120 QUANTITATIVE PLANKTON INVESTIGATIONS [PART II 



and the latter will also sink a little below the surface of the sea 

 and so fish in a different water layer. If she travels fast then 

 more water will pass through the net but this will then rise to 

 the surface, or even a little above the latter. Wherever there is a 

 rapid tidal stream additional complications are introduced, for if 

 the vessel travels against the stream more water will pass through 

 the net than if she travels with the stream. Then the velocity of 

 the tidal streams varies with the time between the neaps and the 

 springs, and on a much indented coast its direction also varies 

 considerably. If the net fishes at the surface the effect of the 

 wind must also be considered, for the force of the latter is a factor 

 of some importance in augmenting the velocity of the tidal streams. 

 It is theoretically possible so to regulate the speed of the 

 vessel as to counteract the effects of the variable flow of water, but 

 anyone with experience of this work will see how difficult this must 

 be. One is perhaps safe in saying that it is practically impossible 

 to avoid the effect of the tides, and to ensure that the same amount 

 of water always passes through the tow-nets in each haul of the 

 latter. This is one obstacle to the employment of the ordinary 

 surface tow-net for quantitative estimates of the abundance of the 

 plankton, but there is another which is even more inconvenient 

 and difficult to avoid. It is well known that there is a vertical 

 movement of the organisms composing the plankton and that thi& 

 vertical migration may take place very rapidly, so that planktonic 

 creatures present at the surface at one time of the day may be 

 absent an hour or two afterwards. The alternation between day 

 and night, and even between a dull and a bright sky, will produce 

 these differences, and there is probably a more or less regular 

 diurnal variation in the plankton of the different water strata of 

 the sea. Changes of temperature will also lead to considerable 

 variations, and so it is necessary that we should consider the 

 direction of the wind if we wish to employ surface tow-nets to 

 obtain estimates of the abundance of the plankton. In our seas a 

 north or east wind will chill the surface and a south or west wind 

 will warm it, and we will usually find corresponding differences in 

 the nature of the plankton taken. Taking everything into 

 consideration it is evident that the surface tow-net is of little use 

 in quantitative plankton fishing. 



