26 MARINE FISHERIES OF NORTH CAROLINA 



coast at this point (Figure 2) and warms the Cape waters in spite of the 

 seasonal cooling of shelf waters to the north and south. ^° 



Parr (1933) plotted the individual temperature averages for five-day- 

 periods at the Diamond Shoal Lightship off Hatteras (Figure 10) and noted 

 that by far the widest fluctuations occur during winter, early spring, and late 

 fall, while there is little deviation from the normal during the summer. This 

 is what would be expected in a region warmed by the near-by Gulf Stream 

 System, which is forever shifting position. In contrast to this, a station such 

 as Five Fathom Bank Lightship off Delaware Bay, which is influenced by 

 seasonal temperature changes that affect water very slowly, shows a much 

 smoother record as plotted on the same graph. The fluctuations at Hatteras 

 are very significant ecologically in that: 



1. They subject the biota to pronounced short term fluctuations, which 

 are generally difficult for organisms to adapt to, even though the gross aver- 

 age temperatures indicate year-around warmth. 



2 . They may offer temporary breaks in the barrier effects of the average 

 temperature conditions. 



From the above discussion it is apparent that with increased proximity to 

 the Hatteras region there is a decrease in seasonal change, also an increase in 

 temperature fluctuations during winter. Much the same effects must also 

 occur with increased distances from shore along the continental shelf to the 

 southwest of Hatteras, where the Florida Current lies at the shelf's edge (see 



10. The following extracts from the "Summary of Annual Geographic Temperature Cycle" 

 on pp. 61-62 of Parr's (1933) paper will, with the aid of Figure 8, orient the reader as to coast- 

 wise temperature relationships for the entire coastline; 



". . . we thus have seasonal temperature barriers established at Cape Hatteras during the 

 winter and in the neighborhood of Cape Cod during the summer, but the impression often given, 

 that there should be a set of more or less permanent temperature barriers at these two points, 

 is entirely erroneous and contrary to the facts. In the winter there is a free access for all the 

 migratory cold water forms to penetrate as far south as to the neighborhood of Cape Hatteras, 

 and in the summer the southern forms may move as far north as Cape Cod encountering only 

 a slow and gradual decline in temperature on the way. . . . 



"It is also suggested that the transition from the midwinter temperature conditions in the 

 southern section of the Atlantic coastal waters of the United States to the midwinter tempera- 

 tures of the Straits of Florida may take the form of a relatively abrupt warm front somewhere 

 in the neighborhood of the region between Jupiter Inlet and Cape Canaveral, Florida, rather 

 than the form of a gradual and equally distributed increase in temperature. On the basis of this 

 assumption we should, therefore, expect to find a critical and perhaps limiting point for the 

 winter migrations of subtropical species in the vicinity of Cape Canaveral. During the summer 

 the temperatures in the Straits of Florida are uniform with the shallow water temperatures to 

 the northward as far as the region of Cape Hatteras, from which point a gradual change begins 

 to become noticeable. [Author's note — The description by Green (1944) of a summer cold area 

 at Daytona (Figure 9) seems to indicate a summer barrier to northward coastwise migration 

 in the Cape Canaveral region in addition to the winter "warm front" described by Parr.] Although 

 no abrupt temperature barrier is to be found at Cape Hatteras during the summer time, it is, 

 therefore, nevertheless to be expected that this point will form the northern limit of distribution 

 for the stenothermal tropical forms during the warm season. 



"In the Straits of Florida we find the northern boundary of the Tropical American Seas with 

 a winter temperature above 70° F. in which the stenothermal tropical forms may remain all 

 year around." 



