Estimated fish age

Figure 1. A. Whole barramundi (Lates calcarifer) otolith; and B. Whole sand whiting (Sillago cilata) otolith
© Queensland Government

To estimate the age of fish, scientists interpret the growth structures of otoliths and count annual growth bands. Knowing the age of fish helps to assess the status of fish stocks.

Using otoliths to estimate age

Otoliths (ear bones) help fish orientate themselves and maintain balance, acting like our middle ear. Otoliths are composed of a form of calcium carbonate and protein which is laid down at different rates throughout a fish's life. This process leaves bands (alternating opaque and translucent bands) on the otolith 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 (see figures 1A and 1B). Otoliths 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).

Otoliths can either be interpreted whole or they may need to be sectioned, in which case a thin slice is cut from the otolith through the core (see sectioning axis in Figure 2A). Sectioning otoliths enables a clearer view of the banding patterns in some species.

How old is the fish?

Otoliths are interpreted (i.e. they are read) by counting the number of opaque bands between the core and the otolith edge and classifying the otolith margin (the distance between the last opaque band and the otolith edge) (Figure 2B). The width of the otolith margin tells us how long since the fish deposited the last opaque band.

This information is considered along with the date the fish was caught, the "birth date" for the species, and the period during the year when opaque bands are deposited on the otolith, to estimate an age for the fish.

Note: The "birth date" assigned for a species is usually the middle of the spawning period.

Example: The otolith section from a snapper in Figure 3A shows six opaque bands with the last band appearing on the otoliths edge, and it is estimated that this fish was six years old. The Spanish mackerel otolith seen in Figure 3B shows two opaque bands and was estimated at two years old.

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
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

How are age data used?

Being able to work out how old fish are is fundamental to knowing how fast they grow, how old they are when they reproduce and how long they live.

Knowing the relative abundance of fish in different age groups within the population and how this abundance changes over time is very important for assessing the status of that population and the sustainability of the fishery for that species.