METHODS OF INVESTIGATION. 19 



refer the reader to the characteristic scales depicted in Figs. 22 and 23 

 (Plate VIII.). 



The spawning mark, which in the case of salmon we found to be a 

 result of and also a clear indication of spawning, sometimes occurs also in 

 the case of trout, but its appearance is generally of a different character. 

 In salmon it is the result of the excessive emaciation which this large and 

 rapidly-growing fish experiences owing to the terrible fasting and privations 

 connected with a sojourn of many months in fresh water prior to and after 

 spawning. Most trout spawn amidst surroundings and food conditions similar 

 to those to which they are accustomed, and their spawning journey, as a rule, 

 is short and unaccompanied by serious privations. Consequently, in most 

 of the smaller forms of trout we find no spawning marks on the scales, 

 even though the fish are of considerable age and have spawned on many 

 occasions. On the other hand, we sometimes find one or more spawning 

 marks on the scales of trout, whose growth has been considerable as com- 

 pared with the food conditions which prevail in those surroundings where the 

 spawning grounds are situated, and whose spawning journey is long and 

 arduous. As an instance of this we may mention large sea trout and also 

 very large lake trout, such as are found in the Mj0sen Lake and also in the 

 lakes of Bavaria. Such spawning marks, however, of which an illustration 

 is given in Fig. 24 (Plate IX.), as far as I can ascertain, do not always 

 appear on the scales as a matter of course as is the case with salmon, 

 and, in my opinion, cannot be depended on as reliable evidence of the 

 frequency of spawning. 



In investigations as to the age of fish and especially with reference to 

 estimates of the rapidity of growth, great care is needed when such spawn- 

 ing marks are found ; and, further, if they are difficult to read one must 

 endeavour to avoid mistakes, which might easily creep into an analytical table 

 if one's material were not handled in a critical manner. 



3. GENERAL OBSERVATIONS ON THE USE OF SCALES AS A MEANS OF 

 DETERMINING THE GROWTH OF FISH. 



If we wish to study the rate at which salmon or trout grow, it is 

 necessary to investigate the ages of a sufficiently large number of fish and 

 to divide them into year-classes according to length or weight. We then 

 find that the various year-classes include fish varying very much in size, 

 and, in order to get an idea of the general rate of growth, we must calculate 

 the average size or weight for each year-class, and on these averages we can 

 base an opinion of the rate of growth. 



For this purpose a large amount of material is necessary, with a con- 

 siderable amount of labour for its investigation. As will be seen later, the 



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