284 



Fishery Bulletin 98(2) 



58 



57 



Longitude 



56 55 



54 



34 S 



53 W 



35 



-a 



3 





36 - 



37 



Figure 1 



Sample locations for female Brazilian menhaden taken in the Rio de la Plata estu- 

 ary, during November 1994, 1995, and 1997. PE = Punta del Este, M = Montevi- 

 deo, PP = Piedras Point, SB = Samborombon Bay, SAC = San Antonio Cape. 



batch fecundity and spawning frequency must be 

 estimated to determine annual fecundity (Hunter 

 and Goldberg, 1980). 



Through histological analysis of the ovaries, we 

 investigated the reproductive biology of B. aurea from 

 the Rio de la Plata estuary to determine the pattern 

 of spawning and estimate spawning frequency and 

 batch fecundity during the main spawning peak. 



Materials and methods 



Brevoortia aurea females were collected from the Rio 

 de la Plata estuary during three fortnightly research 

 cruises in November 1994, 1995, and 1997 (Fig. 1). 

 Fish samples and oceanogi'aphic data (temperature 

 and salinity) were taken for each trawl station from 6 

 to 20 m depths. Fork length ( FL, cm ) and total weight 

 (TW, g) were recorded for each fish sampled. Length 

 distributions obtained for the three years were com- 

 pared with a Kolmogorov-Smirnov Test (KS) (Sokal 

 and Rohlf, 1969). During 1994, only gravid females 

 with hydrated oocytes («=54) for fecundity estima- 

 tion were sampled. Females collected in 1995 (n = 169) 

 and 1997 in=92) were randomly selected including 

 different maturity stages. The ovaries were removed 

 and fixed in W^r neutral-buffered formalin for one 

 week. In the laboratory, the gonads were weighed, 

 and a portion of tissue (about 2,0 g) was removed 



from the center of each ovary, dehydrated in metha- 

 nol, cleared in benzol, and embedded in paraffin. Tis- 

 sues were cut into 4-pm sections, and stained with 

 Harris's hematoxylin followed by eosin counterstain. 

 Classification of ovaries was based on the stage of 

 oocyte development and on the occurrence of post- 

 ovulatory follicles (POF) according to Hunter and 

 Goldberg ( 1980). Our description of the stages of POF 

 degeneration was adapted from that given by Fitz- 

 hugh and Hettler (1995) for B. tyrannus and these 

 stages were classified as day-0 and day-1, according 

 to the elapsed time from spawning. A day-0 POF 

 (elapsed time from spawning <24 h) has an irregular, 

 convoluted shape; the granulosa cells are aligned, 

 and many folds and the lumen are clearly visible (Fig. 

 2, A and B). A Day-1 POF (elapsed time from spawn- 

 ing >24 h) shows degenerative process, the linear 

 appearance of the granulosa cells is not distinct and 

 the lumen becomes reduced (Fig. 2, C and D). 



Oocyte diameters for five gravid ovaries were mea- 

 sured after fixation ( 40 ±pm ) with an ocular microm- 

 eter («=749). Spawning frequency was estimated 

 from samples (n = 169) collected during November 

 1995. Daily fraction of spawning females was esti- 

 mated by the incidence offish with day-0 and day-1 

 POFs (Hunter and Goldberg, 1980) and spawning 

 frequency was determined from the average of the 

 percentages of day-0 and day-1 spawning females 

 (Fitzhugh et al., 1993; Macchi, 1998). 



