CH. IX] THE PRODUCTIVITY OF THE SEA 179 



approximate values. We may not be able to find out, Hensen 

 says, whether our neighbour's income is £1000 or £1200 per year, 

 but we are interested in knowing whether he is a millionaire, is 

 wealthy, or is only in comfortable circumstances. 



Our estimates of the abundance in the sea of diatoms, protozoa, 

 copepods, fishes and so on, are of just this degree of value. But 

 even when we have obtained these approximate figures for the 

 population of the sea it is not enough, for such populations are 

 continually changing. Organisms die and fall to the sea bottom 

 and decompose, or are devoured by their enemies or are captured 

 by man. Birth-rates in the sea var}^ with each kind of organism 

 and change with the season, and the rates of growth undergo 

 corresponding fluctuations. Death-rates too change with the season, 

 and with changes in the density of inimical organisms. Not only 

 must we attempt to estimate the density of population in the sea 

 at a given time, but we must also try to find out what mass 

 of living substance is periodically generated. 



It is much more difficult to attempt such estimations of 

 the productivity of a sea area, than merely to attempt to ascertain 

 the mass of life at one particular time. Man has hardly at all 

 cultivated the sea in the way that he cultivates the land. When 

 compared with the present position of agriculture there is hardly 

 any science of aquiculture. With the exception of certain 

 attempts by Hensen and Brandt to estimate the productivity 

 of certain inshore sea areas in the Baltic, and a very incomplete 

 study, by the Germans, of the conditions of culture in carp-ponds, 

 the science of aquiculture is practically non-existent. It is true 

 that both the French and Dutch cultivate oysters on quite a large 

 scale, and the Americans deal very largely with the artificial 

 fertilisation and rearing of marine and fresh-water fishes, and in 

 this country trout and salmon hatching is carried on to some extent. 

 But the French and Dutch oyster farms are conducted in 

 apparently a purely empirical manner, and both American and 

 English fish culture is as yet quite experimental in its aims. 

 Agriculturists have acquired much information as to the conditions 

 of cultivation of crops, the metabolic processes involved in the 

 rearing and fattening of live stock, and so on; and it is known what 

 mass of produce in the form of cereal and other crops and live 



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