PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 249 



at the same time as the mackerel : in the Schelde it is taken with the 

 anchovies in May and June ; I have seen it at Brightlingsea in the 

 same two months, and I also saw several specimens brought in with 

 the mackerel at Lowestoft on October 12. The saury-pike {Scomhresox 

 saurus) has habits similar to the gar-pike, but occurs only occasionally, 

 apparently not every year. Van Beneden states that he saw a shoal 

 swim ashore near Ostend. The anchovy fishery in the Zuyder Zee 

 and the Schelde takes place principally in June and July, although 

 young specimens are found there for some time afterwards. They 

 appear to depart entirely before the end of October. 



There can be little doubt, I think, that the annual incursion of these 

 migratory fishes into the North Sea along the continental coast depends 

 primarily on temperature. In the regions we are considering the depth 

 is less than 20 fathoms, and there can be little question of difference 

 between bottom and surface temperature. In February, which is the 

 month in which the water is coldest, as we have seen the west side 

 of the North Sea is at G°, the east side, east of the Texel, 4° to 5°. We 

 may consider that this is cold enough to drive out mackerel and the other 

 species mentioned. At this time the limit of 9° C. lies to the west 

 of the western entrance of the Channel. Mackerel in January and 

 February are caught in the western part of the Channel as far east as 

 Start Point. In May the limit of 8° has advanced eastwards and runs 

 across the North Sea from the southern part of Denmark to the Firth 

 of Forth. In August temperatures above 13° prevail over the whole 

 of the North Sea, and in November we find the cold again advancing 

 from north to south and from east to west. 



These surface temperatures do not, however, explain the absence of 

 the fish in question from the coast of Britain in summer. The restric- 

 tion of the fish to the continental coast may be due to two causes: firstly, 

 a movement of water in that direction ; secondly, the extension of the 

 deeper portion of the North Sea along the east coast of Britain. The 

 salinity of the water has little to do with the matter. We have seen 

 that in 1894 the oceanic water was more extended in February than at 

 any other time of the year. Concerning the question of an infiow of 

 water from the Straits of Dover, this distribution of surface salinities 

 does not indicate that it is greater in summer than in winter. The 

 report of an expedition for physical investigation, to which no reference 

 has yet been made, namely, that of the Pomerania in 1872, does 

 mention that the outfiow of the warmer water from the southern area 

 of the North Sea was traced as a current which flowed along the coasts 

 of Schleswig-Holstein and Jutland to the Skagerack. This was in 

 summer, but it seems to me that the surface salinities indicate that this 

 current is due rather to the greater outflow of fresh water from the 



