quality control?

Answer:  What is done for ageing quality control? Typically 20% of our ages are blind read by a second age reader. For a detailed descriptions of our quality control methods review the paper by Kimura and Anderl (2005) Quality control of age data at the Alaska Fisheries Science Center.


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