August 26, 1920] 



NATURE 



819 



Apstein have made some notable calculations based 

 on the occurrence of these in certain hauls taken at 

 intervals across the North Sea, whkh 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 1,200,000,000 

 spawners, enabling them to calculate that the total 

 fish population of the North Sea (of these six species) 

 at that time (the spring of 1895) amounted to about 

 10,000,000,000. Further calculations led them to the 

 result that the fishermen's catch of these fishes 

 amounted to about one-quarter of the total popula- 

 tion. 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 represent only i 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 assump- 

 tion, 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 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 from 

 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 Schutt 

 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 333 c.c, but the individual catches ranged from 

 15 c.c. to 6-5 c.c, and the divergence from the 

 average may be as great as -I-3-2 c.c. ; and, after 

 deducting 20 per cent, of the divergence as due to 

 errors of the experiment, Schutt estimates the mean 

 variation of the plankton at about 16 per cent, above 

 or below. This does not seem to me to indicate the 

 uniformity that might be expected in this " halistatic " 

 area occupying the centre of the "North Atlantic Gulf 

 Stream circulation. Hensen also made almost simul- 

 taneous hauls with the same net in quick succession 

 to test the amount of variation, and found that the 

 average error was about 13 per cent. 



As so much depends in all work at sea upon the 

 weather, the conditions under which the ship is 

 working, and the care taken in the experiment, with 

 the view of getting- further evidence under known 

 conditions I carried out similar experiments at Port 

 Erin on four occasions during last April and on 

 a further ©ccasion a month later, choosing favour- 

 able weather and conditions of tide and wind so as 



NO. 2652, VOL. 105] 



to be able to maintain an approximate position. On 

 each of four davs in April the Nansen net, with 

 No. 20 silk, was hauled six times from the same 

 depth (on. two occasions 8 fathoms and on two occa- 

 sions 20 fathoms), the hauls being taken in rapid 

 succession and the catches emptied from the net 

 into bottles of 5 per cent, formaline, in which they 

 remained until examined microscopically. 



The results were of interest, for although they 

 showed considerable uniformity in the amount of the 

 catch— for example, six successive hauls from 

 8 fathoms being all of them 02 c.c, and four out of 

 five from 20 fathoms being 06 c.c— the volume was 

 made up rather differently in the successive hauls. 

 The same organisms occur for the most part in 

 each haul, and the chief groups of organisms are 

 present in much the same proportion. For example, 

 in a series where the Copepoda average about 100, 

 the Dinoflagellates average about 300 and the Diatoms 

 about 8000, but the percentage deviation of individual 

 hauls from the average may be as much as plus or 

 minus 50. The numbers for each organism (about 40) 

 in each of the twenty-six hauls have been worked out, 

 and the details will be published elsewhere, but the 

 conclusion I come to is that if on each occasion one 

 haul only in place of six had been taken, and if one 

 had used that haul to estimate the abundance of any 

 one organism in that sea-area, one might have been 

 about 50 per cent, wrong in either direction. 



Successive improvements and additions to Hensen's 

 methods in collecting plankton have been made by 

 Lohmann, Apstein, Gran, and others, such as punrip- 

 ing up water of different layers through a hose-pipe 

 and filtering it through felt, filter-paper, and other 

 materials which retain much of the micro-plankton 

 that escapes through the meshes of the finest silk. 

 Use has even been made of the extraordinarily minute 

 and beautifully regular natural filter spun by the 

 pelagic animal, Appendicularia for the capture of its 

 own food. This grid-like trap, when dissected out 

 and examined under the microscope, reveals a sur- 

 prising assemblage of the smallest protozoa and proto- 

 phyta, less than 30 micro-millimetres in diameter, 

 which would all pass easily through the meshes of 

 our finest silk nets. 



The latest refinement in capturing the minutest- 

 known organisms of the plankton (excepting the bac- 

 teria) is a culture method devised by Dr. E. J. Allen, 

 director of the Plymouth Laboratory." By diluting 

 half a cubic centimetre of the sea-water with a con- 

 siderable amount (1500 c.c.) of sterilised water treated 

 with a nutrient solution, and distributing that over a 

 large number (70) of small flasks in which after an 

 interval of some days the number of different kinds 

 of or|«anisms which had developed in each flask was 

 counted, he calculates that the sea contains 464,000 of 

 such organisms per litre ; and he gives reasons why 

 his cultivations must be regarded as minimum results^ 

 and states that the total per litre nay well be something- 

 like a million. Thus every new method devised seems 

 to multiply many times the probable total population 

 of the sea. As further results of the quantitative 

 method, it may be recorded that Brandt found about 

 200 Diatoms per drop of water in Kiel Bay, and 

 Hensen estimated that there are several hundred 

 millions of Diatoms under each .square metre of the 

 North Sea or the Baltic. It has been calculated that 

 there is approximately one Copepod in each cubic 

 inch of Baltic water, that the annual consumption 

 of these Copepoda by herring is about a thousand 

 billion, and that in the sixteen square miles of a cer~ 

 tain Baltic fishery there is Copepod food for more than 

 530,000,000 herring of an avierage weight of 60 grams. 



n Journ. ^ir. Biol. Assoc, vol. xii., p. i, July, 19*^ 



