PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 239 



If it were a mere question of oxygen, there would be no reason why 

 the mackerel should migrate to the Swedish coast in summer, and the 

 herring in winter. Moreover, the coast-water flows in at the surface, 

 and displaces the Baltic current in winter, and so far from being richer 

 in oxygen, contains less of that element than the surface water of the 

 Baltic. In a foot-note on the latter, Pettersson states : — 



" This singular fact tliat the waters of the Baltic upper layers may 

 occasionally be supersaturated with oxygen like Arctic water, is worthy 

 of attention. I have discussed this phenomenon with the eminent 

 specialist on diatoms, Professor Cleve of Upsala. We both arrived at 

 the conclusion that supersaturation with oxygen, as well as deficiency 

 of oxygen, is probably due to the influence of organic life. The pre- 

 dominance of vegetable Plankton, which is characteristic of Arctic as 

 well as of Baltic water, may cause the former, the respiration of 

 animals and of animal Plankton the latter phenomenon. 



"Be this as it may, it is certainly a fact to be borne in mind by 

 biologists, that the conditions of organic life are very different in the 

 Baltic and in the North Sea, on account of the relatively high amount 

 of dissolved oxygen in the upper layers of the Baltic. Owing to the 

 low salinity, the perfect aeration of the water down to a considerable 

 depth, and the low temperature at which their water is saturated with 

 air in winter, the upper layers of the Baltic contain about 30 per cent, 

 more oxygen than the waters of the North Sea." 



In view of the above statement, it can scarcely be maintained that 

 the visits of migratory fish, like herring and mackerel, to the mouth of 

 the Baltic, are to be attributed to the abundance of oxygen present in 

 the sea-water which flows into the entrance of that sea at certain 

 seasons of the year. 



What Pettersson does prove with regard to the herring is, that its 

 presence on the west coast of Sweden depends on the presence of water 

 having certain qualities, and derived from a certain source. In the 

 fifth of his series of papers, in the Scottish Geographical Journal for 

 1894, there is a special discussion of the changes in the water on the 

 west coast of Sweden, and tlieir effect upon the herring fishery. In 

 1877, after an absence of seventy years, herring again appeared on the 

 Swedish coast. It appears to be a historical fact that, time after time, 

 herring have entirely deserted this coast, have remained absent for a 

 period of about seventy years, and have then resumed their annual 

 migration to the region, giving rise to a valuable and important fishery. 

 The cause of this regular irregularity does not concern us here, nor is 

 it discussed by Pettersson ; we are considering merely Pettersson's 

 conclusions concerning the changes in the water which determine the 

 arrival and departure of the herring on the coast during the period 



NEW SERIES. — VOL. IV. NO. 3. S 



