GERMAN PLANKTON INVESTIGATIONS. 315 



of an adult West Baltic herring as being 60 grams, and 

 allowing that every herring uses in 50 days its own weight 

 of organic substance, we find that every herring consumes 

 annually 438 grams. In the 16 square miles of the 

 Eckenforde fishery district there exists food in the shape 

 of Copepoda for 534 million herring of an average body 

 weight of 60 grams. This result may of course be largely 

 problematical, but it is at any rate extremely interesting. 

 The North Sea, in a similar manner to the Baltic, contains 

 an abundant wealth of Copepoda. The open ocean, on 

 the other hand, contains much less. 



The estimation of the number of free swimming larvae 

 of the larger edible Crustacea, such as the crab and lobster, 

 is also of practical interest, as on almost every coast a large 

 number of the adult individuals may be met with. When 

 applied to previously investigated coasts the method 

 would yield results which would enable one to ascertain 

 whether or not a fishery for such Crustacea could be 

 successfully established. The larvae of the edible 

 mollusca, the mussel, for example, are also frequently 

 present in almost incredible quantities, and especially in 

 the neighbourhood of coasts where the adults prevail. In 

 the greater depths of the North Sea and in the open ocean 

 such larvae are rarely found. 



On one occasion in the West Baltic it is calculated 

 that in the case of the larvae of Mytilus edulis, 

 the number present was 170,000 for each square 

 metre of surface water. If all these developed into 

 adult animals, we should have for each square centi- 

 metre of bottom 17 adult mussels. This is, of course, 

 physically impossible. It therefore follows that only an 

 exceedingly small proportion of the larva) ever become 

 mature, the greater portion being devoured by other 

 animals. It is thus seen that the larvae of the various 



