Fish age and growth can be determined by various methods which have an element of subjectivity and therefore require knowledge of the fish species in order to select the option which makes the most sense, biologically speaking.
Age determination from seasonal rings is feasible in most species. The method offers information on length variability in each year class, making it possible to establish whether length-distribution at each age is normal. Normal distribution is an assumption basic to many length-frequency analysis methods (Morgan, 1988). Although the otoliths of many tropical fish species have seasonal rings, ring formation is not universal. Frequently, in the same area, some species have rings with seasonal periodicity and others do not (Morgan, 1987).
Slow growth rings are deposited following periods of environmental or physiological stress such as spawning, migration or environmental fluctuations. In some species a link has been established between ring formation and the period of sexual maturity. The reproductive process involves striking changes in feeding and metabolism which are mirrored in bodily growth. Although sexual cycles may be one of the most important causes in ring formation, ring formation in immature fish implies some other triggering factor. A deeper understanding is needed of fish metabolism and physiology if the mechanisms of otolith deposition are to be established.
The rings in tropical fish are usually rather indefinite, and the interpretation of the pattern often overshadowed by the presence of false and multiple rings. Methods such as otolith burning, which enhance the contrast of the real, and (normally) structurally different rings, therefore need to be used.
The consistency of an interpretation must be tested by the available statistical techniques (Beamish and Fournier, 1981). In otolith interpretation, neither the size of the specimen nor other factors which might influence the result must be known. Once the presence of rings which can consistently be interpreted has been established, the temporal significance of their deposition must also be established. The type of sample available will determine the applicable age validation method. The most useful method is probably to follow the evolution of rings on the edge of the otolith, in that it allows the time of formation and possible causes to be established.
Increment depositions in adult fish during slow growth periods may be very fine (0.1–0.4 μm) which are undetectable with the light microscope (approximate amplification 1 μm), and will appear to the reader as bands with no clear increments. The resolution may cause rhythmic increments to be perceived as a single increment and lead to underestimations of age.
The presence of these fine increments is a serious limitation to the application of light microscopy in determining the age of adult fish. In starting the study of a species it is advisable to read with the SEM some otoliths which cover the length range and thus to determine whether there are fine increments which would rule out the use of the light microscope. The cost of the scanning electron microscope and the time needed to prepare the otoliths and view them, however, make the application of this method impracticable for the numerous otoliths which must be interpreted in a fishery consultancy. In species with minimal growth periods, use the light microscope to determine age in the early growth phases and the scanning electron microscope to determine the rate of growth in adults and to validate findings obtained by other methods.
The first phase of study should determine the best method of otolith preparation, the presence and clarity of the growth rings, and the criteria for interpretation. When growth rings are found, their periodicity must be established before going on to determine the age.
When you wish to use growth increments, you must study other adult fish otoliths with the SEM to determine whether light microscopy is applicable. A combination of electron and light microscopy is the most cost-effective and time-saving approach.
