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Fishery Bulletin 101(2) 



34°S 



35 



36 



34S 



35 



36 



54°W 



Figure 4 



Spatial location of the M funnen spawning area during Novem- 

 ber 1995 (A) and February 1996 (B). The size of the squares is 

 proportional to the percentage of gravid females (with hydrated 

 oocytes). In both maps, the isohalines expressed as psu) represent 

 the bottom salinity field. PE = Punta del Este, Uruguay; PP = 

 Punta Piedras; and PR = Punta Rasa, Argentina. 



1996, spawning occurred in the outer part of the estuary, 

 and gravid females were more scattered compared to No- 

 vember 1995 ( Fig. 4B). 



Postovulatory follicles and spawning frequency 



The new POFs (between and 6 h after spawning) have 

 an irregular shape with many folds; the granulosa cells are 

 aligned with a prominent nucleus, and the lumen is clearly 

 visible (Fig. 5, A-B). A 12-h-old FOF shows degenerative 

 processes with fewer convolutions, the walls of the granu- 

 losa cells are not well defined, and the lumen becomes 

 reduced (Fig. 5C). A 24-h-old FOF shows pronounced signs 

 of degeneration and forms a compact structure much 



smaller than that at age-12-h. The granulosa layer is still 

 evident but consists of a few cells (Fig. 5D). Postovulatory 

 follicles older than 24 h are difficult to distinguish and may 

 be confused with atresia stages. 



Only females collected during the season 1995-96 were 

 used to estimate spawning frequency of M. furriieri because 

 during 1997-98 spawning females (with POF or hydrated 

 oocytes) were scarce. For the samples taken in November 

 1995 (Table 2), the mean percentage of mature females with 

 postovulatory follicles was 31.51% (CV=0.18), equivalent 

 to a spawning interval of about 3 days. During February 

 1996, the percentage of mature females with postovulatory 

 follicles was 25.35'/, (CV=0.27), which would indicate that 

 each female spawned about every 4 days (Table 3). 



