Sarre and Potter; Variation in age compositions and growth rates of Acanthopagrus butcher/ 



789 



1 opaque zone 



SONDJ FMAIVIJJASONDJFMA 



1993 1994 1995 



Month 



Figure 2 



Mean monthly marginal increments +1SE for sec- 

 tioned sagittal otoliths of Acanthopagrus Imtclicn in 

 the upper Swan River Estuary. The mean marginal 

 increment is expressed as a proportion of the dis- 

 tance between the primordium and the edge of the 

 opaque zone, when only one such zone was present, 

 and as a proportion of the distance between the outer 

 edges of the two outermost opaque zones, when two 

 or more such zones were present. Sample sizes are 

 given for each month. 



Number of growth zones on hard structures 



The number of opaque zones detected on a sectioned otolith 

 of A. butcheri was always the same as the number observed 

 on the same otoHth prior to sectioning, when six or less 

 such zones were visible on the whole otolith (Fig. 3A). How- 

 ever, the use of whole otoliths would have underestimated 

 by one year IS'/f of seven- and eight-year-old fish, collec- 

 tively, and, by one or two years, SV^f of nine- to 13-year-old 

 fish, collectively. The use of whole otoliths would also have 



underestimated the age of two 14-year-old fish by three 

 years, two 1.5-year-old fish by two years, and one 19- and 

 one 21-year-old fish by five years each (Fig. 3A). 



The numbers of annuli observed on the scales of A. 

 butcheri differed from those recorded in 27, 67, and 40% 

 of the sectioned sagittal otoliths of the same fish, when 

 the otoliths possessed one, two, and three opaque zones, 

 respectively (Fig. 3B). The number of annuli exceeded that 

 of the number of opaque zones in 34% of all cases. The 

 number of annuli on the scales of fish, in which the sec- 

 tioned otoliths possessed eight to eleven opaque zones, 

 exceeded by one or two the number of opaque zones on 

 those otoliths in 70% of cases (Fig. 3B). On the basis of the 

 number of opaque zones on their sectioned otoliths, one 

 fish that was estimated as 19 and another as 21 years old, 

 displayed seven more annuli on their scales than on their 

 otoliths. Although the number of sectioned otoliths with 

 more than six opaque zones, that were used for compari- 

 sons with scales, was only 18, it is still noteworthy that 

 the number of opaque zones on more than half of those 

 otoliths was less than the number of circuli on the scales 

 obtained from the corresponding fish. 



The number of opaque zones recorded independently 

 by a second "'reader" for sectioned otoliths of A. butcheri 

 with 0-3 zones (50 fish), 4-6 zones (50 fish I and 7-10 

 zones (40 fish), were always the same as those recorded 

 by the senior author for the same otoliths. Furthermore, 

 the second reader recorded the same number of opaque 

 zones on all but two of the 22 sectioned otoliths that the 

 senior author had recorded as having 11 or more opaque 

 zones. Moreover, after reviewing and discussing the two 

 discrepancies, the second reader agreed that he had failed 

 to detect one of the least conspicuous opaque zones near 

 the periphery of the two otoliths for which there were 

 discrepancies, and therefore his counts agreed with the 

 counts made by the senior author. However, the number of 

 annuli counted on the scales by the second reader, that had 

 previously been recorded by the senior author as having 

 0-3 annuli (50 fish), 4-10 annuli (30 fish), and >11 annuli 

 (7 fish), differed in 20%, 43%, and 86% of cases, respec- 

 tively, which reflects the difficulty in detecting annuli on 

 scales. The differences between counts ranged from one on 

 scales with 0-3 annuli to more than five on scales with >11 

 annuli. 



Trends exhibited by length-frequency data for 

 different age classes 



The data presented earlier demonstrated that the number 

 of opaque zones on whole otoliths of A. butcheri could be 

 used for aging this sparid when there were six or less 

 opaque zones present (Fig. 3A). However, the data in Fig. 

 3A showed that otoliths had to be sectioned to consistently 

 reveal all of their opaque zones when they displayed seven 

 or more such zones prior to sectioning. Thus, to reduce the 

 margin for producing invalid counts to a minimum, esti- 

 mates of the age of individual A. butcheri were made by 

 using whole otoliths, when five or less opaque zones were 

 present, and by employing sectioned otoliths, when six or 

 more such zones were present. 



