New technique aids in estimating age of fishery stocks

No, an otolith is not an eight-sided monolith or a newly discovered mineral. It is a calcareous concretion found in the otocyst of an invertebrate or in the internal ear of a vertebrate, and it is of interest because it helps to determine the age of fish.

Ageing fish is of vital importance to the assessment and management of stocks. Specifically, it helps to measure mortality from natural causes as well as from fishing activity, and thus to estimate whether a stock is fully exploited or overexploited. It also helps to set the best time to begin and to end the fishing effort on a stock, allowing the stock to renew itself by reproduction or growth, thus remaining an economically viable enterprise. Almost every skeletal structure has been used in the attempt to age fish, including bones, vertebrae, and spines. But scales and otoliths are the most popular because they are easy to collect and to store. Fish have three pairs of otoliths, which appear as small shiny bodies somewhat lateral to the brain. These can usually be extracted fairly easily under a dissecting microscope with fine needles.

Either in transparency or through the grinding down of a cross-section, the three-dimensional structure will show a pattern composed of a number of concentric rings, very much like the rings of a tree trunk. Each ring is made up of a light or hyaline zone and of a darker or opaque zone.

Scientists consider the rings to be a response to changes is environmental conditions, such as temperature, light, and feeding. Indeed, any major change in he environment in which a fish lives is likely to produce a ring. In the temperate zones, where differences between summer and winter are marked by changes in both water temperature and amounts of available food, age determination of most species is relatively easy. But other events, such as plankton blooms, sudden unseasonal changes in temperature, accidental discharge of pollutants, or any stress, such as reproduction or a sustained lack of food, may also produce rings identical to annual ones.

In the tropics, the changes from one season to another are less regular and marked, which made ageing fish from the reading of otoliths more difficult. In the early 1970s, however, it was discovered that the otoliths of some fish contained primary growth increments that seemed to be formed on a daily basis, presumably as a result of the light/dark cycle and probably reflecting periods of feeding activity. This provided the basis for the development of a new technique that holds particular promise for ageing young or short-lived tropical or subtropical species for which the annual ring technique does not provide satisfactory results.

In any case, reading otoliths is not like reading a calendar, says Jorge Csirke-Barcelli, senior Fishery resources Officer at FAO. "It is a code, established for convenience, our convenience." Thus, otoliths must be read over a certain period so as to establish the time-scale over which the rings are laid down, and each reading has to be confirmed by a later reading or by another person.

Further confirmation can be sought by various means. Fish in tanks can be injected with tetracycline, which is readily absorbed and deposited in bony structures where calcification is taking place, and checked at regular intervals. In the field, fish must also be tagged and then later retrieved which can become an expensive process. Once a definite time pattern for ring formation has been established, the age of the fish can be determined.

Because the technique is costly in equipment and time-consuming when it involves daily rings, it is used in developing countries mostly for purely biological purposes. But, according to FAO, it has potential as a tool to be used in support of stock assessment in tropical and subtropical areas, particularly to age highly valuable species, and to provide reliable age estimates to be used in conjunction with other methods, including length/ frequency analysis.

Marie-Christine Comte