THE SALT-WATER FISHERIES OF BOHUSLAN. 215 



period. The chief difficulty consists in finding a '■^primus motor'''' or the 

 original cause wbich makes the great races of herrings move ; and until 

 a better explanation is found I would assign as this cause the change of 

 biological conditions produced by the great size of a race of herrings, 

 and a supposed periodicity of meteorological and hydrological causes, 

 and possibly a combination of both. 



63. The other and perhaps simpler way of explaining the periodicity 

 of the herring-fisheries by the size of the race of herrings, may be reached 

 by considering the very evident effect of this cause, viz, that the herrings 

 are compelled to seek their food on a larger territory, farther from the 

 coast, and more dependent on the changes of weather and current ; and 

 here Prof. G. 0. Sars^s theory regarding the visits of the herrings at dif- 

 ferent times during a fishing-period would come in well. In accordance 

 with this theory it might well be supposed that the herrings would finally 

 have to seek their food at such a distance that they could not reach their 

 old spawning-places at the right time, but would have to select other 

 spawning-places which were within easier reach. But as the herrings 

 chiefly live on small crustaceans floating about in the water, we must, in 

 following this theory, suppose a change in the occurrence of this " herring- 

 food," which could scarcely be explained except by a periodicity of the 

 currents and by the changes in the weather which principally jiroduce 

 this periodicity. No one has so far, however, been able to show the 

 existence of such a periodicity, although it has been supposed to exist, 

 and although there are facts which jjoint in this direction. This hypo- 

 thetical explanation is, therefore, nothing but a further development of 

 the old opinion that the periodicity of the herring-fisheries is caused by 

 physical changes, and its chief merit consists in indicating by the very 

 point from which it starts the cause why not all herring-fisheries are 

 periodical in consequence of these changes. 



It will be clear, liowever, that this explanation can easily be harmo- 

 nized with the regular changes of time and place in the so-called "land- 

 ing " of the herrings, and this consideration should by no means be lost 

 sight of. When the herrings are near the coast they can also land sooner 

 and go farther along the coast (in Bohusliin and Western Norway farther 

 south) than when they are far from the land in the open sea. Eegular 

 changes in the one will, therefore, also produce regular changes in the 

 other. It will be more difficult, however, to explain in this way the ex- 

 ceptions from this regular course of changes in the fishery during a 

 fishery-period. And such exceptions have occurred both during the last 

 Norwegian spring-herring fisheries and during the latter part of the last 

 Bohuslan fisheries. This theory may also be further developed by com- 

 bining it with the other theory that the one race of herrings has to give 

 way to the other so that the great races of herrings would be uninter- 

 ruptedly moving backward and forward. 



64. If, as I have supposed, two great herring-fisheries should be inti- 



