The Use of the Scales of Rainbow Trout (Salmo Gairdnerii) to make Direct Comparisons of Growth

No one has apparently made use of efficient statistical procedures for the “back-calculation” of the growth of fish from the measurements of the scales. If it can be shown that the scale provides an unbiased estimate of the length or “size” of the fish, then there is no real necessity for the conversion of scale size into body size for comparisons of the growth. A study of data obtained from the rainbow trout population of Paul Lake, British Columbia, showed that scale size could be regarded as an unbiased estimate of the length of immature 2-year-old male and female rainbow trout under the ordinary conditions of sampling by angling. An example is given to show how the growth of rainbow trout in 4 separate years could be compared by the direct use of the scales. It is known from previous studies that a significant reduction in size occurred during these years. A corresponding reduction in the size of the scales was also found. It is suggested that the method provides a direct index of the growth of the fish which can be used to compare growing conditions from year to year.