460 DISCOVERY REPORTS 



an extensive survey by tow-net methods it is not always possible to place stations very 

 close together ; particularly is this so when making wide ocean traverses. An observer 

 may doubt whether he is obtaining a true picture of the plankton distribution ; he may 

 at one point have taken observations within a concentration of zooplankton and between 

 two other stations missed an important zone of phytoplankton. 



As a supplement to the tow-net used at intervals it seemed desirable to construct a 

 machine which could be towed at full speed at any required depth below the surface 

 and sample the plankton mile by mile so as to give a continuous record of the main 

 changes in its composition along the line of tow. This is the purpose of the Continuous 

 Plankton Recorder. It enables one to study the size, density and frequency of patches 

 and to compare the irregularity of distribution in different areas. By having a con- 

 tinuous record of the changing abundance of the different species in the plankton 

 community one may be able to detect ecological relationships between different species 

 more readily than by observations at isolated stations ; it provides one with a continuous 

 line of ecological observation through the sea. 



The recorder is not intended to supplant or to compete with the tow-net as a method 

 of investigation, but rather to supplement it. On the other hand, it is possible by its 

 use to carry out over a wide area surveys which could not be made by one research ship 

 using tow-net methods. By employing several continuous recorders simultaneously on 

 different commercial steamship lines across an area such as the North Sea it is possible 

 to chart the broad changes taking place in the plankton month by month, or week by 

 week, just as it is possible to chart meteorological changes if one has sufficient obser- 

 vations taken at the same time over a wide area. 



The machine then has two main functions. Firstly it enables one to study in detail 

 the changes in the composition of the plankton along a continuous line of observation, 

 and secondly, if several are used simultaneously, they enable one to carry out broad 

 periodic surveys using commercial ships. There are limitations to the method, as indeed 

 there are to nearly all methods ; since they are dependent upon the nature and working 

 of the machine they will be discussed later (p. 498) when the machine has been 

 described. 



A number of modifications of these uses suggest themselves. The following have not 

 yet been tried but are perhaps worth mentioning here to show the possible application 

 of the recorder to other problems. When towed slowly the machine will sink by its 

 weight to much greater depths than when towed at speeds of 6 knots and over (see p. 463). 

 It could be used for a slow oblique haul, with altered gear ratios, to give a continuous 



siderable variation at this one station. As the nets were taken at the surface, midwater and near the bottom, 

 and the first and last set of samples were taken at the same time of the day (1900), vertical migration could 

 hardly account for the differences found. They must be attributed to concentrations of post-larval fish and 

 plankton being moved by the different rates of water flow of the lower water masses in relation to the position 

 of the ship in the surface current. Such results would seem to point to the necessity of making obser- 

 vations on the patchiness in the distribution of different organisms before embarking upon a survey of their 

 distribution by means of stations placed at considerable distances apart. In the 1926-7 'Discovery' survey at 

 South Georgia the patchiness was investigated by series of consecutive nets (Hardy and Gunther, 1935, p. 255). 



