CH. IX] THE PRODUCTIVITY OF THE SEA 187 



Altogether I do not doubt that the productivity of this area, 

 in terms of dry organic substance, cannot be much less than 

 1500 kilogrammes per hectare. 



The absolute productivity of a sea area. What then is the 

 probable quantity of organic substance which can be produced 

 in an inshore sea area ? All the estimates quoted refer only to the 

 quantity of directly utilisable produce which may be taken from 

 the sea — a very different thing indeed. We wish to know what 

 total mass of life is generated throughout the year per unit area of 

 a sea such as the Baltic or Irish Sea. Now the estimation of this 

 quantity is a problem of such very great practical difficulty that 

 one may well despair of its solution. But if we assume that 

 it is possible, by means of quantitative plankton investigations, to 

 deduce the quantity of life present in a sea area at a definite time, 

 then the calculation of the absolute productivity is theoretically 

 possible. At the beginning of the year a certain mass of life 

 is present, and at the end of the year much the same quantity 

 is still present. But in the meantime all organisms have been 

 reproducing and growing. The mass of life present at the 

 beginning of the year is the capital ; the mass generated during 

 the year by the reproduction and growth of the capital is the 

 interest. At the end of the year the capital remains the s^me : 

 the interest has been eaten up, or otherwise destroyed. > What 

 is the rate of interest ? 



In order to determine what is the interest we require to know 

 (1) the rate of reproduction of each species of organism under 

 different conditions (temperature, weather, &c.); (2) the rate 

 of growth of the individuals of each species (also under different 

 conditions) ; (3) the average duration of life of the individuals 

 of each species ; (4) the duration of reproductive activity in 

 the life-history of each species ; and (5) the amount of natural 

 destruction due to enemies. Some of these things we do know : 

 for instance, we know what numbers of eggs are spawned by most 

 fishes, and some invertebrates ; the rate of birth in some inverte- 

 brates (thus from the relation between eggs, larvae and adults in 

 the plankton, Hensen ascertained that the birth-rate of copepods 

 (all) was about 134 per 1000) ; the duration of life in many marine 



