METHODS 



(Plate III). There is a wide range of meshes made, but the most practical 

 ones for plankton work are {a) the fmest with about i8o to 200 meshes to 

 the inch for fine work, (/)) 'No. 3' with 60 meshes to the inch for most of 

 the small creatures of the zooplankton, and (c) a 'grit-gauze' of about 

 26 meshes to the inch if you want to lose nine-tenths of the smaller and 

 commoner forms and only look for some of the larger varieties in a lot of 

 water. The finer the net the smaller the organisms retained, but the water 

 will pass through it only slowly and a fine net is easily clogged. Because 

 of its greater resistance it must be towed more slowly or the water in its 

 path will not pass through the net but spill round its edges. The larger 

 animals capable of darting about will be able to avoid such a net and their 

 numbers in the sample will not be representative. There is thus no ideal 

 mesh for all purposes. 



However, for casual interest one does not want representative samples, 

 but merely a diversity. An irregular mesh of cheese cloth or muslin will give 

 a nicelv mixed sample — but do not use too delicate a fabric or it will 

 burst under strain. A simple plankton net is illustrated in Fig. 3 ; i. It is merely 

 a conical net attached to a light metal or cane ring, the size of the opening 

 depending on the size of sample wanted and the facilities for using it. One 

 foot diameter will give quite a strong resistance behind a rowing boat and 

 6 or 9 inches would be preferable. Power driven research ships use larger 

 ones of I or even 2 metres diameter (Plates II, Wa). Because the flow of water 

 through the net depends on the ratio of total area of the holes to the opening, 

 the finer the mesh the longer the net in relation to the diameter, and a net 

 from three to seven times as long as the diameter is usual. This shape also 

 has the advantage of washing the catch during towing down to the tip where 

 it is concentrated and so the more easily extracted. To keep the catch in the best 

 condition, it should be protected as far as possible from the strain of being 

 pressed against the meshes, and so a 'bucket' or small jar is usually attached 

 to the end (Fig. 3 ; ic). A metal one is less fragile than glass, but a clear plastic 

 one has the advantage of both. The bucket also keeps the catch from drying 

 before it is transferred to the sample jar. If the condition of the organisms 

 is less important than their numbers, as it so often is in plankton survey work, 

 then a small detachable silk bag is more efficient and more convenient than a 

 'bucket'. It should be of the same or slightly smaller mesh than the net itself. 

 Most plankton organisms collected in such a bag are in remarkably good 

 condition but the delicate siphonophores (p. 54) and similar fragile forms 

 can be rather badly damaged. 



A net of this sort usually needs to be handled from a boat, which adds to 

 the pleasure of collecting if it is a nice day, but an equally effective way near 

 the shore is to let the tide provide the energy and to stream the net from the 



II 



