This is because there is a sharp difference between the temperatures of two seasons-summer-the period of faster growth, and winter-the period of slow growth or no growth. Fish of temperate regions shows clear rings, which are true marks. This abnormal function is due to the fact that the fish has not grown during one growing season (one summer) either in length or in weight.ġ. This type of annulus by position coincides with the annulus of the preceding year, with no normal circuli forming in between. The overlapping of scales can occur due to a slow growth during the growing period, which is represented by an increase in the length but not in the weight of body. The position of these annuli in posterior field coincide with the annuli of the preceding year while, in anterior part, it is separated from the preceding year’s annuli by 4 or 5 circuli. These false annuli resemble the true annuli but they take position on the scale closer to the true annulus of the preceding year than the normal annulus for the next year which appear in case of normal growth (Fig. Sometimes false annuli appear on the scales of fishes due to undesirable factors like retarded growth due to paucity of food, starvation, injury, disease, and fluctuation in temperature. The calculated age should be in agreement with the age determined by other methods. The length frequency distribution should coincide with the calculated age from the scales.Ĭ. When there is a correlation between the calculated age from the scale and the size of the fish.ī. The annuli are best seen at anterior part of the scale.Īnnuli are considered as year-marks in the age determination of fish in the case of the following facts.Ī. Thus annuli are very useful in counting the age of fish and serves as year-marks on the scale for age determination. Annuli show slow growth in a year but in many fishes, during winters, annuli grow remarkably and are added yearly as fish grows. In this way every year many such circuli and grooves are formed.Ī characteristic bone material, ichthylepidin, is deposited in circuli and thus their height increases which depends on the calcification. They also indicate osteoblastic activity as a result of which secreted material is deposited around the focus. The grooves and circuli represent growth activity. As scale grows older, other structures are added and perform their functions. 14.1).Īt the time of development of scale, focus is established first and represents the original size of the scale. The number of annuli represents age of a fish in years (Fig. Each trough contains a few incomplete and narrow circuli different from the circuli outside it, which are complete and more widely spaced. These are wide circular troughs found in aged fish over one year. Radii cut the circuli present in their path. These are grooves found radially, viz., they run from focus to margin of scale.
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