530 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [6] 

 Warmth of the surface water (t) and the bottom icater (T) in degrees C. 



From an analysis of this table it appears : 1. The warmth of the 

 atmosphere is followed by the warmth of the surface water with some 

 slight delay, so that in the water, February is the coldest month, and 

 not January, as in the atmosphere. 2. In the deep water the influence 

 of the atmospheric warmth is delayed, and this is most noticeable in deep 

 water where there are no strong currents, for example, in the deep water 

 near Kiel, October is the warmest and March the coldest month. 



The spawn which, during the first months of the year, is by the fish 

 ejected in shallow water would therefore first be in cold water, but would 

 soon find the temperature rising, whilst the spawn ejected in autumn in 

 deep water would find a high temperature which would favor a rapid 

 development. 



In the surface water, which in winter may reach as low a temperature 

 as zero, and even lower, the fish could not live, whilst in deep water 

 they find warm places of retreat during winter. 



We must mention another important circumstance which is caused 

 by a remarkable quality of the water. 



Like all other bodies, water becomes specifically heavier the colder it 

 gets; but in water there is a limit to this. When fresh water has cooled 

 down to a temperature of 4° it grows continually lighter until it freezes. 

 The water having a temperature lower than 4°, therefore, continues to 

 float above the heavy water of 4°, and the deep water of deep fresh- 

 water lakes, consquently, never reaches a temperature lower than 4°. 



This quality of fresh water is somewhat changed by the salt contained 

 in the sea water, and the extreme limit of density is found in water 

 having a low temperature. Thus, water containing f per cent, salt, 

 such as is found in the eastern portion of the Baltic, reaches its greatest 



