288 THE VAUIATIUN, ItACES AND MIGRATIONS OF THE MACKEREL. 



to the coast-Hue. A moderately deep gully of more than 50 fathoms 

 depth separates Euglish from Irish territory. 



Accordiugly our division of British mackerel into an Irish and a 

 Channel race coincides with the geographical division of the sea-bed 

 by the 50-fathom gully which runs up St. George's Channel. 



On the other hand, the 100-fatliom line approaches the coast only off 

 the south-west coast of Ireland and separates a considerable plateau of 

 ground off the west coast of Ireland from the great quadrangular 

 plateau which lies to the south of Ireland, and the southern boundary 

 of which coincides with the latitude of Ushant. 



Deeper contour lines, up to 1000 fathoms, leave the boundary of the 

 southern plateau practically unaffected, but considerably increase the 

 area of that off the west coast of Ireland, which at 200 fathoms includes 

 the Porcupine Bank, a distance of 250 miles from the mainland. On 

 the other hand, the study of deeper contour lines renders still more 

 evident the geographical distinctness of the two plateaux, which are 

 connected merely by a narrow shelf off the south-west coast of Kerry, 

 50 miles wide at 100 fathoms, and only 120 miles wide at 1000 

 fathoms. Thus the separation of the Irish race into a west coast and 

 south coast stock coincides with the division of the Irish submarine 

 plateau into a western and a soutliern portion by the deep 1000-fathom 

 rift which has been described. As these banks probably form the 

 winter quarters of the mackerel of the adjoining coasts, we can under- 

 stand how the segregation of the Irish stocks has been induced, and 

 how a certain amount of mixture between the Kinsale stock and the 

 Channel race has taken place. The southern Irish stock is more closely 

 related to the western Irish stock than to the Channel race on account 

 of its proximity to the former during the breeding season. But it 

 approaches the Channel race in character because it shares the same 

 submarine plateau for its winter quarters ; and, although this area is too 

 large to bring about complete mixture of the two races, it is not large 

 enough to prevent a certain amount of mixture from taking place. The 

 annual amount probably depends upon the severity of the winter season, 

 which determines the extent and depth to which the fish retire from 

 the shore. 



