RECENT HYDROBIOLOGICAL 

 INVESTIGATIONS 1 



THE GULF STREAM— AND SEA FISHERIES OF 

 NORTHERN EUROPE 



By JAMES JOHNSTONE, B.Sc. 



Fisheries Laboratory, University of Liverpool 



The Marine Fisheries and Hydrographic Research 



The evidence that the productivity of the marine fisheries 

 varies according to the physical conditions of the sea during 

 the same, or preceding, years is now fairly strong. Apparently 

 there are various ways in which physical changes in the sea- 

 water may affect the abundance of fishes, (i) There are 

 optimum conditions of temperature and salinity for many 

 species of fish, and these conditions restrict their distribution. 

 But changes due to the periodicity of the Gulf Stream drift, 

 or the annual sea-temperature-wave itself may enlarge or restrict 

 a sea-area exhibiting uniform, or approximately uniform, con- 

 ditions as regards temperature and salinity. With the variation 

 of this sea-area so will the distribution of certain species of fishes 

 be affected. The annual summer migrations into the Irish Sea 

 of the Bass {Labrax lupus), the Garfish {Beloue vulgaris), and the 

 Mackerel are probably instances of this kind of fish-migration. 

 These fishes normally inhabit a sea-area to the south of the 

 Irish Sea, and as the temperature in the latter rises during 

 the summer they migrate to the north. (2) Certain conditions 

 of temperature and salinity appear to favour the spawning 

 acts of some fishes, and when the annual spawning period 

 approaches, such species migrate in order to reach the sea- 

 areas characterised by these physical conditions. Cod, for 

 instance, are present to some extent in the Irish Sea throughout 

 the entire year, but they enter this sea in greater numbers in 



1 The preceding portion of this article, " The Gulf Stream — and Climate 

 and Crops in Northern Europe," appeared in Science Progress, January 1910, 



4,474-9L 



612 



