BIOLOGY OF THE ATLANTIC MACKEREL 



163 



to be desired, present information supports the view that the present survey has cov- 

 ered the most important spawning ground. 



The existence of large regions with little spawning near the middle of the spawning 

 range of the species is a peculiarity that may be explained by hyclrographic conditions. 

 It will be noted from the diagrammatic representation of relative intensity of spawning 

 in figure 3 that the regions of greatest intensity are the southern and northern quarters 

 of the spawning range. That of the least intensity is the middle balf of the range. 

 The places of intense spawning, that is, the great oceanic bight between Cape Cod 



Figure 3.— Relative intensity of mackerel spawning in various regions along the Atlantic coast of North America, as indicated 



by the average number of eggs caught in plankton nets. 



and Cape Hatteras, Cape Cod Bay, and the southern half of the Gulf of St. Lawrence, 

 have this in common: they are all bodies of relatively shoal water overlying relatively 

 flat bottom, where topography and circulation favor vertical stability, and vernal 

 warming of the upper strata proceeds rapidly, producing temperatures suitable for 

 mackerel spawning earlier than in the intervening areas. On the other hand, the 



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