-> Catches 



ORDER OF INCREASING C^ 



Fig. 34. Catches ot herring compared with the number of Calaiius taken at the same time and 



place by the 'Hardy' Plankton hidicator. The samples are arranged in order of increasing numbers 



of Ccilauus, and divided into two blocks of eleven samples each, to show the greater average 



catch of herring in the richer water. Adapted from Hardy. 



It docs not always work out that way, indeed the highest catch of the sample 

 was in the lower Calaiitis group and no herring at all were caught with the 

 sample of maximum Calaiius. Why is this? There are several reasons. 



A big shoal of herring will graze down the Calaiius so that the shoal may 

 be found where Calaiius have been abundant, but bciore the herring have 

 moved on to new feeding grounds. If food is abundant there may be plenty on 

 the edge of the Calaiius patch so that the richest areas are not reached. When 

 herring are about to spawn they are then most concentrated and most easily 

 caught, but they are then just not interested in food. The Yitvnngj Calaiius 

 relationship has yielded even better results in the Icelandic area. The method 

 has fallen out of favour as a commercial venture since the introduction of the 

 echo-sounder which can locate the actual shoals very accurately and indepen- 

 dently of biological theories and practice. 



Calaiius is not, of course, the only food of herring and most other species 

 of zooplankton arc taken and young sandeels are particularly important as 

 they average about 40 per cent of the total herring food. Incidentally, about 

 70 per cent of sandeel food is Calaiius. 



Herring arc migratory fish, moving in shoals in complicated and not too 

 well understood paths between feeding and spawning areas. These paths 



132 



