SOME RESULTS OF THE INTERNATIONAL FISHERY INVESTIGATIONS. 479 



returns made by England, Holland, Germany, Denmark, and Belgium 

 with regard to the quantities of small plaice landed in these countries. 

 These returns are, of course, very imperfect, but the result giving the 

 proportion of small plaice to the total quantity landed is probably 

 approximately correct. About 2h per cent of the plaice landed in 

 Holland and about 1 per cent of the plaice landed in England were 

 under 8 inches in length, an estimate which does not greatly encourage 

 the pessimistic view held in various quarters as to the detrimental effect 

 of steam-trawling upon the plaice population of the North Sea. 



The connexion between Hydrographic and Biological 

 Phenomena in the Sea. 



We have seen that there is a well-marked periodicity in the 

 hydrographic changes taking place in the sea. When the temperature 

 and salinity of the sea water are regularly determined, it is found that 

 the variations are not irregular or casual ones, but are repeated with a 

 certain amount of regularity from year to year. At any one place in 

 the seas of Northern Europe the temperature of the water gradually 

 rises from a minimum some time in the winter or spring, to a maximum 

 in the summer or autumn ; and so also with the quantity of salt in the 

 water ; this too varies with more or less regularity, though the maxima 

 and minima may not correspond with those of temperature. Then we 

 have also seen that there is an analogous periodicity in the fisheries of 

 the same area. These are not carried on with perfect regularity all the 

 year round. At certain times in the year different fisheries are pre- 

 dominant. For any one fishing area there are seasons in the year which 

 are characterized by the abundance of a certain kind of fish. Herring 

 fisheries, cod fisheries, sole fisheries, and others have their seasons, 

 which are repeated from year to year with a certain uniformity. 

 Leaving aside for a moment the commercial fisheries, we find that the 

 same periodicity of occurrence and abundance also obtains with the 

 microscopic life of the sea. The waters of the sea always contain a 

 certain amount of drifting microscopic life, bacteria, diatoms, protozoa, 

 coelenterates, and the eggs and larval forms of the larger animals living 

 at the sea bottom or swimming about in the water. If the occurrence 

 and abundance of this jJ^ankton are studied throughout tlie year, it will 

 be found that it too is not always the same, neither in nature nor 

 quantity. There is a more or less regular sequence of forms of aninud 

 or vegetable life, each of which has its maximum and minimum of 

 abundance. Further, if the reproduction of any animal or plant in the 

 sea is studied, it will be found that the breeding season is periodic, and 

 occurs with very great regularity from year to year at about the same 



