Acha and Macchi: Spawning of Brevoortia aurea in the Rio del la Plata estuary off Argentina and Uruguay 



231 



ID 



Salinity (psu) 

 



J) 10 20 10 20 10 20 10 20 ,, 



766n/rrf I 29 



n/ni 



J 



Q 



40 60 



Distance (km) 



Figure 3 



Vertical distribution of Brazilian menhaden eggs. (A) Vertical distribution of eggs at the head of the salt wedge, 

 where the upper and bottom layers were sampled separately. Numbers in the white areas show eggs/m' above 

 the halocline, numbers in the gray areas show egg concentrations below the halocline. Black lines show salin- 

 ity profiles. (B) Location of the transect in the estuary. (C) Salinity section in the estuary. Isohalines are each 

 2.5 psu; vertical dots indicate CTD observations. Numbers above the top axis show eggs/m' in oblique plankton 

 tows from bottom to surface. 



the echo sounder as a scattering layer, owing to the 

 concentration of zooplankton ( Madirolas et al. , 1997 ). 

 Brazilian menhaden appears to spawn close to the 

 bottom, at the salinity front. The mean position of 

 this front is strongly related to bottom topography 

 (Guerrero et al., 1997a), having a relatively fixed 

 location. This spawning area extends into the strati- 

 fied waters of the estuary between Montevideo and 

 Punta Piedras, and along Samborombon Bay. How- 

 ever, because waters of the inner zone of the bay are 

 vertically homogeneous, or weakly stratified (Guer- 

 rero et al., 1997a), some high egg concentrations oc- 

 cur in nonstratified waters. 



Menhaden eggs occurred in a wide range of salini- 

 ties and temperatures. Because the bottom front is a 



zone of major salinity changes, the eggs are exposed 

 to a wide range of salinity values, and because the 

 reproductive season extends over a long period, eggs 

 are exposed further to a wide thermal range (Fig. 

 4). Low egg densities at low temperatures and high 

 salinities represent samples taken in continental 

 shelf waters, which were cooler than the estuarine 

 waters during the warm season (Guerrero et al., 

 1997a, 1997b). 



Presence of Brazilian menhaden eggs in the plank- 

 ton year-round (Fig. 1) is strong evidence that they 

 have a protracted reproductive season. Major spawn- 

 ing activity, based on incidence of eggs and hydrated 

 ovaries (gravid females), occurs from September to 

 January. Other reports of menhaden eggs, based on 



