— IX — 



HOEK— INTRODUCTORY 



any which penetrate into brackish waters in order 

 reproduce in waters of high salinity. 



to spawn, from those which 



These two forms (herring and cod) have therefore to be closely investigated, 

 so that, firstly, a true picture may be obtained of their migrations in relation to their 

 development, life-history and reproduction, and secondly, that light may be thrown 

 on the general problem of migration. That these things are of the greatest im- 

 portance in the economical working of the fisheries, appears from the great value, 

 of the fishes mentioned; the more we understand of their natural history, the sooner 

 we shall succeed in reaching a rational fishing of the extremely valuable fishing 

 grounds. 



From the beginning of the international investigations, a valuable working- 

 hypothesis, as explanation of the migrations of fishes, has been at the disposal of 

 naturalists, namely, the hypothesis of the Scandinavian naturalists: Pettersson, 

 Ekman and Cleve. According to this, it is the marine currents, arising from 

 the reciprocal action and opposition of the oceanic Gulf Stream, the cold Polar 

 Stream coming from the Arctic Ocean and the freshwater effluents of the continent, 

 which are of first importance for the explanation of the distribution of the migra- 

 tory fishes, just as they are of extreme importance and even of essentially 

 predominating influence, for the climate 'of middle and northern Europe. As is 

 well-known, this hypothesis of the close connection between the migrations of fishes 

 and the oceanic currents has been elaborated by these authors with great care for 

 the exceedingly interesting case of the Bohuslän herring. The international inves- 

 tigations accomplished up to the present, do not yet lead to any conclusion on 

 this point, so far as can be judged from the results to hand. 



That the physical conditions of the sea, temperature, salinity, currents etc. are 

 of extraordinary importance for the migrations of fishes is generally admitted — 

 how close the connection is, however, if any parallel or pei'haps causal connection 

 exists between the phenomena, is still a matter over which opinions differ. Mean- 

 time, Prof. Pettersson has endeavoured (see, Appendix A to this General Report) 

 to give a comprehensive sketch of the periodicity in the fluctuations of the Atlantic 

 Stream and their connection with meteorological and biological phenomena, espec- 

 ially the migrations of fishes. This will certainly contribute to throw light upon 

 these difficult points. 



Our knowledge of the life-history and development of the principal fishes 

 coming into consideration here, has also made great advances, however, in other 

 directions. This holds especially for the cod, as appears from the fisheries inves- 

 tigations carried out by Dr. Joh. Hjort and Dr. C. G. Joii. Petersen, especially in the 

 Danish and Norwegian waters. So far as it concerns thp cod, this paper (Ap- 

 pendix G) may also be regarded as a preliminary report over the work of Com- 

 mittee A conducted by Dr. Hjort. 



These authors have been able, more especially from their own investigations 

 and those of Dr. Jon. Schmidt in the waters of Iceland and the Faeroes, but also 

 from the observations of other members of Committee A, to give a comprehensive 

 picture regarding the spawning of the cod, its spawning time, spawning places, 



Pettersson- 

 Ekman's 



hypothesis 



Hjort-Pctcrsen's 

 investigations 



