of the Fishery Board for Scotland. 



Ixvii 



Apart altogether from the ordinary phenomenon of high and low 

 tide, there is found to be a difference in the mean level of the North 

 Sea amounting on the average to about six inches between May and 

 December ; and even at the same season there may be a difference 

 in mean level from one year to another amounting to two or three 

 inches. Such changes of level must involve the movements into and 

 out of the North Sea of an immense body of water, and must appreci- 

 ably affect not only the currents, but also the composition and salinity 

 of the sea itself. It forms a part, therefore, of those hydrographic 

 studies which must be undertaken if we are ever to have a proper 

 understanding of the physical conditions of the sea, and of their 

 relation to the life-history and the fluctuating abundance of fish. 

 Among many other fluctuations in sea-level which are discussed in 

 this paper, there are three of particular interest. One of these is a 

 small tide with a period of about nineteen years. It depends upon a 

 well-known phenomenon in the moon's motion, by which the moon's 

 inclination to the ecliptic varies in a period of 18 J years. A small 

 tide of this period is accordingly set up, and, small as it is, its regular 

 19-yearly ebb and flow can be detected throughout the last fifty years 

 upon the Aberdeen tide-guage records. Another small tide is found 

 to have a period of about fourteen months. This is a very remarkable 

 tide, which was very recently discovered for the first time by certain 

 Danish observers. It depends upon a very small periodic movement 

 of the earth's axis, by which the whole earth wobbles (so to speak) 

 in its otherwise steady rotation, like a spinning top. Though this 

 little tide causes a rise and fall of less than an inch in fourteen months, 

 it can still be detected in both the Aberdeen and Dundee tide records. 

 Lastly, there is a very curious fluctuation, hitherto quite unknown, 

 by which the sea-level is found to have fallen steadily (though very 

 slightly) from about 1870 to about 1893, and to have risen with the 

 same steadiness since that time. The whole period of this fluctuation 

 must be at least fiity years, and the cause to which it is due is mean- 

 while quite unexplained. 



The S fawning of the Plaice and the Distribution of the Eggs. 



A Report, by Dr. Alexander Bowman, was published last year* 

 on the occurrence and distribution of the eggs of the plaice in the 

 northern parts of the North Sea, the result of the investigation of the 

 -collections made by the research steamer " Goldseeker " during the 

 months of November to March in the years from 1904 to 1913 inclusive. 

 This Report contains much valuable information on the life-history of 

 the plaice. It is shown that the northern part of the North Sea is a 

 spawning area of minor importance for this fish compared with the 

 southern parts of the North Sea, and that within the northern area 

 the Moray Firth is by far the most favourable locality for plaice during 

 the spawning season. The general study of the distribution of plaice 

 •eggs shows that the region within which they occur is well defined. 

 In the northern part of the North Sea, this region is bounded by a line 

 which runs almost parallel and adjacent to the 50-fathom line ; then 

 parallel to the coast of Scotland, from Rattray Head to the Firth of 

 Forth, including practically all localities with a less depth than 40 

 * Fisheries, Scotland, Scientific Invest., 1914, ii. (August 1914). 



