24S PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 



Heincke places the mackerel amonf^ the common resident forms, and 

 in general only considers the comparative abundance and rarity of the 

 forms, while I am endeavouring to trace the relation between the 

 temporary and permanent presence of certain species on the one hand, 

 and temporary and permanent physical conditions on the other. The 

 anchovy, according to Heincke, has not been yet found at Heligoland, 

 but we know that it is abundant in summer in the Zuyder Zee, and 

 Ehrenbaum found its pelagic ova further to the east near the Island 

 of Nordeney. 



We must remember that these forms might reach the Heligoland 

 Bight from the north, since all of them occur in the Skagerack and 

 Western Baltic. It is probable that the hake does come from this 

 direction, as it is a deep-water fish, and has scarcely ever been taken 

 in the shallow part of the North Sea south of the Texel ; it is 

 not mentioned in Van Beneden's "Fishes of the Coast of Belgium."* 

 It is not common in the Heligoland Bight, being placed by Heincke 

 among the rarer residents. The tunny is still more rare, and is classed 

 by Heincke as an occasional visitor. It probably comes both from the 

 south and the north, as it has been occasionally taken on the coast 

 of Kent and of Norfolk. But on the whole, as it is an oceanic fish, 

 it is more likely to reach Heligoland from the north. The other four 

 species are more important, because they visit the region in question 

 in greater numbers, and they undoubtedly come from the south through 

 the Straits of Dover, appearing regularly every summer in the narrow 

 southern part of the North Sea between the coasts of Holland and 

 Belgium and the English coasts, as well as on the south coast of 

 England. The anchovy, however, is practically absent from the 

 English side north of Kent. 



All these four species are so scarce as to be practically absent along 

 the east coast of England, but become somewhat more abundant 

 towards the north along the east coast of Scotland, owing clearly to 

 their incursion from the north. 



Although more complete observations are required concerning the 

 time at which these fish are present in the southern and eastern part 

 of the North Sea, we have evidence that they occur there only in 

 summer and autumn, and disappear in winter. Mackerel fishing at 

 Lowestoft begins at the beginning of May, and lasts till the end of 

 June, and there is a second or autumn fishery in September and 

 October. It is difficult to understand why these fish are not present 

 in the same neighbourhood in July and August. Van Beneden states 

 that the scad appears regularly on the coast of Belgium towards the 

 end of April, before the arrival of the mackerel. The gar-fish occurs 

 • AUmoires de VAcad. Roy. de Belgique. Tom. XXX VIII. 



