CH. VIIl] A CENSUS OF THE SEA 173 



Here then we have a preliminary estimate of the total numbers 

 of some kinds of fishes inhabiting a definite sea area. To what 

 extent does it represent the actual conditions ? I think that it 

 can only be regarded as a first essay at such a census. In the first 

 place the accuracy of the results depends on the accuracy of the 

 estimate of the average number of eggs present in the sea per 

 square metre of surface — that is, on the exactitude of the fishing 

 operations involved in the capture of these eggs ; and on the validity 

 of the deduction of the average from the individual numbers — 

 that is, on the degree of inequality of distribution of these plank- 

 tonic fish eggs. Now if the reader will consult the Appendix, still 

 better if he consults the original tables given by Hensen and 

 Apstein, I think he will conclude that this inequality of distribution 

 was not so great as to rob the estimated average of a certain 

 amount of value. If we take the total number of eggs of our six 

 species I think that the average number found per square metre 

 may be regarded as an approximately accurate one, and that the 

 probable error of the estimate may be calculated. If this is the 

 case, then the pelagic estimate of the total number of fish eggs in 

 the North Sea must be regarded as a rough approximation to the 

 truth ; and again, if this is the case, then it is justifiable to calcu- 

 late from this the number of adult fishes in the same area. 



But when Hensen and Apstein made their investigations the 

 characters of the various species of pelagic fish ova were not ac- 

 curately determined. It is always difficult to identify a planktonic 

 fish egg, and in 1895 our knowledge of the life-histories of fishes 

 was not exact enough to enable this identification to be made 

 with certainty. Hensen and Apstein, therefore, confused eggs 

 belonging to different species^ — the cod and plaice for instance; 

 and we must conclude that the numbers representing the pro- 

 portions in which the six species, cod, plaice, haddock, flounder, dab 

 and long rough dab occur, have little or no real value. But so far 

 as concerns the total number of all the six species, it is probable 

 that the results are rough approximations to the truth. 



How close is the approximation ? We have reasons for con- 

 cluding that the formidable numbers given in the previous pages 

 are not blind guesses at the truth, but have some degree of prob- 



1 See Hensen, Wiss. Meeresunt. Kiel Komm. Bd. v. Abth. Kiel, p. 157, 1901. 



