410 IDS die ANDADIDINTE 
formed in spring and early summer, the dark rings in late summer and 
autumn. During the winter practically no growth of the otolith takes 
place. The first white ring is formed in the spring following the year of 
birth, that is when the fish is just one year old, and the total number of 
white rings will therefore give us the number of years of life which the 
fish has completed. 
As the Plaice gets older, however, the rings at the edges of the otoliths 
are so crowded together that it becomes impossible to count them. For 
these older tishes Heincke (9) has shown that an examination of the bones, 
after special treatment, gives the information required. 
It is clear that if we can determine the age of individual fishes, and if 
we do this on sufficiently large samples, we at once obtain some informa- 
tion as to their rate of growth, for if, say, the average size of the two-year- 
old Plaice in a particular locality is 20 em. and the average size of the 
three-year-olds is 25 cm. we shall not be far wrong in concluding that a 
Plaice of 20 em. will grow about 5 cm. during the next year of its life. 
This conclusion, however, assumes that the conditions of growth are the 
same each year, and from information now available we know that this 
is not always true, but that some years are more favourable for growth 
than others. Growth in different localities, also, even though they may 
not be very distant from each other, may differ greatly. What we can 
obtain by the use of the methods already described, if the observations 
are repeated for a number of years, is the average rate of growth for each 
year of age. 
There are, however, other methods by means of which we can get an 
idea of the actual rate of growth in a particular area at a particular time. 
The one which has been most used and has yielded the most reliable 
results is the method of marking experiments. In these experiments a 
healthy fish is measured soon after being caught and a small metal label 
is attached to it, generally at the base of the dorsal fin. The label is 
numbered and the fish is returned to the sea. When it is subsequently 
caught again by the fishermen it can be identified by its number and 
measured a second time. The actual amount of growth will then be 
known. A great many experiments of this kind have been made on both 
Plaice and Cod, and large numbers of the fish have been caught and 
returned by the fishermen to the various laboratories. Even these 
experiments, however, which were carried out primarily to give informa- 
tion as to the migrations of the fish, are subject to at least one possibility 
of error, owing to the fact that the future growth of a fish may be con- 
siderably retarded by any slight injury it may have sustained when it 
was first caught. This source of error has to some extent been overcome 
by keepmg the fish in tanks of running sea-water for some time before 
