480 SOME RESULTS OF THE INTERNATIONAL FISHERY INVESTIGATIONS. 



time. All these changing phenomena — the temperature and salinity of 

 the sea water, the occurrence and abundance of different planktonic 

 species, and the reproductive phases of all animals and plants in the sea 

 — occur from year to year with a certain periodicity. The later direc- 

 tion of fisheries research has been to correlate them and to find out how 

 the one phenomenon depends on the other. More precisely, one of the 

 main objects of the International Fishery Investigation has been to 

 determine in what manner hydrographic changes in the sea are con- 

 nected with the productivity of the various commercial fisheries. 



In all cases plankton observations have been carried on simul- 

 taneously with the hydrographic work. The plankton investigations 

 consist of periodical fishing of the sea over wide areas, and both at the 

 surface and at deeper levels, with fine-meshed silk nets, so designed as 

 to catch all but the very smallest organisms in the water. As a rule, 

 these plankton observations have been so carried out as to secure only 

 samples of the kinds of species in the sea, but in some of the countries 

 quantitative observations have been made ; that is, the " townets " are 

 designed to filter a known quantity of water, so that if the numbers of 

 individuals of each species caught are determined, it can be estimated 

 how many organisms of each species were present in a certain bulk of 

 sea water ; for instance, in a column of water one square metre in 

 section and extending from the bottom of the sea up to the surface. 

 These quantitative plankton observations are the most laborious which 

 have been included in the International programme, and the results 

 obtained so far are not yet fully worked out and discussed. In the ordin- 

 ary plankton investigations, only the kinds and relative abundance of the 

 various organisms present have been determined. These qualitative 

 plankton observations have two main objects : (1) to ascertain the 

 sequence of occurrence and abundance of planktonic organisms. Among 

 these are, of course, the eggs and young stages of fishes, and the 

 determination of the occurrence and abundance of these from place 

 to place, or time to time, has obviously the utmost importance in the 

 study of the life histories of the fishes. Then (2) the plankton 

 observations are also of use in confirming the results of the hydro- 

 graphic researches. The main object of the latter is to ascertain the 

 movements of bodies of water of different origins, by a consideration of 

 their temperatures and salinities. But oceanic streams and currents have 

 also characteristic plankton organisms, and the recognition of the latter 

 is of material value in the determination of the origin of the current. 



In any part of the sea of our coasts there is a regular sequence in the 

 occurrence of the organisms of the plankton. It would be quite impos- 

 sible to give this in detail for the various regions investigated by the 

 International steamers. Speaking quite generally, we should find such 



