._ 7 _ APPENDIX A : PETTERSSON 



ßornholmdeep Undercurrent 



of the Baltic 



Nov. 6t'' 1893. i6°2'Long. E. 55=23' Lat.N. Oct. nth igoi. i5°39' Long. E. 55=20' Lat.N. 



Tlie above tables show this phenomenon, as observed in the autumns 1893 and 

 1901. The situation is probably that of normal years, although exceptions are on record. 

 During the invasion of this warm undercurrent, the biology of the sea assumes some 

 interesting features. East and west of Bornholm, the trawl brought a quantity of flat fish 

 to the surface, among which spawning plaice 260 — 263 mm. in dimension and flounders 

 of 270 — 280 mm^ were found. The eggs of these fishes were found floating in depths, 

 where the water was supplied directly by the undercurrent. A number of flatfish in 

 spawning condition were again found by the German fishery-expedition December 1903 

 in the southern Baltic. On the contrary, all experiments with the trawl in July and 

 August, when the cold bottom water still fills this depression in the bottom of the Baltic, 

 have given a poor return of flat fish, of which the majority were out of spawning con- 

 dition or not yet ripe for it. The biology of the deep layers of the southern Baltic is 

 thus regulated by the hydrographie conditions, especially with regard to the immigration 

 and spawning of flat fish from the Kattegat. 



The undercurrent must enter the depression by the sound between Scania and Born- velocity 

 holm, its passage south of that island being barred by shoals, such as the "Adler-Grund" un^^jrJurrjn, 

 etc. In October 1901 measurements in 50 m. depth in the Sound between Scania and 

 Bornholm showed that the water flowed into the inner Baltic with the extraordinary 

 velocity of 57 cm. a second^. The surfacelayer (o — 40 m.) moved in the opposite direc- 

 tion with a velocity of 19 — 23 cm. a second. 



In February, the warm waterlayer of the deep basin east of Bornholm is generally 

 replaced by cooler water. In some years, however, as in, 1896, 1898, 1899, the deep 

 layers keep their high temperature longer. 



1 P. Trybom, lakttagelser om fisk m. m, trän SkagL'rack och Kattegat i August! 1901 ocli Fcbruari 

 1902 samt frân Östersjön i Oktober 1901. p. 10. Svenska Hydrografisk Biologiska Kommissionen.s Skrifter. 

 Hafte I. 1903. 



2 Hvdrographic-biological Researches etc. bv P. T. Cleve and O. Petter.sson. p. 12. Svensk Hydrogr. 

 Biol. Koramiss. Skrifter. Hafte I. 1903. 



