2G0 PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 



We have seen that Pettersson traces a distinct connection between 

 the herring fishery in the Skagerack and Cattegat, and the inflow of 

 coast water. But there appear to be two periods of inflow, one from 

 the south of warm water 15° C. to 16° C. in temperature in August and 

 September, and one from the north of cold water 4° to 5° C. in January, 

 February, and March. Herring fishery is associated with both of these, 

 but principally, it would appear, with the northern water, which con- 

 tained northern forms of plankton. It is well known that there are 

 winter spawning herring in various localities, which must be considered 

 to be races quite independent of the summer spawners. With regard 

 to the relation of the fish to temperature, it is suggestive that on the 

 south-west coast of England, in the neighbourhood of Plymouth, there 

 are no herrings in summer or autumn, but only from about the end of 

 November till the end of February, that is at the time when the water 

 is coldest. 



Dr. John Murray considers that there is evidence that the herrings 

 of Loch Fyne and the Firth of Clyde reside there permanently, and 

 do not merely make periodical visits, and believes that they feed 

 chiefly in the deep depressions near the bottom. Whether these are 

 the herrings which spawn on the Ballantrae Banks in early spring, we 

 cannot definitely decide at present. But enough has been said to show 

 that the introduction of Atlantic water with a greater supply of oxygen 

 is not a sufficient explanation of the annual migration of summer 

 herrings into the North Sea, and that probably some important and 

 interesting discoveries have yet to be made concerning the relation 

 between the food, breeding, and movements of herrings, and the 

 temperature of the water in which they are found at different seasons. 



(.3) Different sizes of fish of the same species at different parts of its 

 habitat. Mr. Holt's observations, together with my own, as published in 

 previous numbers of this Journal, have shown the different sizes of 

 plaice in (1) the northern and western part of the North Sea (2), on 

 the south coast of Iceland (3), in the southern shallow part of the 

 North Sea and in the English Channel. In the two latter cases the 

 difference has been precisely exhibited in the lengths of the smallest 

 mature and largest immature specimens. This is probably the best 

 method of testing the matter, for the average size of mature specimens 

 as a standard is liable to the objection that it depends on the extent to 

 which older and larger fish are captured. Mr. Holt's observations in 

 the Journal, and Petersen's * in the Annual Eeport of the Danish 

 Biological Station, refer to small races of plaice in the Baltic. There 

 are three points to be taken into consideration in relation to these 



* Dr. C. G. Joh, Petersen, the Danish biologist, is not to be confounded with Prof. 

 Pettersson, the Swedish hydrograplier. 



