Section IV, 1918 [47] Trans. R.S.C. 



The Scale Method of Calculating the Rate vf Growth in Fishes. 

 By A. G. Huntsman, B.A., F.R.S.C. 



(Read May Meeting, 1918.) 



Comparatively recently the discovery was made that the age of a 

 fish could in many cases be found out by examining its scales. In a 

 tree can be seen revealed in the wood the story of- its growth. As 

 winter alternates with summer, so does dark zone alternate 

 with light zone, as shown in a cross section of the trunk of the tree. 

 Similarly in the scale of a fish the alternation of the season is reflected 

 by an alternation of zones of rapid and slow growth. By counting 

 these the age is readily determined. 



Successive zones on the scale often differ greatly in width. The 

 scale grows much more in some years than in others. The idea was 

 conceived by certain Norwegian scientists that the variation in width 

 of the zones might reflect the variation in growth of the fish in suc- 

 cessive years. If the scale grows proportionally at the same rate as 

 the fish, it will be possible by measuring the width of a complete zone, 

 representing a year, on the scale, to determine by calculation the 

 amount of growth of the fish in that year. For example, if a fish 20 

 centimetres long and three years old has a scale measuring 2 milli- 

 metres from centre to margin, then, if the yearly zones measure 0-9, 

 0-7, and 0-4 millimetres respectively, the growth in length of the 



0-9 X 20 

 fish during the first year was = 9 cm., during the second 



2 



0-7x 20 ^ , J . , , . J 0-4x 20 

 year, = 7 cm., and durmg the third year = 4 cm., 



Measurements of a certain scale in a large number of herring of differ- 

 ent sizes seemed to show that the scale does grow approximately at 

 the same rate proportionally as the fish. Following this discovery 

 very many calculations have been made of the growth rate in individual 

 fish of many kinds. 



However, the correspondence in rate of growth between the 

 scale and the fish is far from exact. The scale appears only after the 

 fish has attained some size, namely at the time of transformation from 

 the larval condition into that of the adult. The growth of the scale 

 compared with the growth of the fish is at first very rapid, but later it 

 diminishes. The scales do not all appear at the same time nor do 

 they grow at the same rate and all parts of the same scale do not grow 



