PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 259 



important factors. In any case it is important that Dickson concludes 

 from his observations that cold bottom water does How from the north 

 down the north-western depression of the North Sea, and this fact 

 corresponds to the prevalence of northern species of fish in that 

 depression and the east coast of Britain. 



(2) The migrations of the herring. Turning more particularly to this 

 difficult subject, I cannot profess, with the data at present available, to 

 give a complete explanation of these migrations. I propose merely to 

 point out some of the more obvious relations, in the hope that my 

 remarks may be of some use in directing attention to the conditions 

 which have to be investigated. 



On the east coast of Scotland, it appears at first sight that the 

 summer herring arrive and are present when the water is warmest. In 

 the northern part, for instance in the Moray Firth, the chief fishing is 

 in July and August. Further south it gets later, taking place in 

 August and September, while at Lowestoft it lasts from October to the 

 beginning of December. There is a mackerel fishing at Lowestoft in 

 September and October, so that during October, as I know from 

 personal observation, both mackerel and herrings are being landed 

 in numljers at the same time. But it must be remembered that the 

 mackerel are going away to the south, and herrings are arriving from 

 the north, and also that mackerel usually swim near the surface, and 

 herring near the bottom, or at some distance below the surface. It is 

 probable that at this time wheu the herrings visit the neighbourhood 

 of the Norfolk and Suffolk coasts, the bottom water is colder than it has 

 been during the preceding summer, in consequence of the inflovv of 

 bottom water from the north. This does not explain why the herrings 

 are absent in January, February, and March. But the herrings come to 

 spawn, and there is some evidence that they retire northwards into deep 

 water to feed. All that I would suggest is, that we do not know that 

 the bottom water, where the summer herring spawn, is at its warmest 

 when the spawning takes place of!" Lowestoft, although, according to 

 Dickson, the mixed Atlantic water which makes its way down the east 

 coast of Scotland in summer is warmer than the water it displaces. 

 The warmth of the infiowing water, however, is not very great, its tem- 

 perature is not above 9° C. We have seen that, according to the 

 observations of the Pomerania, the temperature at the bottom in the 

 northern part of the North Sea in summer is frequently below 8° C, and 

 we do not know what the winter temperature is. The hypothesis that 

 the arrival of herrings is connected with a greater supply of oxygen, 

 seems to be inconsistent with the fact that there is at all times of the 

 year such an abundance of bottom fish (haddock, plaice, cod, etc.) in the 

 places where the summer herring fishery is carried on. 



