10 A. C. Hardy 



represents the passage of several miles of sea, so that any marked variation in numbers 

 appearing from the analysis of the records is really showing itself through a consider- 

 able " smoothing of results " effected by the very nature of the sampling (Hardy, 

 1936 A, p. 495). The recorder was, of course, in part designed to overcome the errors 

 due to patchiness which may falsify the results of an ordinary net survey. As far as I 

 am aware no other experiments of the kind here described have been made except 

 those in the South Georgia survey already referred to, and two series taken to test the 

 validity of the plankton recorder method: one in the open South Atlantic midway 

 between Gough Island and Cape Town (Hardy, 1936 b, p. 535) and the other in the 

 North Sea (Hardy and Ennis, in an appendix to Hardy, 1936 a). I believe more 

 such experiments might be valuable in the understanding of planktonic ecology. 



The results of the series here published may be shown most easily in graphical 

 form by the use of histograms and so save much description in the text. They are 

 shown in Figs. 1 and 2, and include only those animals of which over 50 have occurred 

 in a single sample; the remainder, including ctenophores, several kinds of amphipod, 

 other species of copepods, and Euphausia frigida were present in only insignificant 

 numbers. Instead of giving the data in tabular form as well as graphically, the actual 

 numbers are inserted against the appropriate histograms. Apart from the marked 

 unevenness of distribution, the effect of vertical migration is clearly seen in the case 

 of the Euphausia superba adults and the copepods. The series began in darkness and 

 ended in daylight, sunrise being at 0645, i.e. in the middle of sample 23 ; we see the 

 animals just mentioned gradually withdrawing from the surface as dawn approaches. 

 It would be valuable, but difficult, to operate such a series of consecutive nets at lower 

 levels, for they would have to be opened and closed at the end of each haul. Regarding 

 the many possible causes of patchiness I still believe that some of the factors suggested 

 in the section on the dynamics of distribution in Hardy and Gunther {he. cit., pp. 

 343-356) are likely to be important. More observations in the field are required and 

 particularly more experiments specially designed to test the different hypotheses; 

 further discussion must await the results of such work. 



Apart from a consideration of causes, not much comment is necessary; the degree 

 of patchiness is obvious. What moral can be drawn from it ? Let us suppose we had 

 been carrying out a survey in this region using similar nets towed for 10 minutes at 

 points say 10 miles apart ; it is clear that we should arrive at very different conclusions 

 as to the distribution of the macroplankton according to exactly where our stations 

 were placed within a circle having a radius of only half a mile. In the consecutive 

 series here described a mile covers three adjacent samples; we see what a contrast 

 there is within any such three we may select. On the evidence provided by this and the 

 only available similar experiments it appears that much of the quantitative plankton 

 distribution work of the past cannot have the degree of validity often attributed to it. 

 Ecological experiments in the field call for a control just as much as those in the 

 laboratory. Each tow-net survey is really in the nature of an experiment; in the case 

 just imagined the experiment was to sample the water at 10 mile intervals to find out 

 the distribution of the more important plankton animals over the area. Before 

 accepting the results as valid a control experiment is necessary to see, if such samples 

 are repeated at several points near the same place, that they give a reasonably con- 

 sistent result : to see in fact if one such net towed at one place can be said to give a 

 fair measure of the plankton lying to five miles on either side of it. 



