FISHERY BULLETIN: VOL, 77. NO, 1 

 Table 2. — Bluefish catches in paired neuston nets during eight weekly cruises off Cape Hatteras, N.C., April, May 1972. 



Temperature-Salinity Regimes 



During the survey, bluefish larvae occurred in 

 two distinct temperature-salinity regimes. One 

 regime was characterized by surface temperatures 

 of 18°-26=C and salinities of .30-32%« (Figure 6i. 

 These conditions prevailed from late spring 

 through the summer above the thermocline in 

 coastal waters of the Middle Atlantic Bight. Blue- 

 fish spawning evidently did not begin there until 

 late July or early August, judging from the small 

 number of large larvae taken in August. Thus, 

 spawning of bluefish in the Middle Atlantic Bight 

 seemed to be influenced partly by features of envi- 

 ronment other than temperature and salinity. 



The other regime was associated with the inner 

 edge of the Gulf Stream and was characterized by 

 surface temperatures of 20°-26°C and salinities of 

 35-38%(i. As mentioned above, few bluefish larvae 

 occurred in this water during the fall and winter, 

 considerable numbers during the spring, and none 

 during the summer. 



Sea.S(>nal Surface Temperature Relations 



Regardless of season or area, nearly all larvae 

 were taken in waters between 17° and 26°C. Lar- 

 vae appeared on the shelf throughout the South 

 Atlantic Bight in spring where the surface water 

 temperatures ranged from 19° to 24.5°C. North of 

 Cape Hatteras where we took no larvae in spring, 

 shelf water was <15°C, but near the edge of the 

 Gulf Stream where we did take larvae, tempera- 

 tures were >15°C. At the stations where bluefish 

 larvae were taken during August, surface tem- 

 peratures ranged from 18.8° to 25.7°C. Surface 



water covering most of the Middle Atlantic Bight 

 south of eastern Long Island was within this 

 temperature range (Figure 3). However, south of 

 Cape Hatteras no bluefish larvae were taken in 

 July when temperatures were mostly >26°C. Sur- 

 face water temperature had decreased between 

 our September and October cruises. The 20°C sur- 

 face isotherm was off Long Island in September, 

 but had moved south to Virginia by October. The 

 bluefish larvae were taken in 20.3°C water in Sep- 

 tember and 16.4°C water in October. The few 

 bluefish larvae taken near the edge of the conti- 

 nental shelf off Florida in October were in water 

 >25°C. In winter, all occurrences were in water 

 >20°C, which was limited to the outer portion of 

 the continental shelf from North Carolina to 

 Florida at that time. 



Diel Cycles of Vertical Distribution 



The number of larvae caught in shallow tows 

 (0-15 m) when compared with deep tows ( 18-33 m) 

 during day and night provided limited informa- 

 tion about diel cycles of vertical distribution. The 

 catch rate was highly dependent on net depth. At 

 the 46 stations where both nets were towed and 

 either caught bluefish larvae, more occurred in the 

 shallow net at 37 stations indicating that the lar- 

 vae were more abundant in the shallow layer i sign 

 test, P<0.001). Nearly all of the catch of the 

 deeper net may have occurred as it passed through 

 the surface layer during setting and retrieving. 



Figure 3. — Distribution of surface temperatures (left) and lar- 

 val bluefish (right) in July-August. Transects A-P sampled Au- 

 gust 1966; AA-PP sampled July-August 1967. 



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