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Fishery Bulletin 101(2) 



ing season and between years. In the 1995-96 

 season, a higher percentage of gravid females 

 (with hydrated oocytes) was observed than in 

 1997-98. In both cases, we found that near the 

 end of the season (February-March) the number 

 of reproductively active females decreased when 

 compared to the main spawning peak (November), 

 and at the same time increased rates of atresia 

 were observed. High levels of atresia have been 

 used to identify regressing ovaries and to estab- 

 lish the cessation of spawning in other species 

 (Hunter and Macewicz, 1985; Hunter et al., 1986; 

 Dickerson et al., 1992; Barbieri et al, 1994). 



Whitemouth croaker spawn in the estuarine 

 waters of the Rio de la Plata. During November, 

 spawning occurs in the innermost part of the estu- 

 ary, coinciding with the bottom salinity front (Mac- 

 chi et al., 1996; Macchi, 1997; Acha et al., 1999). 

 In the outer sector of the estuary, the females 

 remain in recovering stage, eventually returning 

 to the inner part where they complete maturation 

 and spawn (Macchi et al., 1996). During February 

 1996, at the end of the reproductive season, the 

 salinity front shows a more seaward position and 

 a weaker horizontal salinity gradient than during 

 November 1995, especially on the Uruguayan coast. Thus 

 gravid females show a more scattered distribution, and the 

 spawning area seems to be located in more coastal waters 

 (25-30 psu). This may indicate that oceanographic condi- 

 tions take priority over salinity in spawning-site selection, 

 as had been suggested by Acha et al. ( 1999). 



We observed a diel spawning periodicity in whitemouth 

 croaker at Ri'o de la Plata estuary, which may allow fish in 

 spawning condition to concentrate at the same time and 

 maximize fertilization (Holt et al., 1985). The estimated 

 time of spawning peak for M./i/rn/eW was near dusk, based 

 on the incidence of females with new postovulatory follicles 

 (less than 6 h after spawning). Spawning at dusk is a gen- 

 eral reproductive pattern for different sciaenids, which are 

 not dependent on light for courtship behavior because they 

 use sound to locate mates (Holt et al., 1985). Maximum 

 sound production for different sciaenids occurs during 

 spawning, generally between 17:00 and 22:00 h (Saucier 

 and Baltz, 1993). Spawning in darkness may reduce preda- 

 tion on eggs by visual feeders and may minimize the delete- 

 rious effects of sunlight on eggs (Saucier and Baltz, 1993). 

 In the highly turbid waters of the Rio de la Plata estuary, 

 however, these advantages may not be as important. 



Postovulatory follicle degradation in M. furnieri of the 

 Rio de la Plata was faster than that observed for E. mordax 

 (Hunter and Goldberg, 1980). This more rapid degenera- 

 tion may be a reflection of water temperature in the spawn- 

 ing area of whitemouth croaker, which is higher (20-25°C) 

 than that recorded for the northern anchovy (Hunter et 

 al., 1985). This result is coincident with that reported by 

 Fitzhugh and Hottlor (1995), who analyzed the effect of 

 temperature on POF degeneration in Atlantic menhaden 

 (Brevoortia tyrannus). 



Spawning frequency estimated during the main repro- 

 ductive peak in November 1995 (31.51%) was slightly high- 



28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 

 Total length (cm) 



Figure 8 



Relative egg production of A/ furnieri by length class (line), during 

 the spawning peak (November 1995). Solid bars represent females 

 in reproductive state (maturity stages 2 to 5), white bars correspond 

 to inactive females. 



er than that estimated at the end of the spawning season 

 in February 1996 (25.3%). These percentages indicate that 

 the spawning interval of M. furnieri in the Rio de la Plata 

 estuary, ranged between three and four days during the 

 season 1995-96. Daily spawning fraction estimated for this 

 species was similar to that reported for Pogonias cromis 

 from the Gulf of Mexico (Fitzhugh et al, 1993), but higher 

 than those estimated for other sciaenids, such as Seriphus 

 politus (DeMartini and Fountain, 1981), Genyonemus 

 lineatus (Love et al., 1984), Cynoscion nebitlosus (Brown- 

 Peterson et al., 1988) and Cynoscion guatucupa (Macchi, 

 1998). There is only one previous report of spawning fre- 

 quency forM. furnieri of the Rio de la Plata area (Macchi et 

 al., 1996). However, this estimate is not definitive because 

 at that time the duration of the POF was unknown, and the 

 authors used only a fraction of the females with postovula- 

 tory follicles to estimate this parameter. 



Analysis of covariance applied to the batch fecundity 

 and length data from November (1995-97) and February 

 1996-March 1998 showed similar slopes. However, the re- 

 gression intercept for November was significantly greater 

 than the intercept estimate for February-March. This re- 

 sult indicates that batch fecundity of Af. furnieri decreases 

 at the end of the breeding season, coinciding with an in- 

 crease of atresia. The number of oocytes per female weight 

 showed the same pattern, decreasing in February-March. 

 The mean relative fecundity for whitemouth croaker 

 during the reproductive peak (196 ±55 oocytes) was less 

 than that obtained for other sciaenids, such as Cynoscion 

 nchulosus (451 ±43 oocytes); (Brown-Peterson et al., 1988), 

 Cynoscion regalis (200-750 oocytes); (Lowerre-Barbieri et 

 al., 1996) and Cynoscion guatucupa (210 ±53 oocytes); 

 (Macchi, 1998). Nevertheless, this variable varies among 

 years, depending on environmental or nutritional factors 

 (Nieland and Wilson, 1993). 



