12 PRESIDENT S ADDRESS. 



Again, take the case of an interesting oceanographic observation 

 which, if estabUshed, may be found to explain the variations in time 

 and amount of important fisheries. Otto Pettersson in 1910 discovered 

 by his observations in the GuUmar Fjord the presence of periodic sub- 

 marine waves of deeper Salter water in the Kattegat and the fjords 

 of the west coast of Sweden, which draw in with them from the Jutland 

 banks vast shoals of the herrings which congregate there in autumn. 

 The deeper layer consists of ' bankwater ' of salinity 32 to 34 per 

 thousand, and as this rolls in along the bottom as a series of huge 

 undulations it forces out the overlying fresher water, and so the 

 herrings living in the bankwater outside are sucked into the Kattegat 

 and neighbouring fjords and give rise to important local fisheries. 

 Pettersson connects the crests of the submarine waves with the phases 

 of the moon. Two great waves of salter water which reached up to the 

 surface took place in November 1910, one near the time of full moon 

 and the other about new moon, and the latter was at the time when 

 the shoals of herring appeared inshore and provided a profitable fishery. 

 The coincidence of the oceanic phenomena with the lunar phases is 

 not, however, very exact, and doubts have been expressed as to the 

 connection ; but if established, and even if found to be due not to the 

 moon but to prevalent winds or the influence of ocean currents, this 

 would be a case of the migration of fishes depending upon mechanical 

 causes, while in other cases it is known that migrations are due to 

 spawning needs or for the purpose of feeding, as in the case of the 

 cod and the herring in the west and north of Norway and in the 

 Barents Sea. 



Then, turning to a very fundamental matter of purely scientific 

 investigation, we do not know with any certainty what causes the great 

 and all-important seasonal variations in the plankton (or floating minute 

 life of the sea) as seen, for example, in our own home seas, where there 

 is a sudden awakening of microscopic plant life, the Diatoms, in early 

 spring when the water is at its coldest. In the course of a few days 

 the upper layers of the sea may become so filled with organisms that a 

 small silk net towed for a few minutes may capture hundreds of 

 millions of individuals. And these myriads of microscopic forms, after 

 persisting for a few weeks, may disappear as suddenly as they came, 

 to be followed by swarms of Copepoda and many other kinds of minute 

 animals, and these again may give place in the autumn to a second 

 maximum of Diatoms or of the closely related Peridiniales. Of course 

 there are theories as to all these more or less periodic changes in the 

 plankton, such as Liebig's ' law of the minimum,' which limits the 

 production of an organism by the amount of that necessity of existence 

 which is present in least quantity, it may be nitrogen or silicon or 



