230 



Fishery Bulletin 98(2) 



waters, depth of bottom layer (measured from the 

 halochne to the bottom) was less than 3 m, and men- 

 haden eggs were located below the halocline. 



Menhaden eggs occurred in a wide environmental 

 range (Fig. 4). Densities of > 100 eggs/m^ were found 



34 S 



35 - 



36 



3 





37 



34 S 



33 - 



36 



37 



53 W 



75-100% 



Figure 2 



Location of the spawning area of the Brazilian menhaden, (A) The size of 

 the symbols is proportional to the egg density from the plankton samples; 

 the arrow shows the study site for examining the vertical distribution of 

 eggs (see the text). (B) The size of the .symbols is proportional to the per- 

 centage of females with hydrated oocytes. In both maps, the isohalines 

 (expressed as psui represent the bottom salinity field for spring-summer. 

 PE = Punta del Este; A = Atlantida; M = Montevideo; PP = Punta Piedras; 

 SB = Samborombon Bay; SAC = San Antonio Cape. 



in a salinity range of 10 to 25 psu; minor densities 

 were located in waters with salinities reaching 33 psu 

 (continental shelf waters). The low salinity boundary 

 for egg distribution seems more abrupt than the high 

 salinity boundary; very few eggs were detected below 

 10 psu. Most eggs produced over 

 the prolonged spawning season 

 were found at temperatures be- 

 tween 13 and 23°C. 



Gravid females 



A total of 375 sampling stations 

 were analyzed, in which 1084 

 gravid females were identified. 

 All the gravid females were 

 caught from September through 

 December. In these months, the 

 total percentage of females with 

 hydrated oocytes ranged from 

 72.8% to 89.4'7r (Fig. 1). 



Gravid females were detected 

 across the river between Mon- 

 tevideo and Punta Piedras, and 

 in Samborombon Bay (Fig. 2B). 

 In that region, percentages of 

 gravid females in each tow were 

 >50%. The highest percentages 

 of females with hydrated oocytes 

 appeared to be associated with 

 the highest horizontal salinity 

 gradient (the salinity front). No 

 females with hydrated oocytes 

 were detected in continental 

 shelf waters. 



Discussion 



The distribution of eggs and 

 gravid females reveals that B. 

 aurea spawn in the estuary (Fig. 

 2). Gravid females were found 

 in a more restricted area than 

 that where eggs were found, 

 thus providing a more accurate 

 picture of the spawning spatial 

 pattern, because currents may 

 move eggs to more distant 

 areas. An echo sounder showed 

 Brazilian menhaden below the 

 halocline on all cruises, includ- 

 ing the occasions when spawn- 

 ing individuals were collected. 

 The halocline was detected by 



South 

 America* 



J 



\/ 



( M Study 

 H" Area 



53W 



