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



233 



little importance as a fishery species (Argentine 

 landings in 1997 were 893 t (Anonymous, 1998). 

 Life history characteristics of Brevoortia aurea are 

 probably more similar to those of B. smithi and B. 

 gunteri, which tend to be more nearshore and estu- 

 arine-oriented species, form loose aggregations in 

 coastal waters, and have protracted spawning peri- 

 ods during fall-winter months (Ahrenholz, 1991). 

 Not enough information, however, on their reproduc- 

 tive biology exists to make further comparisons. 



Drift of early life history stages is a major feature 

 in the life cycle of 5. patronus, and especially of B. 

 tyrannus. On the contrary, egg retention seems to 

 be a main property of the life cycle of the Brazilian 

 menhaden in the Rio de la Plata estuary. However, 

 in southern Brazil, B. aurea does not seem to be an 

 estuarine spawner ( reported as B. pectinata , Weiss et 

 al., 1976; Weiss and Ki-ug, 1977; Weiss, 1981; Sinque 

 and Muelbert, 1997 ). The greatest number of eggs 

 occurred in nearshore waters, in a saline range of 

 33.04-35.50 psu. Several eggs were also found in the 

 access channel to Lagoa dos Patos (a large coastal 

 lagoon in southern Brazil, 32''S) during high salinity 

 events, although its larvae and juveniles were dis- 

 tributed throughout the estuary in low salinity con- 

 ditions (Weiss et al., 1976; Weiss, 1981; Sinque and 

 Muelbert, 1997). The spawning area of M. furnieri, 

 the most abundant fish species in the Rio de la Plata, 

 partially overlaps that of S. aurea, and the eggs of M. 

 furnieri are retained in the estuary below the halo- 

 cline (Acha et al., 1999). Like B. aurea, M. furnieri is 

 an estuarine spawner in the Rio de la Plata (Macchi 

 et al., 1996) but seems to be a saltwater spawner in 

 southern Brazil (Sinque and Muelbert, 1997). Both 

 species have the potential for estuarine reproduc- 

 tion, but the eventual spawning site depends on the 

 dynamic properties of the environment. 



Major reviews of the role of estuaries for fishes 

 state that spawning of pelagic eggs within estuar- 

 ies is an unusual episode (e.g. Day et al., 1981; Day 

 et al.,1989; Haedrich, 1992). Owing to the net sea- 

 ward movement of estuarine waters, export of early 

 life-history stages from the estuaries seems to be a 

 major problem for estuarine spawners ( Boehlert and 

 Mundy, 1988). 



In the Rio de la Plata estuary, at least two fish 

 species (S. aurea and M. furnieri) spawn pelagic 

 eggs, taking advantage of the retention properties at 

 the head of the salt wedge. These retention proper- 

 ties are not a feature unique to the Rio de la Plata 

 because estuaries with salt wedges are well known 

 features (e.g. Mann and Lazier, 1991; Officer, 1992); 

 however spawning of pelagic eggs at those locations 

 is an uncommon event. Retention of eggs and larvae 

 in the convergence zone may not be complete and 



the net nontidal flows (residual currents) may drive 

 them seaward. Given the large size of the Rio de la 

 Plata estuary, distance from the spawning area to 

 the offshore limit of the salt wedge may reach 250 

 km (Guerrero et al., 1997a, 1997b). Therefore, larvae 

 probably have enough time to develop to the stage 

 where they become able to control their vertical posi- 

 tion in the water column. The migratory behavior of 

 larvae is manifested mostly in the vertical rather than 

 horizontal plane (Norcross and Shaw, 1984). Thus, 

 larval menhaden could take advantage of the two-lay- 

 ered circulation pattern and through vertical migra- 

 tions compensate for the horizontal transport. This 

 movement would be an important adaptation to main- 

 tain population (as the reproductive unit) coherency 

 (Boucher, 1988; Sinclair and lies, 1989). 



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