D— ZOOLOGY 93 



hydrographical changes is corroborated from farther afield. Since 1926 

 continuous records of the currents in the Straits of Dover have been made 

 from the Varne Lightship with the Carruthers drift indicator. Water 

 can enter the North Sea both from the English Channel and round the 

 north of Scotland and Dr. Carruthers infers that these v^^ater-masses are 

 opposed to one another and act in a sort of ' buffer relationship.' At the 

 Varne Lightship the relative strengths of these two forces are indicated 

 by a change in the direction of the current. Dr. Carruthers ^" has calcu- 

 lated the direction of the residual current for each year since 1926 and the 

 figures which he has given show that from 193 1 onwards this residual 

 current has svioing towards the north and has considerably less of the 

 easterly component which it possessed in the earlier years when high 

 winter values for phosphate were observed at Plymouth. 



A point worthy of consideration is whether a similar series of adverse 

 years has occurred in the past, but on this unfortunately we have no 

 reliable data. The statistics of the herring industry are almost the only 

 source open to us, for we have no regular observations on fish larvae 

 prior to 1924, and it was not until five years later that the importance of 

 Sagitta was recognised. But before the War, the herring industry was 

 conducted on different lines, from sailing vessels, and we have figures 

 only for the aggregate catch from which it is not possible to draw any 

 conclusions. 



In 1915 and 1916 Mr. D. J. Matthews first began the determination 

 of phosphate in Channel water. His results, though not obtained by the 

 methods now in use, have a high degree of accuracy, and they suggest 

 that in those years there was a deficiency of phosphate comparable with 

 that in recent times. Unfortunately the Plymouth herring fishery was 

 greatly reduced during the period of the war and we have no reliable 

 statistics for comparison. 



We may suppose that this long-period fluctuation at the mouth of the 

 Channel will end in due course, but we have no means of knowing when 

 this will happen. When the change comes it will be heralded, we believe, 

 by the return of Sagitta elegans in large numbers, and by a marked increase 

 in the winter phosphate maximum. The fisherman will presumably not 

 find any immediate improvement in the bottom fish. As yet he has 

 perhaps scarcely realised the full extent of the depression which started 

 some years ago, and when there is a return to better conditions he must 

 wait until the increased numbers of larvae grow to fish of marketable 

 size. It is possible, however, that bottom-living fish have been migrating 

 into the area and that he may thus in some measure escape the worst 

 effects of the depression. If the younger herring have forsaken their 

 spawning grounds and gone elsewhere, we may hope that they will at 

 once return in force when conditions improve, and that the Plymouth 

 fishery will rapidly be re-established. If, however, they have throughout 

 held to their former migration routes, and the present dearth is due to 

 lack of suitable conditions for the larvae, they are in the same position as 

 the bottom fish and a number of years must elapse before the fishery can 

 be resumed. 



" J.N. Carruthers, ' The Flow of Water through the Straits of Dover," Part II. 

 Min. Agric. Fisheries, Fishery Invest., ser. ii, XIV, pp. 15, 64, Table VI (i935)- 



