790 



Fishery Bulletin 98(4) 



A. Number of 



opaque zones 

 Number of otoliths 



Sectioned otoliths 

 1 23456789 10 1112 1314 1518 19 21 



56 26 25 27 30 13 6 7 19 8 6 4 5 3 2 1 1 



B 



Number of 

 opaque zones 12 3 4 

 Number of otoliths 55 24 10 7 



Sectioned otoliths 



6 7 8 9 10 11 12 1314 1518 1921 



44520310001 1 1 



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§ § 8 



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£ 5 



+7 



+2 

 + 1 



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-1 



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Figure 3 



Comparisons between (A) the numbers of opaque zones on whole and sectioned 

 otohths and (B) the numbers of annuh on scales and the numbers of opaque 

 zones on sectioned otoliths of Acanthupagrus butchcn from the Swan River 

 Estuary. 



Although seine netting over sand in nearshore, shallow 

 waters of the upper Swan River Estuary in the summer and 

 early autumn of both 1993-94 and 1994-95 yielded many A. 

 hutchen of 50 to 180 mm, it produced only a few smaller indi- 

 viduals of this species. However, sampling of beds of the mac- 

 roalgae Gracilaria verrucosa in the same period in 1998-99 

 yielded considerable numbers of A. butcheri that ranged in 

 length from 16 to 60 mm. Once A. butcheri reached lengths 

 of ca. 60 mm, they moved from the beds of G. verrucosa to 

 reside over sand. The lengths of these A. butcheri. which 

 represented the 0+ recruits resulting from spawning from 

 November to December of 1998 (Sarre and Potter, 1999), and 

 thus represented the 1998 year class, increased from 16-32 

 mm in December 1998 to 26-63 mm in January 1999 to 

 44-86 mm in February 1999 and to 36-112 mm in March 

 1999 (Fig. 4). The mean length ±1SE in the latter month, i.e. 

 76 mm, did not differ significantly (P>0.05) from that of the 

 0+ age class in March 1994, when a substantial number of 

 these small fish were also caught (Fig. 5). 



Because A. butcheri spawns predominantly between 

 October and January (spawning peaking in November 

 ISarre and Potter, 19991), the cohort of small A. butcheri 

 (81-115 mm), that was caught by seine net in the shallow 

 waters of the Swan Estuary in September 1993 and whose 

 otoliths did not possess an opaque zone, was the product 

 of the spawning that took place in the spring to early 

 summer of the previous year, i.e. 1992 (Fig. 5). The lengths 

 of this 1992 year class increased from 81-115 mm in 

 September 1993 to 100-152 mm in November 1993, by 

 which time a single and narrow opaque zone could be 

 discerned on the edge of the otohths of this year class. 

 Because spawning peaks in November, the members of 

 this cohort were thus now about one year old. By Novem- 

 ber and December 1994. the lengths of this 1992 year 

 class, now representing early 2+ fish, had increased to 

 170-220 and 189-224 mm, respectively The 1993 year 

 class first appeared in seine net catches in March 1994 at 

 lengths of 67-102 mm and was caught until May, when its 



