The otolith thus prepared is examined under the microscope to count the annulations for age determination and the growth rate analysis Graeme, (2007). For this, first of all the otolith is broken and cut in transverse plane followed by polishing with liquid of high refractive index, such as immersion oils, creosote, etc. To get the otolith, fish is killed and dissected and otolith is taken out and annulations are counted. The growth rate of fish is faster in the summer as compared to the winters, thus annulations formation is faster in the summer. The formation of annulations varies with the growth of fish, and the growth of fish varies with the season. The otolith has annulations formed by the regular deposition of calcium salts. The otolith or ear-stone is present in the internal ear of the fishes and helps in balancing the body. The rings on the centrum are counted under the microscope. For counting the rings on the centrum, it is exposed by removing the tissues attached to it using solution of 0.7% pepsin in 0.2% of hydrochloric acid. The centrum of fish vertebrae possess rings, which are used in age determination. Similarly, the centrum of fish is also helpful in calculating the age of fish. Number of annual rings are helpful in calculating the age of fish. The growth rate is different in different seasons. These annuli are increased in number with the age of fish. The important bones such as operculum, vertebra, supra occipital and scapula are provided with annulations. Therefore, the calculation of the age of fish by annuli is most reliable in temperate fish.Īnnuli are also present on some bones. 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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