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Fishery Bulletin 102(4) 



60 



40 



I ° 



0.1 2 3 4 5 6 7 



Predicted maximum change in daily tidal level (m) 



B 



Number of hours between high tide and 

 estimated time of spawning completion (06:00 h) 



Figure 7 



Relationship between prevalence of spawning and 

 (A) the predicted maximum change in daily tidal level 

 and (Bi the difference between the time of high tide 

 and the time at which spawning in Rhabdosargus sarba 

 is estimated to be completed in the lower Swan River 

 Estuary I 06:00 h). Each point represents the data for 

 a separate sampling occasion. 



Spratelloicles robustus was particularly numerous in 

 some of our seine-net catches, a movement of the eggs 

 ofi?. sarba out of the estuary would also enhance their 

 chances of avoiding predation by that species. 



A downstream transport of eggs would account for 

 the relatively few young 0+ juveniles that are recruited 

 into the nearshore shallow waters of the estuary ( Hesp 

 et al., 2004). Indeed, substantial recruitment into these 

 nearshore waters, presumably as a result of immigra- 

 tion from coastal marine waters, does not occur until 

 R. sarba is about one year old and about 140 mm in 

 length (Hesp et al.. 2004). Because R. sarba settles at a 

 length of ca. 12 mm (Hesp et al., 2004) and ca. 30 days 

 of age (Neira 1 ), this immigration back into the estuary 

 does not occur until 11 months after settlement. In 

 contrast to the situation in the Swan River Estuary, R. 

 sarba elsewhere typically spawns in marine waters and 

 their larvae often enter estuaries on flood tides (e.g.. 

 Miskiewicz, 1986; Neira and Potter, 1992). 



a marine environment in which salinity remained con- 

 stantly at ca. 35'</, rather than in one in which sudden 

 rainfall could result in sudden marked declines in salin- 

 ity. However, the possession of spawning cycles linked 

 to lunar and tidal periodicities can reduce the likeli- 

 hood of predation (Taylor, 1984). For example, Johannes 

 (1978) pointed out that, because the spawning of many 

 reef-dwelling fishes is synchronized with the lunar cycle 

 and occurs on high or ebbing tides, their eggs would be 

 transported away from reefs, where the concentration 

 of predators is high, and consequently the likelihood 

 of predation during the early stages of life would be 

 reduced. Because planktivorous fishes are abundant in 

 estuaries (Johnson et al., 1990; Morgan, 1990), includ- 

 ing the Swan River Estuary where the planktivorous 



Potential annual fecundity 



The estimates of potential annual fecundity derived for 

 R. sarba during the present study, which ranged from 

 109,000 to 2,417,000 eggs for fishes of 188 and 266 mm 

 total length, respectively, greatly exceed those of El- 

 Agamy (1989), which ranged from 23,000 to 99,000 eggs 

 for fishes of 170 and 260 mm total length, respectively. 

 However, because El-Agamy ( 1989 ) based his estimates 

 on the number of large oocytes present in the ovaries of 

 individual R. sarba, he did not take into account the fact 



1 Neira. F. J. 2004. Personal commun. Australian Maritime 

 College. Faculty of Fisheries and Marine Environment. 

 PO Box 21, Beaconsfield. Tasmania 7270. Australia. 



