APPENDIX A ■. PETTERSSON — 8 — 



Great Belt Observations of more than lo years, 1889— 1899, show the mean temperature in 



the depths of the Great Belt to vary as follows ^ : 



May 4°-3 C 



August .... 8°-i C 

 November. . ii°-5 C 

 Kattegat In the Kattegat, the fluctuation between a warmer undercurrent in the autumn and 



a cooler in the winter and early spring is very marked. 



All along the deep channel of the Kattegat lightships are anchored. The most 

 northerly of these is anchored off Skagen, the one farthest south, off" Schultz Grund, 

 just at the northen entrance to the Great Belt. On board these vessels hydrographie 

 obsei-vations have been made daily since the year 1881. Comparing the mean tempera- 

 tures in 23 m. depth, we find that: 



The temperature at the entrance to the Kattegat is about 4°-3 C. higher than at the 



entrance to the Great Belt during July— August — September. During October — November 



—December the temperature of the Belt-entrance is about o°7 C. higher than at Skagen. 



Heatwave in ^ heatwavc ftom the Skager Rak thus penetrates into the Baltic during the 



the Kattegat 



autumn. 



In the spring, the undercurrent carries cold northern Bank-Water through the Kattegat 

 into the Baltic. This annual fluctuation regulates the biology and fisheries of that sea. 

 In proof of this statement, we need only to mention the mackerel-fisheries of spiing and 

 summer and the herring-fishery with drift-nets in winter, which begins as soon as the 

 southern Bank-Water sets in. The first hint of the existence of such a fluctuation was 

 given in 1877 by the German investigators Möbius and H ein eke. These observers 

 communicated their discovery, that among the migrating fishes, fishes that periodically 

 Northfishes visit the Kattegat without spawning there, one group (10 species) only appear there during 

 soutrfllhes ^^^^ ^^^^ ?^^^ °^ "-^^ y^^'^' whereas another group (18 species) appear only during the latter 

 half of the year. Further it became evident, that the former group migrated from the 

 Norwegian Sea, while the latter had their origin from the Atlantic 2. This phenomenon 

 was explained in 1890 by the discovery of the annual hydrographie variation of the 

 Kattegat 3, In 1899, the connection between the fluctuations of the undercurrent, and the 

 thermal periodicity of the deeper layers was elucidated +. Since then, another phenomenon 

 of this kind has been discovered by the Danish biologist Dr. C. G.J oh. Petersen. Cod 

 above a certain size is scarce in the Kattegat in summer , but abounds there in winter. 

 There are, however, exceptions to this rule, of which later will be spoken. Generally 

 speaking, the relations between hydrography and fish-migrations have been first, and most 

 thoroughly, studied in the Kattegat. 



North Sea A clear conception of the growth of the Atlantic current in autumn can be formed 



Summer 'and ^o™ ^^e accompanyiug section (Fig. 4 and 5), which from the slope of the North Sea 

 Autumn ^ j^^ Knudsen. Beretning 1. c. 



2 Möbius und Heincke, Die Fische der Ostsee. Berichte der Commission zur Untersuchung der deut- 

 schen Meere, 1877— 1881. IL p. 278. 



3 O. Pettersson och G. Ekman, Grunddragen af Skageracks ocli Kattegats hydrografi. Kgl. Sv. Veten- 

 skapsakademiens Handl. Bd. 24. (1891). Nr. 11, p. 147. 



4 M. Knudsen, Beretning fra Kommissionen for videnskabelig Unders0gelse af de danske Farvande. 

 II. 1899. p. 49. 



