of the Fishery Board for Scotland. 



45 



an examination of these rings, therefore, it is held to be possible to 

 determine the age of a particular fish, and if a sufficiently large 

 sample of herrings from a particular shoal is thus examined, the 

 percentage composition of the shoal as to age Gan be ascertained. 



The practical value of this theory can be shown by a purely 

 hypothetical illustration: — if, in a certain area, we know from 

 past experience that the bulk of any catch will consist of herrings, 

 say, six years old, and it has been shown during the routine of the 

 investigations that the herrings which will be six years old next 

 year were hatched in a bad spawning year or for other reasons are 

 scarce, then it can be predicted that next year large catches will not 

 be made at this particular area. The practical importance to the 

 industry of such prediction will be at once apparent. The actual 

 problem is, unfortunately, never so simple as this, but the illustration 

 given will show the trend of the idea. 



• The validity of the theory indicated above of determining age by 

 scale reading has not passed unchallenged, and the scientific staff in 

 collaboration with certain mathematical and statistical experts are 

 making a very careful survey of the premises on which it is founded. 



The summer of 1921, like that of 1920, was a disastrous one for 

 the great herring fishing on the East Coast of Scotland, and in view of 

 reports as to abnormal " marine growths " found on certain of the 

 grounds, the alleged poor quality of the herrings caught, and the 

 successful operations of the herring trawling fleets on the grounds 

 beyond the range of the drift-net fishermen, a comprehensive hydro- 

 graphic survey of the area was made both in August and September. 



That conditions were unusual at this period of the year was 

 indicated by the presence of enormous numbers of jelly-like forms 

 (Salpce, and more particularly Salpa fusiformis) floating in the 

 different water layers. These organisms, some of which reach a 

 length of 3 J inches, are not to be confused with jellyfish ; although 

 much like them in general appearance, they are a much more highly 

 organised form of life. They may readily be distinguished from 

 ordinary jellyfish, if found adhering to the meshes of the drift-net, 

 by their barrel-shaped form — many are pointed at both ends — and 

 by the presence in their centre of a red and orange body about the 

 size of a pea. The point of importance, however, is that these 

 passively floating forms belong to the Atlantic, and their presence in 

 the North Sea, at any time, is an indication that the warmer water 

 masses in which they live have been transferred from the Atlantic to 

 the North Sea. The accompanying chart shows concisely the general 

 conditions prevailing on the Scottish east coast during August 1921 ; 

 it will also serve to illustrate the area flooded by these forms in 

 August 1920, although the character of the invasions in the different 

 years was not precisely similar. The broken line shows the eastern 

 boundary of the area invaded by Salps. 



