Sarre and Potter Variation in age compositions and growth rates of Aconthopagrus butcheri 



791 



1998 



1999 



20 

 15 - 

 10 - 



December 



(40) 



January 



(57) 



February 



{51) 



March 



{70) 



^^jr-^ U "-^tJ"] 



-J 



10 20 30 40 50 60 70 80 90 100 110 120 



Total length (mm) 



Figure 4 



Length-frequency histograms for juvenile Acan- 

 thopagrus butchen caught with a 5.5-m seine net 

 (1-mm mesh) in the shallows l<l m) of the upper 

 Swan River Estuary between December 1998 and 

 March 1999. Numbers in parentheses represent the 

 number offish measured. 



lengths had reached 104-139 mm. When representatives 

 of the 1993 year class reappeared in catches in September 

 1994, their lengths were still only 96-135 mm, indicating 

 that growth had not occurred during the immediately pre- 

 ceding winter months. The lengths of the 1993 year class 

 subsequently increased to between 136 and 183 mm in 

 January 1995 (Fig. 5). Although the 1994 year class first 

 appeared in March 1995, i.e. in the corresponding month 

 to when the 1993 year class appeared in the previous year, 

 its numbers were small and it was not represented in the 

 following month, i.e. April (Fig. 5). 



The 1991 year class was well represented in the major- 

 ity of months (Fig. 5). The lengths of this strong cohort 

 increased from 163-235 mm in September 1993 to 220-296 

 mm in September 1994 and to 222-325 mm in November 

 1994, at which time the fish were entering their fourth 

 year of life. The 1990 year class was a particularly strong 

 cohort (Fig. 5). The lengths of this year class increased 

 from 208-304 mm in September 1993 to 268-349 mm in 

 September 1994 and 259-360 mm at the commencement 

 of their fifth year of life in November. The numbers offish 

 belonging to earlier year classes, i.e. the 1989, 1988, 1987 

 year classes, etc., were very low (Fig. 5). Thus, the number 

 of older fish collectively in the Swan River Estuary was 

 also low. 



Maximum size and age and von Bertalanffy growth 

 parameters 



The spawning activity of A. butcheri peaked in early 

 November in the Swan River, Moore River, and Nornalup 

 Walpole estuaries and in early October in the Wellstead 

 Estuary (Sarre and Potter, 1999). The von Bertalanffy 

 growth cui-ves for A. butcheri were thus derived by using a 

 birth date of 1 November for the first three estuaries and 

 1 October for the Wellstead Estuary. Because many fully 

 mature fish were found in Lake Clifton in early November, 

 and this lake was located near the Swan River Estuary, 

 a birth date of 1 November was likewise assigned to the 

 population of A. butcheri in that system. 



The lowest maximum lengths of female and male A. 

 butcheri were 377 and 365 mm, respectively, which were 

 recorded for fish caught in the Wellstead Estuary, whereas 

 the greatest maximum lengths of each sex was 480 mm 

 recorded for a female in the Swan River Estuary and 

 485 mm recorded for a male in Lake Clifton (Table 2). 

 The maximum weights of A. butcheri in the five systems 

 ranged from a low of 862 g for a female in the Wellstead 

 Estuary to 2196 g for a female in the Swan River Estuary 

 (Table 2). The maximum age of both sexes in each estuary 

 was at least 15 years and the maximum age attained in 

 any system was the 21 years recorded for both a female in 

 the Swan River Estuary and a male in the Nornalup Wal- 

 pole Estuary (Table 2). 



The growth curves of male A. butcheri in the Swan River, 

 Nornalup Walpole, and Wellstead estuaries and Lake Clif- 

 ton followed similar overall trends and thus never crossed 

 one another, and the same was true for the growth curves 

 for females in the above three estuaries (Figs. 6 and 7). 

 The rates of increase in the lengths of both sexes in the 

 Moore River Estuary were initially slower than in each of 

 the above three estuaries, and the rate of increase in the 

 length of males in the Moore River Estuary was also ini- 

 tially less than that of this sex in Lake Clifton. (Note that 

 there were insufficient data for the females in Lake Clifton 

 to make similar comparisons with this sex in this system.) 

 The far slower rate at which length initially increased in 

 the Moore River Estuary is illustrated by the fact that, 

 when male fish were 35-37 months old, i.e. three years 

 in age, their mean length in this estuary was only 151 

 mm and significantly lower (P<0.001) than the 272 mm 

 in the Swan River Estuary, 162 mm in the Nornalup Wal- 

 pole Estuary, 204 mm in the Wellstead Estuary, and 339 

 mm in Lake Clifton. Likewise, the mean length of females 

 of A. butcheri in the Moore River Estuary at this age was 

 only 144 mm and thus significantly lower than the 285, 

 161, and 212 mm recorded in the Swan, Nornalup Walpole, 

 and Wellstead estuaries, respectively. It is also notewor- 

 thy that the above-mean lengths of males in each of the 

 five systems were significantly different from each other 

 in all cases, except for A. butcheri from the Nornalup Wal- 

 pole and Wellstead estuaries. Corresponding results were 

 obtained for females in the four estuaries. As A. butcheri 

 in the Moore River Estuary reached an older age, the 

 gi'owth curves for males and females in this estuary then 

 crossed those for fish from the Nornalup Walpole and Well- 



