o 



c 



40 r 



30 



20 



10 



< 



- -- 



1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 22 



SURFACE TEMPERATURE °C 



° 40 r 



= 30 



a. 

 < 



20 



>- 

 U 



5 lO 



o 



.. I-... 



10 20 30 40 50 60 70 80 90 100 120 



DEPTH ( m ) 



140 



Figure 3.— Water depths at which Atlantic silversides were 

 captured during inshore and offshore bottom trawl surveys of 

 the National Marine Fisheries Service over the eastern North 

 American continental shelf, 1963-79. 



>■ 

 U 



o 



50 r 



40 



30 



20 



10 



B 



_u_u_ 



1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 22 



BOTTOM TEMPERATURE °C 



Figure 2.— Water temperatures at stations where Atlantic 

 silversides were captured during inshore and offshore bottom 

 trawl surveys conducted by the National Marine Fisheries 

 Service during 1963-79 over the eastern North American con- 

 tinental shelf. A. Surface temperatures (n = 141). B. Bottom 

 temperatures (w = 135). 



Discussion 



The results of this study demonstrate that 

 populations of M. menidia north of Cape Hat- 

 teras undergo an offshore winter migration from 

 inland to inner continental shelf waters. Atlantic 

 silverside winter habitat probably also includes 



Table 3.— Chi-square analysis of diel variations in catch fre- 

 quencies of Atlantic silversides in combined inshore (5-27 m) 

 and offshore (27-366 m) trawl surveys conducted by NMFS, 

 1963-79, over the continental shelf of eastern North America. 



" P<0 01 

 "*P<0 005 



deep inland waters not sampled by NMFS sur- 

 veys, as Hildebrand and Schroeder (1928) and 

 Richards and Castagna (1970) have noted. Since 

 the lower lethal temperature for M. menidia in 

 short-term experiments was 1°-2°C (Hoff and 

 Westman 1966; Conover unpubl. data), the off- 

 shore migration may be promoted by potentially 

 stressful or lethal low water temperatures in 

 shallow inland waters during midwinter. Con- 

 over and Ross (in press) and Warfel and Merri- 

 man (1944) found that Atlantic silversides leave 

 the New England shore zone in November as 

 water temperatures drop to about 6°-8°C. The 

 timing of Atlantic silverside disappearance from 

 shallow inland waters corresponds closely with 

 their appearance in deeper offshore waters. 



If the offshore migration of Atlantic silver- 

 sides is primarily motivated by low temperature 

 stress, than offshore movements in warmer 

 waters, such as south of Cape Hatteras, would 

 not be expected. Even though our data cannot 



148 



