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ESTIMATION OF AGE BASED ON OTOLITHS
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Otoliths
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The otoliths of fish are analogous to the otoconia of other vertebrates.The functions of otoliths include balance, hearing. Three pairs of otoliths occur in teleosts namely, Lapilli, Sagittae and astersci. Fish otoliths (Sagitta) are accrete layers of calcium carbonate and gelatinous matrix throughout their lives. The accretion rate varies with growth of the fish – often less growth in winter and more in summer – which results in the appearance of rings that resemble tree rings . By counting the rings, it is possible to determine the age of the fish in years.
Position of Otolith in fish
Source: misclab.umeoce.maine.edu
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By measuring the thickness of individual rings, it has been assumed (at least in some species) to estimate fish growth, because fish growth is directly proportional to otolith growth. However, some studies disprove a direct link between body growth and otolith growth. Otoliths, unlike scales, do not reabsorb during times of decreased energy making it even more useful tool to age a fish. Fish never stop growing entirely, though growth rate in mature fish is reduced. Rings corresponding to later parts of the life cycle tend to be closer together as a result.
Otolith microstructure
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Bony fish otoliths are complex polycrystalline bodies which act as organs of balance in the inner ear. The otoliths are primarily composed of crystallized calcium carbonate in the form of aragonite and of a fibrous, collagen-like protein: otoline.
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Partly or wholly abnormal otoliths made up of calcite are relatively common. These crystalline otoliths are transparent and lack clearly defined growth marks.
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In addition to age determination, increments have been used to validate annulae periodicity to determine changes in growth, to detect life transitions, to estimate recruitment and mortality and in taxonomic studies.
Otolith selection and removal
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The otoliths must be removed as soon as the fish dies, barring which the fish must be frozen or duly fixed to avoid the loss of the growth structures present in the otoliths.
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Larvae and juveniles must be handled with great care, because the area to otolith volume ratio makes them much more susceptible to deterioration.
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There is a certain amount of body loss in fish storage, which is of particular importance for larvae and juveniles. The corrective factors should be calculated by measuring and weighing a series of specimens, before and after preservation, covering the size range to be studied. Shrinkage is proportionate to fish size and depends on storage time and preservatives. The period of time between death and fixing must also be reckoned for juveniles and larvae.
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Freezing is a good storage method where there is no risk of partial thawing due to temperature changes, which would degrade otolith quality.
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To remove otoliths the cranium must be sectioned to reach the chambers of the inner ear. In round fish a transversal cut is usually made in the head a little behind the eyes. The cut must be deep enough to open the skull without damaging the otoliths. When the saccule is exposed the otoliths are withdrawn carefully with forceps so as not to break them. With small otoliths, it is better to remove the semicircular canals and separate the otoliths under a binocular microscope. Any adhering tissue can be removed from the otoliths by rubbing them gently between the fingers or with tweezers under a magnifying glass. Immersion in a 5% sodium hypochlorite solution facilitates the cleaning operation.
Source: www.southbaytech.com
Otolith storage and preservation
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The otoliths must be stored in such a way as to ensure they occupy the minimum space, save money and are well-preserved and easy to identify. As otoliths are acellular bodies with a small proportion of organic matter, the risk of decomposition is minimal. Growth structures are nonetheless more easily visible in newly collected otoliths. They should therefore be read as quickly as possible.
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Otoliths can be stored in test-tubes or vials, dry or with a clarifying liquid when they are to be read immediately. Another method in use is to stick them with transparent nail varnish to a slide or with two-sided sticky tape to a sheet of acetate. The more sturdy otoliths are usually stored dry in properly labelled envelopes.
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