80 LIFE IN THE SEA [CH. 



European seas; and the Arctic tern, which breeds 

 only in the Arctic Circle may yet be found in Antarctic 

 seas. 



But the area of optimal conditions varies with the 

 seasons and we therefore find that the productive 

 distribution of most species fluctuates within certain 

 limits. Flounders live under optimal conditions in 

 the brackish waters of the estuaries when the tem- 

 perature of the water there is higher than it is in the 

 open sea offshore ; but whenever the first winter snows 

 begin to melt and form freshets in the rivers then the 

 fish begin to migrate offshore, where the temperature 

 is now higher than it is in the estuarine areas. The 

 cod which come into British waters during the winter 

 have migrated down from more northerly latitudes 

 because when the temperature of the sea was falling 

 there it was a little warmer to the south. The sea-perch, 

 mackerel and gar-fish (Belone) are species which have 

 their area of productive distribution to the south-east 

 of the British Isles, in the Mediterranean, and off 

 the coasts of southern Europe; they migrate into 

 northern waters because the flooding of the latter 

 with Atlantic water in the spring and the seasonal 

 rise of temperature have enlarged their area of distri- 

 bution. Plaice from more southern latitudes invade 

 the Barentz Sea to the north of Europe in the autumn 

 because the temperature of the bottom water there 

 has been raised by the inpouring of water from the 



