Hesp et al.: Timing and frequency of spawning and fecundity of Rhabdosargus sarba 



657 





^m^ 



Figure 6 



Histological sections of ovaries of Rhabdosargus sarba showing an oocyte at 

 the (A) a and (B) /3 stages of atresia. c = chorion; t= thecal layer; v= vacuole. 

 Scale bars = 100 pm in A and 50 j.tm in B. 



iMicropogonias furnieri) (Macchi et al., 2003). This 

 finding provides further evidence that R. sarba spawns 

 close to dawn. Certainly, the state of degeneration of the 

 newest POFs in mature ovaries of R. sarba at dusk pro- 

 vides very strong circumstantial evidence that spawn- 

 ing could not have occurred during at least most of the 

 previous daylight hours. 



Our results demonstrate that the prevalence of fish 

 with ovaries at mid to late atretic state 1 declined 

 precipitously as salinities increased from their winter 

 minima in July and August and that this decrease 

 was accompanied by an increase in the prevalence of 

 migratory nucleus oocytes, hydrated oocytes or POFs. 

 The implication that the oocytes of R. sarba are often 

 inhibited from undergoing final oocyte maturation when 

 salinities are low parallels the conclusions drawn for 

 the influence of salinity on the gonadal development of 

 Cynoscion nebulosus in estuaries entering the Gulf of 

 Mexico (Brown-Peterson et al., 2002). The resorption of 

 yolk granule oocytes by the ovaries of R. sarba in July 

 and August would help conserve energy at a time when, 

 if those oocytes progressed through to final maturation 

 and were released, they would be exposed to salinities 

 that are known to be lower than those required for op- 

 timal development (Mihelkakis and Kitajima, 1994). 



Frequency of spawning 



Because ovulation lasts for ca. 2-5 hours, the POFs that 

 were present in ovulating ovaries and that showed no 

 detectable signs of degeneration were presumably <3 

 hours old. It then follows that, when POFs at intermedi- 

 ate and advanced stages of degeneration were present 

 in those same ovaries, those POFs were presumably ca. 

 24 and ca. 48 hours old, respectively. Thus, the pres- 

 ence of these three very distinct forms of POFs in the 

 same ovary of a fish implies that individual R. sarba are 

 capable of spawning on at least three successive days 

 during that part of the month when spawning activity 

 is greatest. 



The estimated average frequency of spawning by R. 

 sarba, i.e., once every 2.7 days, is essentially the same 

 as that recorded for several other species, including e.g., 

 spotted seatrout (Cynoscion nebulosus) (Brown-Peter- 

 son et al., 1988. 2002), red drum iSciaenops ocellatus) 

 (Wilson and Nieland, 1994), and common snook (Centro- 

 pomus undecimalis) (Taylor et al., 1998). The resultant 

 conclusion that R. sarba spawns about 45 times during 

 a spawning period is comparable to that estimated for 

 black sea anchovy (Engraulis encrasicholus) (Lisovenko 

 and Andrionov, 1991) and cobia (Rachycentron canadum) 

 (Brown-Peterson et al., 2001). However, spawning fre- 

 quency does vary markedly among species. 



Relationship between spawning time and tidal cycle 



Although seine netting between 00:30 and 05:30 h on a 

 number of days yielded no female R. sarba with newly 

 formed POFs, rod-and-line angling in deeper water 

 between 01:30 and 04:30 h yielded several females in 

 which the ovaries contained both newly formed POFs 

 and concentrations of hydrated oocytes in their oviducts, 

 and also running ripe males. This finding provides 

 strong circumstantial evidence that, just prior to ovula- 

 tion, R. sarba moves from nearshore shallows to offshore 

 deeper waters. 



Because R. sarba typically spawns just prior to the 

 commencement of a relatively strong ebb tide, the fer- 

 tilized eggs would likely be transported downstream 

 and out of the estuary. The conclusion that eggs are 

 swept out of the estuary is supported by the fact that 

 only 15 larvae of R. sarba were caught during extensive 

 sampling of the lower Swan River Estuary and that 

 virtually all of these larvae were caught at its mouth 

 (Gaughan et al., 1990). A downstream movement of 

 eggs would be further facilitated by R. sarba spawn- 

 ing in deeper waters, where the current is greatest. 

 Emigration of eggs from the estuary would enhance 

 the chances of survival of the eggs of this essentially 

 marine species by ensuring that they would develop in 



