FISHERY RESEARCH 



331 



the fish up to boiling point in water so that it is just begin- 

 ning to be cooked. The flesh may then be easily scraped 

 from off the backbone, without the backbone itself falling 

 to pieces. A much more rapid and clean method which 

 may replace the older method, is merely to take X-ray 

 photographs of the herring, when the vertebrae can be 

 counted on the negative (Plate 120). 



The scale of the her- 

 ring is well suited for 

 studying its growth 

 rate, for the yearly 

 rings containing broad 

 and narrow bands, cor- 

 responding to periods 

 of fast and slow growth, 

 are clearly marked out 

 on it (Plate 120). It 

 is known that the scale 

 grows in proportion at 

 a corresponding rate 

 to the body of the fish, 

 and from the distance 

 from the centre of the 

 scale to the outer edge 

 of each yearly ring can 

 be calculated the size 

 of the fish at the end 

 of each year (see Fig 



Examination of scales from large numbers of herring dur- 

 ing the course of many years has thrown very interesting 

 light on the age composition of the shoals of fish. It 

 may, for instance, be found that in one year over seventy 

 per cent, of the herring caught were six years old, the re- 

 mainder being mostly four, five, seven and eight. The fact 



Fig. 62. — Diagram showing how the scale 



growth is proportional to the growth in 



length of the whole fish. 



62). 



