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



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Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun 



Figure 1 



The spawning season of the Brazihan menhaden in the Ri'o de la Plata estuary. Line and 

 dots show monthly distribution of the mean number of menhaden eggs per tow (CPUE). Black 

 bars show the percentage of positive plankton stations for menhaden eggs. Gray bars show 

 the females with hydrated oocytes as a percentage of the total adult females caught each 

 month. Insert; the gray bars show the monthly distribution of plankton (above) and adults 

 (below) samples. Numbers below the bottom axis correspond to cruises on small boats (SB) and 

 research vessels (RV) each month. 



September through January and diminished during 

 late summer, followed by a secondary peak during the 

 fall (April-June). The percentage of positive stations 

 for menhaden eggs had a similar pattern to that of 

 the CPUE. There was no plankton sampling in July. 

 Menhaden eggs were identified in 220 samples 

 that represented 22. 5C^ of all samples analyzed. The 

 eggs were found mainly in estuarine waters (Fig. 

 2A), especially at depths <10 m. Highest densities 

 of eggs were found in Samborombon Bay and west 

 of Montevideo in the innermost part of the estuary, 

 where salinity values ranged between 10 and 20 

 psu. Medium egg densities were found along the 

 Uruguayan coast between Montevideo and Atlan- 

 tida (55'45'W) and in the middle area of the river 

 between Montevideo and Punta Piedras. Lowest den- 

 sities (<10 eggs/m'^) were present in the rest of the 

 estuary and adjacent coastal waters. 



The study area (5 stations) of the vertical distri- 

 bution of B. aurea eggs is shown by an arrow in 

 Figure 2A. Total depth in this area was 6.5-7.5 m 

 and halocline depth was 4.8-6.5 m. Menhaden eggs 

 were present in the water layer below the halocline 

 but were extremely scarce in the upper layer. In 

 the bottom layer, egg density ranged from 50-2100 

 eggs/m'. Eggs were found within a thermohaline 

 range of 9.7-27.3 psu and 18.5-20.2°C. The upper 

 layer was less saline (0.7-10.8 psu) and warmer 

 (19.7-21.7-C). Strong haloclines up to 21.5 psu/m 

 were observed. Figure 3 shows the vertical distri- 

 bution of salinity and B. aurea eggs along a tran- 

 sect in the estuary. This transect includes four of 

 the five plankton stations sampled above and below 

 the halocline. Menhaden eggs were detected in the 

 region where the salt wedge intersects the bottom 

 (the bottom salinity front). In this area of stratified 



