APPENDIX A ; PETTERSSON 



24 — 



Migrations of 

 the lierring 



has a temperature of 6° — 7° C. During summer this mixed water is further heated on 

 the comparatively shallow coastal plateau of northern Iceland till its temperature measures 

 5° — 6° C at the surface and 2° — 3° C. at 200 m. depth. During June — August the fish 

 and fish-brood travelling with the Irniinger current to the north of Iceland find thermal 

 conditions almost equalling those of the south-side during springtime. Besides nourishment 

 is here in abundance consisting partly of plankton, partly of arctic fishes such as the 

 "Lodde". As soon as winter sets in the hydrographie situation in the borderregion of 

 the polar current is entirely changed, a sufficient ground from which to explain the 

 wandering towards east and southeast of the North-fish. 



It is noteable, that the herring of the North-Sea area also migrates in this direction, 

 beginning at the Shetlands gradually passing the Dogger Bank and so on to the Skager Rak. 

 Finally the appearance at times of large shoals of herring off the Swedish westcoast in 

 winter should be born in mind. Since the commencement of the last herring-period in 



Fig. 15 



Fig. 16 



1877 and 1878 the hydrographie conditions prevalent in these parts at such times have 

 been the subject of a careful study. 



The winterherring is a North-fish not inhabiting the Atlantic water proper, but the 

 mixed water (Bank-water) of 32%o— 33%o salinity^. During the "herring-years" the 

 level of the Atlantic water was found to remain at comparatively great depths outside 

 the coastal shoals. These were covered by a layer of Bank-water some 30 — 40 m. in 

 thickness' which through the deep-channels entered into the fjords. Within this layer 

 the herring was always found. 



December 1896 when the herring-fishery failed the level of the Atlantic water had 

 risen so high close to the coast as to leave only a layer of about 7 m. thickness between 

 it and the surfacecover of diluted Baltic water. This is shown in the accompanying 

 hydrographie sections. 



Up to the present time the migrations of the fishes have been ascribed solely to 

 tendencies of food-seeking and propagation. The discovery of the connection of fish- 

 migration with hydrographie conditions is in no way inconsistent with this theory. The 

 fishspecies here in question have distinct periods of feeding and spawning. 



I The connection of tlie December-February Herring fisliery on the Swedish Sl^ager Rak coast with the 

 invasion of the coast-banks by this mixed water of 320/00- 33 °/oo salinity in 1878—79 was discovered by 

 G. Ekman; see G. Ekman , "Hydrografiska Undersölmingar, 1878—79" in; Bih. Göteborgs och Bohusläns 

 Ârskrift 1880 and the description in "Grunddragen af Skageracks och Kattegats Hydrografi" 1. c. p. 112— 

 122, by G. Ekman and O. Pettersson. 



