CH. VIIl] A CENSUS OF THE SEA 159 



organisms of the first category present per unit volume of sea- 

 water in the manner already described. It is also possible to 

 estimate the numbers of those fishes producing pelagic ova which 

 inhabit the same sea area ; and one can also attempt to estimate 

 the numbers of the bottom living invertebrates which likewise 

 produce pelagic eggs and larvae. But there is no means ot 

 estimating the numbers of the fishes and other large animals 

 which produce eggs or young which do not appear in the plankton. 

 One can trawl or dredge for these organisms and calculate the 

 numbers caught per unit area of sea bottom swept by the 

 fishing instrument; but it is impossible to say with certainty what 

 proportion of the animals living on the bottom were caught by the 

 trawl and what proportion escaped capture. Such fishing experi- 

 ments only tell us imperfectly what is the relative density of the 

 things captured in different parts of the sea; and what changes 

 are taking place from season to season. The material does not, in 

 fact, exist for the construction of a " census " of a sea area : and I 

 •only quote such results as will give the reader some idea of the 

 extent to which the sea is populated by some predominant 

 marine organisms. 



Density of plankton in the North Sea. In 1903, and 

 .succeeding years, the German Fishery Research steamer Poseidon 

 made a number of hauls with the Hensen quantitative vertical 

 plankton net in the North Sea on the eastern side and in the 

 Baltic, and the results of these experiments have been published \ 

 A cruise was made every three months and on each of these 

 •quarterly cruises a number of "stations" were visited. The 

 number of stations varies from three to fifteen. The procedure at 

 each station is to lower the net down to the sea bottom, and then 

 haul it up slowly to the surface. The catch made is then preserved 

 and subsequently counted. The column of sea water thus 

 filtered is only a small fraction of a square metre in cross section, 

 but the results are converted into numbers of organisms captured 

 per column of water of one square metre in sectional area. This 

 •does not involve any error of moment, as the distribution of a 



1 Apstein, "Plankton in Nord- u. Ostsee auf den deutschen Terminfahrten," 

 Wiss. Meeresuntersuch. Kiel Komm. Bd. ix. Abth. Kiel, 1906. 



