PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 23o 



The cause of the difference in the amount of the influx cannot be 

 other than the periodic variation in the outflow from the Baltic. 

 When the Baltic outflow decreases, coast water flows into the Skage- 

 rack, where it is found as a thick and relatively warm surface layer 

 in the coldest months of the year. When the outflow of Baltic water 

 increases in spring and summer, the coast water is swept out of the 

 Skagerack again, and at the same time the deeper waters begin to flow 

 in and swell in volume. Professor Pettersson attributes the latter 

 inflow to a reaction upon the deeper strata in the North Sea, due to 

 the energy stored up in the waters of the Baltic, but I must confess 

 that for my comprehension these expressions require further explana- 

 tion. The annual variation in the Skagerack affects the water to a 

 depth of about 50 fathoms. 



The temperature of the North Sea water varies inversely with that of 

 the season ; it is the coldest water of the Skagerack in summer, and the 

 warmest in winter. 



The North Sea water varies much in the amount of dissolved oxygen 

 which it contains ; in July, 1890, it was very deficient in oxygen, while 

 in September, 1893, it was supersaturated with that gas, a condition 

 which has only been found to occur in surface waters from high 

 latitudes. 



The North Sea water begins to flow into the Skagerack in May, and 

 its entrance coincides with the commencement of the mackerel fishery 

 on the Swedish coast. There seems to be a certain connection between 

 the expansion of the volume of 34 per cent, water in the Skagerack, 

 and the appearance of the mackerel and gar-fish. 



The temperature of the coast water, on the other hand, and still 

 more of the Baltic water, varies with that of the season. 



The coast water may flow into the Skagerack from two directions, 

 namely, either from the south along the coasts of Denmark and 

 Germany, or from the north along the coast of Norway. This is 

 important, because there are reasons for believing that the coast water 

 has two periods of influx. The first influx occurs in August and 

 September, and is due to the influence of westerly gales. At this time 

 of the year warm water, whose temperature reaches 15° or 16° C, and 

 whose salinity is 32 to 33 per cent, sets in along the north-west coast 

 of Jutland. It fills the central part of the Cattegat from top to 

 bottom as far to the south as a point between Trindelen and Anhalt, 

 and then dips under the Baltic water. 



In early autumn the herring fishery, with floating nets, in the 

 Cattegat and south Skagerack, coincides with this influx of coast 

 water. In a subsequent part of his paper, Pettersson points out that 

 Mobius and Heincke, in their memoir on the Fishes of the Baltic, 



