SCIENCE AND THE SEA FISHERIES 229 



sea deposits known as the Globigeriaa and Radiolarian oozes. Of 

 the plant organisms the most important undoubtedly are the 

 Diatoms. The great producers of organic matter in salt water, 

 they have been called the " pastures of the sea." Like the Globi- 

 gerina and Radiolaria their accumulated shells form important 

 deep-sea deposits. 



The plankton is collected for examination by means of fine- 

 meshed nets, known as tow-nets, because in their original form 

 they were towed horizontally through the surface layers of water. 

 The net -substance was either muslin or silk bolting cloth. Recently 

 attention has been directed to the accurate estimation of the 

 quantity of plankton in a given volume of sea water, and for this 

 purpose several special nets have been devised, the original, and 

 even now the most successful, being that of Hensen. This net, in 

 the form of an inverted truncated cone, is lowered vertically to a 

 given depth, and then raised to the surface perpendicularly. By 

 this means a cylindrical column of water filters through the net 

 and its planktonic contents are captured. The volume of this 

 cylindrical column of water can be calculated, since the depth to 

 which the net is sunk and the area of the net -opening are both 

 known. The first and most important requirement of the Hensen 

 net is that it should capture the whole of the plankton in a known 

 Volume of water. The correct estimation of the volume of water 

 fished through is not a simple matter, since it is quite obvious that 

 not so much water passes through the net as would pass through 

 a ring of equal diameter to the mouth of the net, which had nothing 

 attached to it. It is also clear that a square centimetre of the net 

 would let through more water if it were composed of a single mesh 

 than if, as is actually the case, it is composed of a large number of 

 meshes, each bounded by a square of silk fibre. The filtration capacity 

 of each separate net has to be calculated, and for the details of these 

 calculations the original papers of Hensen should be consulted. 



If we assume that it is possible to fish out all the plankton from 

 a known volume of sea water, it becomes possible to determine the 

 productivity of the sea and compare it with a similar area of land 

 surface. The estimation of a catch of plankton may be made in 

 one of four ways. The estimation may be by volume, by weight, 

 by chemical analysis or by enumeration of the individual con- 

 stituents. The method of chemical analysis enables us to compare 

 the pastures of the sea with the pastures of the land. Chemical 

 analysis of the plankton consists in the estimation of carbon 

 hydrogen and nitrogen by organic combustion. The fats are 

 determined by making ether extracts. The ash, chlorine and 

 sUica are also usually estimated. 



