188 THE PRODUCTIVITY OF THE SEA [PART II 



organisms (thus Hensen determined that a copepod lived, on the 

 average, for about a week, and so the rate of birth was about 

 134 per 1000 per week). Reasoning in this way, and considering 

 the average numbers of copepods present per square metre of 

 Baltic water, Hensen came to the conclusion that there were 

 produced, per annum, 8,866,000 copepods in every 10 cubic metres 

 of the Baltic. Since in a stationaiy population the birth-rate and 

 death-rate are the same, it follows that in every 10 cubic metres 

 of the Baltic, 8,866,000 copepods must have been destroyed 

 annually, that is, eaten by other animals. 



Now theoretically sound as this method is, it is nevertheless 

 incredibly difficult to make the calculation of productivity ; for we 

 do not know precisely the birth-rates, rates of growth, «Sjc. The 

 accuracy of Hensen's calculation — that 8,866,000 copepods are 

 annually produced per 10 cubic metres of Baltic — is therefore 

 very problematical. But we may check this result roughly, so as 

 to see that it is something more than a mere guess. We know 

 that herrings feed largely (or entirely) upon copepods ; we know 

 how many herrings are caught in the course of a fishing season in 

 a roughly determined area of sea ; we know quite accurately how 

 many copepods are contained in the stomachs and intestines 

 of a sample number of such herrings examined ; and we may 

 estimate roughly, or assume, how often a herring uses up its own 

 weight of organic substance. Thus we may calculate roughly how 

 many copepods have been eaten by the herrings per unit of time 

 and sea. I leave the sceptical reader to make these calculations 

 and recommend him to do so before he concludes that Hensen's 

 estimate is all nonsense. 



Thus by the application of such principles Hensen determined 

 that so much plankton was annually generated in the Baltic 

 per year, under each square metre, as was equal to 150 grams of 

 dry organic substance. He tells us that the value of his estimation 

 lies more in the study of the method than in the result^ There 

 can be little doubt that the result obtained by him is a minimum 

 one. I have shewn by quite a different method, that the 

 productivity of Ringhole, at Morecambe, in dry mussel flesh 



y ^ See " Fruchtbarkeit des "Wassers " : in " Nordsee-Expedition, 1895"; Wiss. 

 -'' Meeresunt. Kiel Komm. Bd. ii. Heft 2, 1897. 



