Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Wildlife Management


Introduction: Knowledge of the age of individuals comprising populations of fish is essential to the fishery biologist. From such information he is able to calculate the rate of growth, determine the age at which sexual maturity is attained and the longevity of the species. These calculations and determinations can be applied in the management of populations and in setting fishery regulations. There are numerous methods being used to determine the age of an individual fish. Of these, the scale method is the most important means of age determination being utilized with species of fish possessing scales (Cooper, 1951). This method assumes that the ratio of body length to scale length is constant for all sizes of fish. As the fish grows, the scale increases in size proportionally and lays down concentric ridges called circuli. If growth of the fish is uninterrupted, no change will occur in the growth pattern of the scale. However, in areas that experience seasonal changes in temperature, the rate of metabolism will decrease in this rate of growth during periods of cold weather. The decrease in the rate of growth results in a corresponding period of interrupted growth of the scale and is recognized by the spacing of the circuli. The circuli tend to be more widely spaced during the summer’s growth and more closely spaced during the winter’s growth. Another growth characteristic of the scale is that with a decrease in the growth rate the circuli will be interrupted and will end at different places along the lateral margin. Then, with the resumption of growth, the new ridges parallel the entire scale margin and hence out across the unfinished circuli previously formed. This phenomenon is called “cutting over” and is very useful in determining the yearmark or “annulus”. In the analysis of a scale for age determination of cutting over and the spacing of the circuli are utilized to locate the annulus. The age of the fish is then determined by counting the number of annuli. The first annulus is measured from the center (focus) of the scale to the outer margin of the first closely spaced circuli encountered and/or to where the cutting-over effects are observed. The other annuli present are measured form the outer margin of the previous year’s growth to the next outer margin where the annular characteristics are located. Then, by measuring the distance between annuli, the rate of growth can be calculated for each year group with this assumption that the distance between the annuli is in direct proportion to the growth in length of the fish for that period (Lagler, 1956). (See more in Text)

Library of Congress Subject Headings

Black bullhead


Includes bibliographical references (pages 37-38)



Number of Pages



South Dakota State University


No Copyright - Non-Commercial Use Only