Otolith annuli
Otolith annuli preparation and interpretation, age validation, mass processing and related links.
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Intact gaspereau otolith showing annuli.
Sectioned cod otolith showing annular growth increments (annuli).
Flounder otolith which has been cracked and burned to show annuli.
Annuli and ageing
To determine the age of fish, we use bony parts of fish that form yearly rings (annuli) like those of a tree. This includes:
- scales
- bones
- otoliths
- fin rays
Otoliths generally provide the most accurate ages, due to their continuous growth throughout the life of the fish and acellular nature. This implies that they don’t reabsorb.
These features give otoliths a significant advantage over scales and other structures, particularly in old fish. Therefore, otoliths have become the preferred structures for age determinations.
Despite the long-term global use of otolith annuli as age indicators, the factors influencing their formation isn’t clear cut. Each annulus has an opaque and translucent zone. In many species, these correspond to fast and slow growth, respectively.
The opposite pattern is apparent in other species. In general, the opaque zone seems to be formed during periods of increasing water temperatures. The translucent zone seems to be formed during periods of reduced growth, or in association with spawning.
Preparation and interpretation
The preferred method for otolith preparation and interpretation differs among species and age readers.
In general, microscopic examination of whole otoliths (immersed in a clear fluid) is acceptable for thin otoliths, while other otoliths require sectioning (the crack and burn method).
Since otolith length may cease to grow in old fish, even while they continue to thicken, some form of cross section is required of old otoliths.
Age validation
In keeping with ages derived from otolith microstructure, annular age determinations are an acquired skill, with a significant subjective component. For this reason, age validation is an important component of any ageing study. It confirms not only that the annular rings are visible in the otolith, but also that they’re being interpreted correctly.
Consistency between age readers or with other bony structures isn’ta measure of accuracy. Ageing errors also occur with the correct interpretation of the innermost (first) annulus, for which independent validation is often required.
Mass processing
The Bedford Institute of Oceanography’s ageing unit processes 5,000 to 10,000 otoliths annually in support of stock assessments for:
- cod
- haddock
- silver hake
They mass-process cod and haddock otoliths by:
- carefully aligning and embedding batches of 100 otolith pairs in a black polyester resin
- cutting transverse cross sections of the resin block with a high-speed, water-cooled saw
- this yields 100 otolith pair cross sections per plate
- applying a transparent coating to the plates
- drying and ageing the plates and otoliths
All ageing is carried out under an image analysis system for quality control assurance.
Related links
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