238 FOUNDERS OF OCEANOGRAPHY 



Diatoms per drop of water in Kiel Bay, and that Hensen 

 estimated that there are several hundred millions of Diatoms 

 under each square metre of the North Sea or the Baltic ; 

 and it has been calculated that there is approximately one 

 Copepod in each cubic inch of Baltic water. 



The floating eggs and embryos of the more important 

 food-fishes occur in quantities in the plankton during 

 certain months in spring, and Hensen and Apstein have made 

 some notable calculations based on the occurrence of these 

 in a series of 158 samples which led them to the conclusion 

 that, taking six of our most abundant fish, such as the cod 

 and some of the flat fish, the eggs present were probably 

 produced by about 1,200 million spawners, leading them to 

 the conclusion that the total fish population of the North 

 Sea (of these six species), at that time (spring of 1895) 

 amounted to about 10,000 millions. Further calculations 

 led them to the result that the fishermen's catch of these 

 fishes amounted to about one-quarter of the total popu- 

 lation. 



Now all this is not only of scientific interest, but also of 

 great practical importance if we could be sure that the small 

 series of samples upon which these colossal calculations are 

 based were adequate and representative, but it will be noted 

 that these samples represent only one square metre in 

 3,465,968,354. Hensen's statement, repeated in various 

 works in slightly differing words, is to the effect that using a 

 net of which the constants are known, hauled vertically 

 through a column of water from a certain depth to the 

 surface, he can calculate the volume of water filtered by the 

 net and so estimate the quantity of plankton under each 

 square metre of the surface ; and his whole results depend 

 upon the assumption, which he considers justified, that the 

 plankton is evenly distributed over large areas of water 

 which are under similar conditions. In these calculations in 

 regard to the fish eggs he takes the whole of the North Sea 

 as being an area under similar conditions, but we have known 



