SEA-FISHERIES LABORATORY. 161 



VARIATION IN SUCCESSIVE VERTICAL PLANKTON 

 HAULS AT PORT ERIN.* 



By W. A. Herdman, C.B.E., F.R.S. 



(Emeritus-Professor of Natural History in the University of 



Liverpool.) 



The degree of uniformity in the distribution of the plank- 

 ton through the water of a sea-area which is under what seem 

 uniform physical conditions is still a vexed question. We are 

 still uncertain as to the degree of validity of our samples, and 

 as to how far they represent more than some proportion of the 

 contents of the actual water which was sampled. 



The Sargasso Sea, surrounded by the North Atlantic Gulf 

 Stream circulation, and of relatively high temperature through- 

 out, is probably the largest area of apparently uniform 

 conditions that is known, and the results of the German 

 Plankton Expedition of 1889 show that the twenty-four plank- 

 ton catches obtained in that area were all small in quantity 

 compared with those from further North and further South 

 in the Atlantic. Schutt, who reports upon these results, shows 

 that the average volume of the twenty-four catches was 

 3-3 c.c, but the individual catches ranged from 1-5 c.c. to 

 6-5 c.c. and the greatest divergence from the average was there- 

 fore + 3-2 c.c, the divergence in the other direction being 

 - 1-8 c.c. After somewhat arbitrarily deducting 20 per cent, 

 of this divergence as due to errors of the experiment, he 

 estimates the mean variation of the plankton in the area 

 investigated at about 16 per cent, above or below the average 

 of the hauls. Even this reduced figure does not, however, 

 indicate a very high degree of uniformity of distribution such 



* I wish to acknowledge, with thanks, the help I have received in this 

 investigation from Mr. Andrew Scott, A.L.S., and Miss H. M. Lewis, B.A. — 

 from Mr. Scott in the examination and estimation of the catches, and from 

 Miss Lewis in making the calculations. I alone am responsible for the 

 work at sea 



