92 SECTIONAL ADDRESSES 



shelf ; and from the evidence I have laid before you it seems probable 

 that the normal water movements off the mouth of the Channel have 

 undergone marked alteration in recent years. Direct proof of this is 

 lacking, for we have no observations in the waters to the west of the 

 Channel, but evidence of it is afforded by the very interesting discovery 

 which Mr. Russell has made that certain planktonic species may be used 

 as indicators of water-masses.^ A relation of this kind has been found 

 in a number of plankton species, but it is here only necessary to refer to 

 those belonging to the genus Sagitta, and these owing to their abundance 

 are the most useful. 



Of the species of Sagitta, S. serratodentata is typical of the open Atlantic, 

 S. elegans of the mixed Atlantic water and S. setosa of the Channel water. 

 The first of these is only to be found on rare occasions off Plymouth when 

 the inflow of Atlantic water is exceptional. 



The importance of the species of Sagitta as indicators of water move- 

 ment was first recognised by Prof. Meek, but Mr. Russell's data from 

 Plymouth only began in 1930, and the records are therefore not as com- 

 plete as could be desired. It is, however, known that for some years 

 prior to this date the offshore plankton in the neighbourhood of Plymouth 

 was of the kind characteristic of the mixed Atlantic water : it was a very 

 rich plankton with such forms as Meganyctiphanes and Aglantha. It was 

 this type of plankton which was found in 1930, and in the regular series 

 of tow-net hauls made in that year Mr. Russell found that there was 

 94 % of S. elegans and only 6 % of S. setosa. In the following 

 year, when the deficiency of phosphate and of summer spawning fish 

 larvae first became manifest, there was, as will be seen from Table II, a 

 conspicuous change in the Sagitta population : of S. elegans there was 

 only 17 % while there was 83 % of S. setosa. Since then S. setosa 

 has always greatly preponderated in the catches, with a percentage of 



93 or over, with the single exception of 1936, when there was 60 % of 

 S. setosa and 40 % of S. elegans. There is no doubt there was a small 

 incursion of mixed Atlantic water in the Channel in this year, but it 

 was apparently insufficient to alter the trend of events. 



Attention may be drawn to the high sensitivity of this new method of 

 distinguishing water-masses. Once the distinctions between the species 

 of Sagitta have been mastered it is an easy method to handle, and it will 

 no doubt be widely employed in the future. 



We thus have evidence from four separate sources of the changed 

 conditions which have prevailed in the Channel since 1930-31. These 

 sources are (i) the winter phosphate maximum ; (ii) the numbers of fish 

 larvae ; (iii) the constitution of the spawning herring shoals ; and (iv) the 

 predominance of one or other species of Sagittn. 



The picture, to my mind at least, is convincing : one gains the impres- 

 sion that if only we had fuller knowledge corroborative data from many 

 biological sources would be forthcoming. 



The view that the large alteration which has occurred is linked with 



' F. S. Russell, ' On the Value of certain Plankton Animals as Indicators of 

 Water Movements in the English Channel and North Sea,' Journ. Marine Biol. 

 Assoc, XX, p. 309 (1935) ; ' Observations on the Distribution of Plankton 

 Animal Indicators ... in the Mouth of the English Channel, July, i935. 

 ibid., XX, p. 507 (1936). 



