320 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



crossed in the offing of Portland, Me., in 1913, 

 and a few miles farther eastward in 1 923 (Bigelow 

 1927, p. 589, fig. 47). A question of more general 

 interest, from the ecological standpoint, is whether 

 the higher summer temperatures of recent sum- 

 mers, as compared with those of the earlier years 

 of record, have appreciably weakened the barrier 

 to the eastward spread of warm-water animals 

 that was formerly set by the low surface tempera- 

 tures that extended southward across the shelf 

 from the elbow of Cape Cod and from the region 

 of Nantucket Shoals. (For a general discussion 

 of this temperature barrier, see Parr 1933, pp. 

 26-34 and 87.) 



There is no apparent reason, so far as summer- 

 early autumn temperatures are concerned, why 

 any species able to maintain itself in the upper 

 20 meters or so of water along the coast westward 

 from Cape Cod during the first decade of the 

 present century should not have been able to 

 do so in the southwestern part of the basin of 

 the Gulf and in Cape Cod Bay in 1953, for maxi- 

 mum surface temperatures were about as high, 

 there that year as they had averaged at Woods 

 Hole, in Vineyard Sound, or in Buzzards Bay, 

 during the period 1902-07 (table 22). Indeed, 

 it had been known long before the upward shift 

 took place that certain shallow, partially enclosed 

 basins on the coasts of New Hampshire and of 

 Maine — where the renewal of water from outside 

 is slow — are warm oases, so to speak, where 

 more-or-less permanent populations of warm-water 

 animals exist, which do not range regularly north 

 of Cape Cod along the open coast. The oysters 

 that support a local fishery in Great Bay, N. H., 

 and the hard clams (Venus) of Casco Bay and 



Table 22. — Surface water temperatures, in southwestern 

 part of Gulf of Maine and westward from Cape Cod, in 

 195S and in 1902-07 



1 For details, see text above. 



2 From Sumner, Osbum, and Cole, 191.1a; pp. 39 and 47. 



other Maine localities fall in this category, while 

 the oysters of the southern shallows of the Gulf 

 of St. Lawrence are a more striking example 

 often cited. 



The upward shift in summer temperatures 

 has involved the coastal belt along southern New 

 England as well as the waters north and east 

 from Cape Cod, the difference between the 

 southwestern part of the Gulf and westward from 

 Cape Cod being of about the same magnitude 

 as formerly. This situation is illustrated by 

 mean temperatures for the 2 warmest months 

 combined for 1950 to 1953, of 65° to 67.2° F. at 

 Boston contrasted with 69.7° to 71.7° at Woods 

 Hole; also, by the distribution of the surface iso- 

 therms to the eastward and to the westward of Cape 

 Cod for early September 1953 (fig. 18). The 

 contrast, too, between higher temperatures to 

 the west and lower to the east, was not only 

 about as great outside the islands (4° to 10° at 

 the surface, 4° to 6° at 15 meters), in September 

 1953 as in the earlier summers of record, but the 

 transition from the one to the other in the general 

 offing of Nantucket Island and of Nantucket 

 Shoals was almost as abrupt (compare fig. 18 with 

 Bigelow 1933, fig. 35). No temperatures were 

 taken in 1953 on Nantucket Shoals, but it is a 

 matter of common knowledge that the surface 

 there may be as much as 7° to 8° colder in summer 

 than it is over the smoother bottom to the south- 

 ward. This difference is the result of the active 

 upwellings caused by the strong tidal currents 

 that run over the shoals and around them. (For 

 further details, see also Bigelow 1927, p. 595, 

 and Parr 1933, p. 31.) 



Continuity between the warm water of Nan- 

 tucket Sound and surface temperatures nearly 

 as high in the southwestern part of the Gulf 

 north of Georges Bank is similarly interrupted by 

 areas of low temperature, caused by strong tidal 

 currents running over the shoals that front more 

 than three-fourths of the total breadth of the 

 eastern exit from the Sound. In late August of 

 1925, for example, surface temperatures were 

 3° to 8° lower on Round Shoal (53° to 59° F.) 

 than on Stellwagen Bank at the mouth of Massa- 

 chusetts Bay (61°) or on Jeffreys Ledge off Cape 

 Ann (62°). "(See Bigelow 1927, p. 1012, table 18.) 

 No data were obtained for the eastern end of the 

 Sound in 1953, but the regional temperature 

 relationship was doubtless of the same order 



