Macchi et aL: Seasonal egg production of Micropogonias furnieri 



335 



80 

 60 

 40 

 20 







November 1995 



February 1996 



iL 



1 2 3 4 5 6 7 



Maturity stages 



November 1997 



1 2 3 4 5 6 7 



March 1998 



12 3 4 5 6 7 



Figure 2 



Maturity-stage frequency of M furnieri during the spawning peak (November 1995 

 and 1997) and the end of the reproductive season (February 1996 and March 1998). 

 Stages: 1 = developing early; 2 = developing late; 3 = fully developed; 4 = gravid; 5 = 

 partially spent; 6 = spent; 7 = resting; modified after Mayer et al. (1988). 



higher than in February 1996 (3% vs. 25%, respectively). 

 In general, we noted an increase in the presence of atretic 

 oocytes as the spawning season progressed (Fig. 3). Fur- 

 thermore, the incidence of atresia was higher in 1997-98 

 than in 1995-96 (Fig. 3); during the last season about 60% 

 of females were classified as in atretic state 1, whereas in 

 1995-96 about 20% of females were in this state. 



Spawning site and oceanographic variables 



Bottom salinity at the estuary ranged between and 30 

 psu. Maximum upriver penetration of salt water during 

 November 1995 and February 1996 was similar, reaching 

 a line between Montevideo and Punta Piedras (Fig. 4). 

 The Argentine coast of the estuary showed a similar salinity 

 pattern during both months, whereas the salt water intru- 

 sion along the Uruguayan coast showed more upriver pen- 

 etration during November 1995, which generated a stronger 

 horizontal salinity gradient (Fig. 4A). In contrast, the salinity 

 pattern along the Uruguayan coast during February 1996 

 showed a weaker horizontal salinity gradient (Fig. 4B). 



Bottom temperature in the estuary ranged from 13.8 to 

 20.2°C during November 1995, and from 20.1° to 25.0°C 

 during February 1996. In the spawning area, bottom tem- 

 peratures ranged from 20.0°C to 20.2°C during the main 

 reproductive period and from 21.5°C to 25.0°C at the end 

 of the breeding season. 



Whitemouth croaker of the Rio de la Plata estuary spawn 

 in the region of high salinity gradients. During November 

 1995, spawning took place near Montevideo, coinciding 

 with the bottom salinity front (Fig. 4A). During February 



