Fish Age Determination

Fish age is a critical input to stock assessments. Growth, reproduction, natural morality and longevity are all informed by age determinations. These key indicators reflect how resilient a species is to changes in population size brought about by fishing or other factors.

Scientists can determine the age of a fish using small crystalline structures on either side of the brain (otoliths or ear bones). Otoliths function rather like the middle ear of humans in maintaining balance, detecting acceleration, and detecting sound. They are made up of a complex of protein and calcium carbonate and grow seasonally, leaving alternating opaque and translucent bands in a cross section of the otolith. Like a tree, the age of a fish can be determined by counting the rings in the otoliths.

In the Southern Ocean, age is required information to assess toothfish population status, bycatch productivity and risk assessment, icefish stock status and sustainable harvest rates.

The Secretariat maintains an age database, along with reference sets of otoliths for which scientists from different laboratories have agreed on the interpretation of the number of rings for each fish. This provides a mechanism to make sure that each laboratory is estimating age in the same way.