227 



Abstract.— Brazilian menhaden, Bre- 



voortia aurea, is the only species of the 

 genus Brevoortia in South American 

 Atlantic waters and is abundant in the 

 Rio de la Plata estuar\'. We found that 

 B. aurea in this area spawns almost ex- 

 clusively in this estuary. We studied the 

 temporal and spatial reproductive pat- 

 tern of this menhaden and related the 

 pattern to the major hydrographic fea- 

 tures of the region. We based evidence 

 of spawning activity on the presence 

 of females with hydrated oocytes and 

 on the occurrence of menhaden eggs 

 in plankton samples. Our results show 

 that B. aurea spawn during virtually 

 every month of the year, but that they 

 spawn mainly from September (late 

 winter) to January (early summer). In 

 the Rio de la Plata estuary, spawned 

 eggs occur in a thermohaline range of 

 13-23"C and 10-25 psu, mainly in strat- 

 ified waters. Breroortia aurea .spawn 

 very near the bottom salinity front, 

 probably in a convergent flow between 

 the riverine and estuarine waters that 

 helps to retain eggs. In contrast to men- 

 haden of the northern hemisphere (B. 

 tyrannus and B. patronus ), which spawn 

 offshore and which drift during early life 

 history stages, Brevoortia aurea in the 

 Rio de la Plata estuary are spawned and 

 held in estuarine waters near spawning 

 sites. The latter reproductive pattern is 

 also shared by Micropogonias furnieri 

 (whitemouth croaker), the most abun- 

 dant fish species in the area. 



Spawning of Brazilian menhaden, 



Brevoortia aurea, in the 



Rio de la Plata estuary 



off Argentina and Uruguay* 



Eduardo M. Acha 



Dto Ciencias Mannas 



Universidad Nacional de Mar del Plata 



Funes 3350 



(7600) Mar del Plata, Argentina 



Present address; Institute Nacional de Investigacion y Desarrollo Pesquero (INIDEP) 



Paseo V Ocampo N° 1 



(7600) Mar del Plata, Argentina 

 E-mail address macha a inidep-eduar 



Gustavo J. Macchi 



Conseio Nacional de Investigaciones Cientificas y Tecnicas (CONICET) 



Rivadavia 1917 



(1033) Buenos Aires, Argentina 



Manuscript accepted 29 November 1999. 

 Fish. Bull. 98:227-235 (2000). 



The Rio de la Plata drains the second 

 largest basin of South America. It 

 flows into the Atlantic Ocean with 

 an average discharge of 22,000 m^/s, 

 generating a large estuary of about 

 35,000 km- and 5-15 m in depth, 

 located at 36°S, 56°W (Framinan 

 and Brown, 1996). Brazilian menha- 

 den, Brevoortia aurea , is abundant in 

 this estuary ( Cousseau, 1985; Boschi, 

 1988); it also inhabits coastal and 

 estuarine environments from 13^S 

 (Brazil) to 40°S (Argentina). Histori- 

 cally, two species of menhaden were 

 thought to inhabit Brazilian-Argen- 

 tine waters (de Ciechomski. 1968; 

 Weiss et al., 1976; Weiss and Krugg, 

 1977; Whitehead, 1985; Lasta and de 

 Ciechomski, 1988); however, Cous- 

 seau and Diaz de Astarloa (1993) 

 concluded that B. aurea is the only 

 species that inhabits South Ameri- 

 can Atlantic waters. 



Little is known about the reproduc- 

 tive biology of S. aurea. De Ciechom- 

 ski (1968) described eggs and early 

 lai"val stages of fi. aurea and reported 

 a period of 86—88 hours from the 

 time of spawning to hatching at 

 13-14'C. Spawning has been detected 

 in the Rio de la Plata estuary (Lasta 



and de Ciechomski, 1988). Although 

 their planktonic eggs also have been 

 found in inshore waters along the 

 Uruguayan and Argentine coasts (de 

 Ciechomski, 1968; Hubold and Ehr- 

 lich, 1981; Cassia and Booman, 1985; 

 Sanchez and de Ciechomski, 1995), 

 Samborombon Bay (a shallow area 

 inside the estuary) seems to be the 

 locus of intensive spawning, where 

 B. aurea eggs are exposed to low 

 salinities (5—15 psu) (Lasta and de 

 Ciechomski, 1988). Estuarine spavin- 

 ing by species producing pelagic eggs 

 is uncommon ( Dando, 1984; Haedrich, 

 1992; Potter et al., 1993 ); however no 

 attempts have been made to describe 

 the basic spawning habitat require- 

 ments of S. aurea. Furthermore, in 

 the Rio de la Plata, Micropogonias 

 furnieri, the species with the largest 

 biomass, releases pelagic eggs in the 

 inner zone of the estuary (Macchi et 

 al., 1996; Acha etal., 1999). 



The life cycles of Northern Hemi- 

 sphere menhaden, mainly Brevo- 

 ortia tyrannus and Brevoortia pat- 



Contribution 1130 of the Instituto Nacional 

 de Investigacion y Desarrollo Pesquero 

 (INIDEP). (7600) Mar del Plata, Argentina. 



