Estimating fish age
Fish age helps assess sustainability
Identifying how old fish are is fundamental to knowing how fast they grow, how old they are when they reproduce and how long they live, so we can estimate the number of fish in different age groups and how this changes over time. This helps us assess the sustainability of fish stocks.
To estimate fish, fisheries scientists examine the otoliths (ear bones) and count the number of opaque bands – like growth rings in a tree.
Using ear bones to estimate age
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 (see image below). 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).
Otoliths can be interpreted whole or they may need to be sectioned by, cutting a thin slice from the otolith through the core (see image below). Sectioning otoliths enables a clearer view of the banding patterns in some species.
Otoliths are interpreted by counting the number of opaque bands between the core and the edge, and measuring the otolith margin, which is the distance between the last opaque band and the otolith edge (see image below). The width of the otolith margin tells us when the last opaque band was deposited.
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.
How old is this fish?
The otolith section from a snapper (image below) 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 below) shows 2 opaque bands and is estimated to be 2 years old.
Email FisheriesMonitoring@daf.qld.gov.auor call 13 25 23