PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NOKTH SEA. 241 



the surface of the water at the entrances than at the inner parts, so that 

 below a certain depth the water in the fjords is cut off from communi- 

 cation with the water in the open sea. The Gulhnar Fjord is described 

 as a typical example. The depth of water on the Swedish coast bank, 

 i.e., from the shore of the mainland as far as the outermost rocks 

 and shoals, does not usually exceed 15 fathoms. But corresponding 

 to each fjord, leading from its entrance towards the S.W. there is 

 a deeper channel, cut off from the deep inner basin of the fjord by the 

 ridge across its mouth. 



In summer the ocean water and the North Sea water in the 

 Skagerack extend from the bottom nearer to the surface than in winter, 

 and above the North Sea water lies the Baltic water, which flows 

 out most abundantly in summer. On the declivity of the coast the 

 layer of Baltic or fresh water is deeper than in the central part of 

 the Skagerack. In consequence of these facts the ocean water never 

 enters the Gullmar Fjord, because its level never reaches to within 

 20 fathoms of the surface on the coast declivity, and that is the depth 

 of the ridge at the entrance of the fjord. The North Sea water, 

 however, whose salinity is 3-1 per 1000, enters the fjord occasionally, 

 or periodically, and it is water of this salinity which fills the inner 

 basin of the fjord. Waters of 33 per 1000 and less flow into or out of 

 the fjord from the surface to the depth of 20 fathoms at all seasons of 

 the year. 



It was found that the bottom water of the Gullmar Fjord in February, 

 1890, was extremely deficient in oxygen, that it was cut off from com- 

 munication with water of the same salinity outside, which contained 

 plenty of oxygen. In June, 1890, the level of the North Sea water 

 outside the fjord rose to less than 20 fathoms from the surface, flowed 

 over the ridge, and displaced or mixed with the stagnant water already 

 there. This did not happen again until 1893. 



The Gullmar Fjord is noted for the fishery for cod and John Dory 

 in its deep water, a fishery which takes place in winter, but not every 

 winter. In the winter of 1889-90 this fishing was very poor, and had 

 been so for some previous years. In the winter of 1890-91 it was very 

 successful. The inference suggested is that the entrance of these fish 

 into the Gullmar Fjord is dependent upon the entrance of North Sea 

 water in summer, which does not always occur. When it does not 

 occur, even if the fish should enter the fjord by swimming over the bar 

 at the entrance, the bottom water of the fjord would be nearly or quite 

 uninhabitable for them on account of the small supply of oxygen it 

 contains. Pettersson expressly points out that he does not consider 

 the herring fishery in the fjord to be affected by this change in the 

 bottom water, but believes it to bo dependent upon the inflow of 



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