102 A HISTORY OF FISHES 



together, recalling the rings of growth exhibited by a cross- 

 section of the trunk of a tree. They represent the new material 

 manufactured by the tissues of the skin, and added to the scale 

 from time to time. But since a fish grows unequally at different 

 seasons of the year, this irregular growth is duly reflected in 

 the scales. In spring and summer, when food is plentiful and 

 the fish grows rapidly, the scales increase in size by the addition 

 of a large number of rings well separated from each other; 

 when growth slows down in autumn or almost ceases in winter 

 the rings added become fewer in number and much closer 

 together. This check in growth once a year enables us to 

 determine the age of any fish by counting the number of winter 

 "zones"; that is to say, of areas of close rings. 



A glance at the accompanying figures of Salmon scales 

 (PI. I) will give an idea of the manner in which this method 

 is applied. The first represents the scale of a 5-lb. Grilse, 

 caught in the River Wye in July when returning from the sea to 

 spawn. The centre of the scale (Fig. a) shows clearly the two 

 years spent in the river from the time of hatching until it 

 descended to the sea as a Smolt, i.e. the two years' Parr life; 

 this is followed by a number of well-separated rings representing 

 the first summer in the sea, after which is a winter zone, 

 followed in turn by an area representing half a second summer 

 in the sea. It may thus be deduced that the age of this fish 

 when caught was three and a half years. The second (Fig, B) 

 was taken from a large spring fish of 22-lb. weight, caught in 

 April, and shows two years' river life and three years' sea life. 

 At the end of a winter's growth an irregular scar will often be 

 seen running right round the scale. This is known to scale- 

 readers as the "spawning mark," and is due to the following 

 cause: after spawning the girth of the fish greatly decreases, 

 and the outer edges of the scales become much frayed and 

 worn; when it again reaches the sea scale-growth recom- 

 mences, but it does not take place in such a way as to repair 

 the eroded parts, and the line of junction between the old and 

 the new growth is necessarily irregular. 



From time to time the scales of a fish tend to wear off or to 

 become otherwise dislodged, a not infrequent occurrence in 

 such forms as the Salmon (Salmo) and. Herring {Clupea). When 

 this happens, new scales, known as replacement scales, are 

 formed to take the place of those lost. Naturally these are of 

 no value for scale-reading purposes, the concentric rings in the 

 centre of the scale being absent. 



