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RFF - How Old is That Fish?
By Robbie Wells
Have you ever wondered how old a fish that you just caught is or how they come up with growth information for each species, for example age for length or weight and growth rate, reproduction age, how long they live for and so forth?
We've often seen social media Fisheries pages wanting frames of certain species for research and it's all part of collating enough information to piece the species together, which in turn helps us in the sustainability of the species and the environmental issues that might arise that could be detrimental to them.
Scientists generally age fish in two ways, the ear bones (Otoliths) or by reading the scales of the fish. The latter has been around since the 1890's and has its flaws in terms of accuracy, especially in older fish as they are often under aged. As the fish gets older their growth rate slows and from fingerling to maturity the amount of scales stays constant, so they don't have more scales when they grow the scale count remains the same. Scale growth is proportional to body growth and as the scales grow, the ridges (circuli) are formed on the edges. As the fish gets older and growth rate slows they are closer together giving the age inaccuracy.
By far the most accurate is the Otoliths similar to counting the rings when aging a tree.
These ear bones (otoliths) act like our middle ear, helping with balance and orientation. Otoliths grow larger over time as calcium carbonate and protein are laid down at different rates throughout the life of a fish, forming alternating opaque and translucent bands, much like the rings we count when aging a tree.
Located below the brain and behind the eyes of the fish, otoliths come in different sizes and shapes across different species. Otoliths are sometimes interpreted whole or they may be sliced through the core to provide a clearer view of the banding.
You can find more information and images on fish aging on the DAF Queensland website at - https://www.daf.qld.gov.au/business-priorities/fisheries/monitoring-compliance/monitoring-reporting/commercial-fisheries/species-specific/estimated-fish-age
Cheers,
Robbie