In many species, age can be determined by examining the periodic markings on meristic structures such as scales, otoliths and opercular bones. The best structure to use varies between species (see Table 9.1). There are many difficulties of interpretation of these markings, and they tend to become less reliable as the age of a fish increases, but the method is useful and widely applied. The age groups of fish are commonly designated as follows:
Group 0 Fish of less than one complete year of life.
Group I Fish between one and two years of age.
Group II Fish between two and three years of age.
Group III Fish between three and four years of age, and so on.
The three parts of the membranous sac in the inner ear of teleost fishes each contain an ear stone or otolith. The largest of these, usually the sagitta, can often be used for age determination. Otoliths grow by the deposition of lime on the outer surface. These layers are deposited at different rates at different seasons. By cutting thin sections of the otolith, the layers can be seen under the microscope as alternating light and dark concentric rings. Each completed year of life is represented by one darker ring. This method has been extensively used in plaice. Fish of less than one complete year of age, i.e. Group 0 fish, show only an opaque central nucleus.
Table 9.1 Usual methods for age determination in various North Atlantic fish species.
Otoliths and scales
Reliable to 6 years. Scales too easily rubbed off and transferred from fish to fish
Otoliths are too small
Only reliable to 3 years
Scales can be used to age many fish such as cod. The surface of the scale bears a large number of small calcareous plates or sclerites (Figure 9.31), the number of which increases as the scale grows. Sclerites formed during the summer are larger than those formed in winter, and the alternating bands of large and small sclerites indicate the number of seasons through which the fish has lived. It is sometimes difficult to distinguish the zones of summer and winter sclerites clearly. In cod, the method is satisfactory up to three years of age, but as the fish become older the accuracy of the scale age becomes less certain.
Annual growth rings are found in the scales of herring but are not often easy to detect. The rings are due to slight differences in refraction of different regions
Figure 9.31 Growth zones in gadoid scale.
of the scale, and are best viewed under the microscope using a low power objective with dark-ground illumination. The rings are usually most clearly seen in scales taken from the anterior part of the trunk region. The rings indicate the interruption of growth of the scale that occurs during the winter months. Fish spawned in late summer or autumn probably fail to record their first winter as a scale ring, and in these the first ring relates to the second winter. Scale markings may be used to determine growth rates because the width of each zone between the rings is closely proportional to the growth in length of the fish during the period in which that zone was formed (see Figure 9.30, page 361). At sexual maturity the growth rate is reduced, and this is indicated on the scales by a narrowing of the growth zones. The pattern of scale rings therefore varies according to the age at which the fish mature. In shelf herring spawning in the southern North Sea, the first narrow zone is usually to be found between the second and fourth rings. In oceanic herring which mature later, the first narrow zone occurs after the fifth ring. In Baltic herring, most of which mature during their second year, there is usually only one wide zone, the first narrow zone being between the first and second ring.
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