238 PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 



occur, on the migrations and distribution of fishes, and consequently 

 on fisheries. ^ 



Mr. Dickson regards the subject in a manner which seems to me 

 peculiar; and while his work in the physical department is well known 

 to be in all respects admirable and sound, I am obliged to express some 

 disagreement with the argument he employs concerning the biological 

 questions. He urges that we cannot discover direct hard and fast 

 relations between the temperature or the salinity or the density of 

 sea-water, and the constant or periodic occurrence of certain animals 

 {e.g., fishes) in particular localities. Yet we have to discover some 

 reason for the fact that the appearance of certain fishes and other 

 marine animals has been frequently observed to be associated with a 

 periodical change in the temperature or salinity of the water. 

 Mr. Dickson says that the peculiarities of temperature or salinity are 

 certainly too slight to seriously affect animal life. But he points out that 

 it is just these two physical elements upon which the oceanographer 

 relies in tracing the circulation of waters, and in identifying the sources 

 from wliich they are derived. 



He proceeds to lay stress upon the alleged fact that Pettersson and 

 his colleagues have collected a mass of evidence shewing that the 

 migrations of herring and mackerel, and other variations in the 

 distribution of not only fishes, but Plankton, are dependent on the 

 amount of oxygen present in the sea-water. The amount of oxygen 

 dissolved in the water depends solely on the temperature and atmos- 

 pheric pressure to which it was exposed when at the surface. The 

 lower layers of water, in enclosed areas, are accordingly liable to 

 become deficient in oxygen, and fresh supplies must be obtained by 

 mixture with water from the ocean, if animal life is to remain healthy 

 and abundant. 



Now, after careful study of Pettersson's papers in the Scottish Geo- 

 graphical Magazine, I cannot perceive that they exactly correspond 

 to Mr. Dickson's description. According to Pettersson, the advent of 

 the mackerel corresponds to the influx of North Sea water in May 

 and the following summer period. During winter the amount of North 

 Sea water present in the Skagerack decreases, and that which remains 

 becomes deficient in oxygen. But it is scarcely possible to hold that 

 the deficiency of oxygen in this layer of water alone causes the 

 mackerel to leave. For in the first place mackerel swim very near 

 the surface, and are therefore not, in all probability, contained in the 

 North Sea water at all, but in the overlying Baltic water, which in 

 summer flows out through the Skagerack in considerable quantity. 

 Then, again, the herring enter the Skagerack, according to Pettersson, 

 in autumn and winter, with the influx of coast-water from the north. 



