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Physical and Biological Conditions in the North Sea. 



By 



J. T. Cunningham, M.A, 



SUMMARY. 



PAGE 



Results of Professor Otto Pettersson's investigations of the conditions and movements 

 of the various layers of water in the Skagerack and Cattegat, and the influence of 

 these conditions and movements on the migi'ation of fishes. Southern Species 

 entering the Baltic with coast water from the south, Northern Species with coast 

 water from the north . . . . ... 233 



Mr. H. N. Dickson's observations in the Fariie-Shetland channel and the northern 

 entrance of the North Sea, and the conclusions he draws from them. Suggested 

 influence upon the migration of fishes of the amount of oxygen in sea- water . 237 

 Mr. Dickson's paper in the Gcograjjhical Journal for March, 1896, describing the 

 surface salinities and temperatures observed in the International Hydrographic 

 Survey of 1893-1894 . . . . ... 243 



Actual peculiarities in the distribution of fishes in the North Sea . . . 246 



Southern Species entering the North Sea from the south . ... 247 



Probable dependence of the immigration of Southern Species on temperature . . 249 

 Southern Species entering at the north. All those found in the south, and a great 

 many more occur on the south coast of Norway, though all do not enter the 

 Baltic. Their presence in this neighbourhood due to the warm Atlantic surface 

 drift, or Gulf Stream . . . . ... 252 



Distribution of Northern Species. These species principally occur in the deeper water 



along the east coast of Britain . . . ... 255 



The general physical conditions determining the distribution of Northern Species . 257 

 The migrations of the herring . . . . ... 259 



Dilferent sizes of fish of the same species at different parts of its habitat . . 260 



In my report, in the preceding number of this Journal, on my 

 observations in the North Sea, I referred briefly to the problem of 

 the relation between the physical and biological conditions. This 

 problem will afford scope for investigation for some time to come,- 

 and the purpose of the present article is to discuss and compare some 

 of the most recent additions to our knowledge of the matter. The 

 paper by Mr. H. N. Dickson, to which I referred in my previous report, 

 was published in the Geographical Journal last March, under the title 

 of " The Movements of the Surface Waters of the North Sea," and in 

 the Scottish Geographical Journal, in 1894, was published a series of 

 papers by Professor Pettersson on " Swedish Hydrographic Eesearch in 



