COLVOCORESSES and MUSICK: CONTINENTAL SHELF DEMERSAL FISHES 



mum (Walford and Wicklund 1968), and therefore 

 it is more appropriate to consider these cruises as 

 having sampled the late winter distribution of fishes 

 (Musick and Mercer 1977). There was a definite trend 

 toward warmer temperatures during the study period 

 for this season which cannot be completely attributed 

 to sampling artifacts (Davis 1979). Bottom isotherms 

 extrapolated from the collection data are shown for 

 two cruises representative of the warmer (1976) and 

 cooler (1969) extremes (Fig. 2). During the 1969 cruise, 

 inshore and mid-shelf temperatures were <4°C north 

 of Delaware Bay and between 4° and 6°C between 

 Delaware Bay and Cape Hatteras, with an increasing 

 gradient present along the entire outer shelf. In 1976 

 temperatures of <6°C were encountered only at 

 northern inshore stations. South of Chesapeake Bay 

 there was a southwardly increasing thermal gradient 

 perpendicular to the shoreline, and the outwardly 



increasing gradient was distributed across a greater 

 portion of the shelf. Bottom temperatures for the other 

 spring cruises exhibited patterns intermediate be- 

 tween these two (Davis 1979). 



Fall sampling cruises were conducted primarily 

 in October. Because of water column turnover, this 

 is the time of maximum temperature for mid-shelf 

 bottom waters in this region (Bigelow 1933); how- 

 ever, coastal waters undergo rapid cooling during 

 the fall (Parr 1933), initiating migrations for 

 many fishes that spend the summer inshore. Bot- 

 tom isotherms for a typical warm (1973) and cool 

 (1971) fall sampling period are shown in Figure 3. 

 In 1971 a strong thermal gradient was encoun- 

 tered along the mid-shelf from New York to Cape 

 Hatteras. A pocket of cooler water (6°-9°C) was 

 present northward and offshore of this gradient, 

 where turnover was in progress or just beginning. 



•" 



Vc 



7**00 



9 



4«M^ 



CA»t "A'TfUAS 



—LLU 



(B) extrapolated from NMFS Groundfish Survey cruises. 



299 



