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a considerable part of the year, and a plausible explanation of occasional dearth was 
furnished by the suggestion that the fish in such poor years neglected, wholly or in part, 
to visit the usual grounds. 
In the course of the extensive herring fishery which for hundreds of years has been 
carried on by Dutch and Scottish fishermen in the North Sea, it was noticed that the 
herring came from a northerly direction. In Norway also, it was observed that the great 
shoals of herrmg which arrived during the winter in order to spawn, came from the north- 
west. As AxEL BoEcK*) has pointed out, this fact gave rise to the idea that the herring 
annually migrated to the Arctic, where they remained during that part of the year in 
which they were not to be found in the waters nearer at hand. The well known theories 
advanced in England by Dorr (1728) and in Germany by JoHAN ANDERSON, Burger- 
meister of Hamburg in 1723, to the effect that the herring annually migrated from the 
Arctic to the coasts of Europe and America, were generally accepted by the majority 
of writers in the 18th and some even in the 19th century, the same hypotheses being 
mentioned by OKEN, BLUMENBACH and Cuvier, (as for instance in Cuvier’s “Règne 
Animal”, second edition, 1830). 
Boeck mentions also another suggestion, put forward by the American writer GIL- 
DING, (American Philosoph. Soc. Transact., Philadelphia 1786) according to which the 
migrations of the herring were considered to be influenced by the declination of the sun, 
the fish being supposed to regulate their movements in order to avoid extremes of heat 
and cold. According to this writer, the field of migration of the fish embraced no less 
than 47 degrees of latitude, extending longitudinally from the coasts of Europe to those 
of America. 
Probable extent of migration essentially reduced by scientific investigation. 
Such theories as the foregoing with regard to the extent of these migrations have 
been rendered untenable by the methods applied by modern science to the study of 
marine biology. In the course of numerous research expeditions it has been ascertained 
that the area of distribution of North-European fish, including the herring and cod, 
is bounded on the south by a line drawn close to the south-west coast of Ireland, and 
that as regards the northern limit these species do not occur in the Arctic. The coastal 
banks in the Atlantic waters of western Europe south of the Channel have an entirely 
different fauna to the waters lying north of the limit above mentioned. The Atlantic 
Ocean has its-own peculiar pelagic fauna, differing entirely from the pelagic species 
encountered in the waters of northern Europe. Roughly speaking, the limit of occur- 
rence of boreal species on both sides of the Atlantic, at any rate as regards the coastal 
banks, may be said to coincide with the isotherm for 10° C. (Fig. 1). Of all the species 
which form the object of fishery in northern Europe, one only, viz. the eel, has been 
shown to extend its migrations across the deep waters of the open Atlantic. Future 
research may possibly demonstrate the existence of a similar extended sphere of move- 
ment in the case of the mackerel; for the present, however, the facts, as far as known, 
do not appear to favour the theory of such ocean migration. 
*) Axeı Borck. Om Silden og Sildefiskerierne; Christiania 1871. 
