president's address. 17 



Based on the occurrence of these in certain hauls taken at intervals 

 across the North Sea, 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 1200 milHon 

 spawners, enabling them to calculate 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 population. Now all this is not only 

 of scientific interest, but also of great practical importance if we could 

 be sure that the samples upon which the calculations are based were 

 adequate and representative, but it will be noted that these samples 

 only represent one square metre in 3,465,568,354. Hensen's state- 

 ment, 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 

 Ee 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 

 since the days of P. T. Cleve and from the observations of Hensen's 

 own colleagues that this is not the case, and they have published chart- 

 diagrams showing that at least three different kinds of water under 

 different conditions are found in the North Sea, and that at least five 

 different planktonic areas may be encountered in making a traverse 

 from Germany to the British Isles. If the argument be used that 

 wherever the plankton is found to vary there the conditions cannot 

 be uniform, then few areas of the ocean of any considerable size remain 

 as cases suitable for population-computation fi'om random samples. 

 It may be doubted whether even the Sargasso Sea, which is an area 

 of more than usually uniform character, has a sufficiently evenly 

 distributed plankton to be treated by Hensen's method of estimation 

 of the population. 



In the German Plankton Expedition of 1889 Schiitt reports that 

 in the Sargasso Sea, with its relatively high temperature, the twenty- 

 four catches obtained were uniformly small in quantity. His analysis 

 of the volumes of these catches shows that the average was 3"33 c.c, 

 but the individual catches ranged from 1-5 c.c. to 6'5 c.c, and the diver- 

 gence from the average may be as great as -f-3'2 c.c. ; and, after deduct- 

 ing 20 per cent, of the divergence as due to errors of the experiment, 



1920 c 



