388 INTERNATIONAL FISIIEKY INVESTIGATIONS. 



grounds in the North Sea and English Channel, and by its means it has 

 been possible to form an estimate C)f the aver.age rate of growth in 

 different localities, a rusult which has direct bearings on many practical 

 problems. 



A large quantity of information has also been collected as to tlie food 

 of fishes of different species and at difl'erent stages of their growth npon 

 the various fishing grounds of the North Sea, the information being 

 derived in part from the examination of the food-contents of many of 

 the fishes captured in the trawl, and in part by an investigation of the 

 fishing grounds with special apparatus. 



III.— Hydrographical and Plankton Investigations in the 

 English Channel. 



Before the commencement of the International Investigations it had 

 been shown that the character of the water filling the North Sea varied 

 greatly from season to season and from place to place. A knowledge of 

 such changes was obviously of the first importance in seeking to under- 

 stand the migrations of the fishes and the fiuctuations of the fisheries. 

 The international programme therefore provided for a co-operative 

 study of these phenomena over the whole area under investigation. To 

 tiie English investigators was assigned the study of the waters of the 

 English Channel, which were important not only in connection with the 

 fisheries in the Channel itself, but also as constituting one of the sources 

 of origin of the waters of the North Sea. 



It was arranged that simultaneous periodic cruises should be under- 

 taken by tlie difTereut co-operating countries, during which observations 

 should be maile at fixed stations scattered over the whole area, and 

 that these oljservations should be as far as possible supplemented by 

 others taken between the stations and in the intervals between the 

 cruises. There are three chief kinds of evidence which, when known 

 over an extended area, are capable of giving an indication of the course 

 of the prevailing currents, and the probable origin of the water at any 

 particular place. These are (1) the salinity of the water, (2) its tempera- 

 ture, and (o) the character of the plankton or minute fiouting organisms 

 which it contains. At each of the stations visited on the periodic 

 cruises, therefore, samples of the sea- water were collected at difl'erent 

 depths and brought to the laboratory, where their salinity was deter- 

 mined ; the temperature of the water at different depths was observed, 

 and samples of the plankton were collected for subsequent examination. 

 Intermediate samples were taken between the fixed stations, and frequent 

 meteorological observations were also made. 



Four quarterly cruises, as arranged in the international programme, 

 were carried out in the English Channel in February, May, August, and 



