REPORT OF COMMISSIONERS OF INLAND FISHERIES. 51 



the mackerel of the British Isles, the difference between the Irish 

 and the Channel fishes makes it extremely probable that no extended 

 coastwise movements can take place; each race of fishes must have 

 its own winter home, its own spawning grounds and its own distinct 

 course of migration, and these must all be so far distinct as to pre- 

 vent any considerable intermingling of fishes of the different races. 

 The geographical configuration of the ocean bed lends further proba- 

 bility to these conclusions. Reference to a contour map of the 

 British fishing grounds show that off the southern and southwestern 

 shores of the British Isles are three submarine plateaus, separated 

 from one another by deep depressions; one of these plateaus is adja- 

 cent to each of the three localities frequented by the separate races 

 of mackerel. On these plateaus in the deeper layers of water the 

 temperature is probably favorable for the mackerel during the 

 winter. The deep depressions between the plateaus apparently 

 form relatively impassable barriers to the mackerel, and thus delimit 

 the area normally inhabited by them. This geographical distinct- 

 ness of their winter habitat probably effectually prevents inter- 

 mixture of the different races while away from the shores during the 

 cold season. But in order to completely preserve the purity of the 

 local varieties the spawning grounds of each race must also be 

 separate from that of the other races and constantly in nearly the 

 same locality from year to year. It is well known in the case of 

 many kinds of fishes that they instinctively return to the waters in 

 which they were spawned. This is doubtless true of the mackerel, 

 and would suffice to prevent intermixture of the local races during 

 the breeding season. With regard to the course of the migrations, 

 they can probably take place only through comparatively short 

 distances; otherwise the chances for intermixture increase and the 

 different races could hardly be maintained. " Indeed the only migra- 

 tion which can, for the most part, be conceded, are migrations from 

 shallow to deep water off the same coast." There is one exception 

 to this, which proves the rule. The North Sea mackerel are racially 

 identical with those of the Channel, and there must therefore be a 



