PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 255 



being- warmed by the sun. It is therefore as warm as the Gulf Stream 

 surface water from the Atlantic, and it seems to me that this fact is the 

 chief condition determining the annual arrival of mackerel, gar-fish, 

 scad, and anchovy on the south-west coast of Norway, and at the 

 entrance of the Baltic. 



The southern forms which have been mentioned occur also to some 

 extent along the east coast of Scotland, but more commonly towards 

 the north. It is therefore evident that they come round the northern 

 end of Britain, and travel southwards. Here, again, detailed informa- 

 tion as to the duration of their stay is at present deficient. It is stated 

 that mackerel do not appear at the Orkneys till July, and in the JMoray 

 Pirth are most abundant in August. The gar-fish, skip-jack, scad, and 

 anchovy are all also recorded as occurring in the Moray Firth and on 

 the east coast of Scotland, the three former as far south as St. Andrew's 

 and the Firth of Forth. Others of the southern species which have 

 been mentioned also are taken as isolated individuals, or in small 

 numbers, as far south as St. Andrew's, and only in summer and autumn. 

 Trigla hirundo, for instance, has been once taken at St. Andrew's, while 

 Pagellus centrodontus is said to be not uncommon there. Zeus fahcr, 

 the dory, is rare in that locality. Labrus macidatus occurs, and the 

 only other wrasse is Crenilahrus melops, which is rare. The southern 

 forms are scarcest or altogether absent between the Firth of Forth and 

 the Wash. 



Distribution of Northern Species. 



The northern species, that is, species whose range extends beyond 

 the Arctic circle but not into the Mediterranean, which occur in the 

 Western Baltic, are chiefly littoral species, or fundicolous species, 

 inhabiting moderate depths. The herring is the chief exception, being 

 almost the only aperticolous species in the list. The species, as given 

 by Mobius and Heincke, are : Cottus scoiyius, Cyclo2)terus lumpus, 

 Centronotus gunnellus, Zoarccs viviparus, Spinachia viUgaris, Gadus 

 morrhua, Gadus aeglefinuSy Gadus merlangus, Ammodytcs tohiamts, 

 Pleuronectcs platessa, Pleuroncdcs limanda, Flcuronedes microccphalus, 

 Ilippoglossoides limandoides, Clupecc harengus. Less abundant are : 

 Cottis huhalis, Agonus cataphradus^ Liparis vidgaris, Gadus po/lachius, 

 Lota molva, Motella cimhria. The occasional immigrants are : 

 Anarrhichas lupus, Gadus virens, Flcuronedes cgnoglossus, Ilijjpoglossus 

 vulgaris, Liparis Montagui, Stiekaeus iskmdicus (Lumpenus lampdrac- 

 formis), Brosmius brosme, Raia radiata. 



As we have seen, Pettersson points out that these northern im- 

 migrants are taken in the Baltic early in the year, from February to 

 April, at which season the Baltic outflow has ceased, the force of the 



NEW SEKIES. — VOL. IV. NO. 3. T 



