102 



The Rate of Growth of a Plaice. 



The age of a plaice is found merely by looking at the 

 otoliths (or earstones). The sex is found by holding the fish 

 up to a strong light and observing whether or not it has a roe : 

 the latter is deeply pigmented and shows through the trans- 

 lucent body and skin. The earstone has always a little opaque 

 spot in the centre surrounded by opaque and translucent 

 rings : thus — 



Nucleus alone — the fish is one summer old. 



Nucleus + translucent ring — one summer + one winter. 



Nucleus + translucent + opaque rings — one summer, 

 one winter, one summer, and so on : a translucent 

 ring is added during each winter and an opaque 

 ring during each summer. 



The plaice in the Irish Sea are always hatched out in 

 February, March, and April, but, for the most part, in March. 

 Therefore, a fish caught in July, and having a nucleus only, is 

 four months old, one caught in September, and with only a 

 nucleus in its earstone, is six months old. So also, a fish 

 caught in October, and having a nucleus, two opaque rings and 

 two translucent ones, is two years and seven months old. 

 Usually we called the plaice 0, 1, II, III, etc., years old, meaning 

 over and less than 1 year of age, over 1 and less than 2, over 

 2 and less than 3, and so on. For the first year we state the 

 age in months, the number of the latter being the number of 

 the months that have elapsed from the middle of Maich up to 

 the date of capture. 



The growth, then, for the first year is as follows : — 



Up to middle of June ... ... 42-2 mms. 



July 



August . . 

 September 

 October . . 



46-5 

 52-5 

 58-6 

 67-4 



