240 PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 



when it pays its annual visit. In the middle of December, 1878, before 

 the herring had appeared, the salt-water of 34 per cent, salinity (North 

 Sea water) reached to within fifteen or twenty fathoms of the surface, 

 and filled the deep channels of the coast. The surface water overlying 

 this consisted of layers of 32 and 33 per cent, salinity, evidently the 

 remnant from the inflow of coast-water in autumn ; the temperature of 

 this was 8° to 9° C. 



In the latter part of December stormy weather occurred, and a 

 new influx of coast water took place, of the same salinity, but with 

 a temperature of only 4° to 6° G. The last influx of this water 

 occurred on the 28th and 29th December, and simultaneously the 

 herring fishery began on the coast to the east of Wiidero Islands. In 

 the middle of January the fishery began to decline at the same place, 

 the cause of which was as follows: On the 11th and 12th January the 

 wind, which had been blowing from the IST.E., began to blow from S. and 

 S.E., in consequence the fresher and colder water belonging to the 

 Baltic stream began to flow northwards along the Swedish coast, and as 

 the Baltic water displaced the coast water the herring disappeared. 

 There can be no suggestion here of a superiority in the supply of 

 oxygen in the coast water. Both the water with which the herring came, 

 and that which drove them away, were surface waters, and both well 

 supplied with oxygen, especially the latter. The difiterence between 

 the two consisted chiefly of salinity and temperature, and the effect 

 upon the fish may have been due rather to the movement of the water 

 than to its qualities. 



On the other hand, it is perfectly correct that Pettersson has 

 discussed in considerable detail the absence of oxygen in the bottom 

 water contained in deep depressions separated by higher ground from 

 similar depths in the neighbourhood. In such cases the water 

 contained in the depression is salter, and therefore heavier than 

 the overlying water, and remains undisturbed and unaffected by 

 the movements of the lighter and fresher water passing over it. When 

 deprived of the dissolved oxygen which it contains, the bottom water, 

 being cut off from contact with the air, can obtain no new supplies, 

 except what it takes up by diffusion from the layers of water above 

 it, and that is scarcely any. The oxygen originally contained in it 

 is constantly being abstracted by the respiration of the animals that 

 live in it, and by algae also in the absence of light, so that if un- 

 disturbed for a long time water in an isolated depression becomes 

 almost entirely destitute of oxygen, and, therefore, incapable of 

 supporting animal life. Pettersson states that these are the conditions 

 to which the deep basins of the fjords on the west coast of Sweden are 

 subjected, because the bottom of the fjords is much higher and nearer 



