Estimating fish age

Identifying how old fish are is fundamental to:

  • knowing how fast they grow
  • how old they are when they reproduce
  • how long they live.

We use this data to estimate:

  • the number of fish in different age groups
  • how this changes over time.

This helps us assess the sustainability of fish stocks.

Otoliths are ear bones

Otoliths (ear bones) help fish orientate themselves and maintain balance, acting like our middle ear. They are composed of a form of calcium carbonate and protein, which is deposited at different rates throughout a fish's life. This process leaves bands (alternating opaque and translucent bands) like the growth rings in a tree.

The otoliths are located within the skull behind the eye and directly below the brain. Otoliths come in different sizes and shapes depending on the species of fish. They can be slender and fragile (e.g. mackerels and cobia), large and chunky (e.g. barramundi and snapper) or symmetrical in shape (e.g. sand whiting).

How we use ear bones to estimate age

To estimate fish age, fisheries scientists examine the otoliths (ear bones) and count the number of opaque bands — like growth rings in a tree.

This information, plus the date the fish was caught, the birth date (middle of the species' spawning period) and the period that opaque bands were deposited, are used to estimate the age of the fish.

Counting opaque bands

Otoliths are interpreted by counting the number of opaque bands between the core and the edge, and measuring the otolith margin (the distance between the last opaque band and the otolith edge).

The width of the otolith margin tells us when the last opaque band was deposited.

Using whole or sectioned ear bones

Otoliths can be interpreted whole or they may need to be sectioned by cutting a thin slice from the otolith through the core.

Sectioning otoliths enables a clearer view of the banding patterns in some species.

Sectioned and whole otoliths (ear bones) from a sea mullet showing orientation, increments and edge.
Figure 2A: Whole sea mullet (Mugil cephalus) otoliths. B) An otolith section from a sea mullet viewed using reflected light. Red arrows indicate annual opaque bands.
© Queensland Government
Sectioned and whole otoliths (ear bones) from a sea mullet showing orientation, increments and edge.
Figure 2A: Whole sea mullet (Mugil cephalus) otoliths. B) An otolith section from a sea mullet viewed using reflected light. Red arrows indicate annual opaque bands.
© Queensland Government
Magnified otoliths of 3 species of fish demonstrating the variability in shape between species
Figure 1. A. Whole barramundi (Lates calcarifer) otolith; and B. Whole sand whiting (Sillago cilata) otolith
© Queensland Government

How old is this fish?

The otolith section from a snapper (image A) shows 6 opaque bands with the last band appearing on the otoliths edge — this fish is estimated to be 6 years old.

The Spanish mackerel otolith (image B) shows 2 opaque bands and is estimated to be 2 years old.

Red arrows indicate annual opaque bands.

Otoliths (ear bones) from a snapper (sectioned) and Spanish mackerel (whole)
Figure 3A: Otolith section from a snapper; and B. The posterior end of a whole Spanish mackerel otolith. Red arrows indicate annual opaque bands.
© Queensland Government

Last updated: 10 Jul 2024