Hesp et al : Age and size composition, growth rate, reproductive biology, and habitats of Claucosoma hebra/cum 



225 



nent reefs at similar lengths, the 

 growth of silver trevally and King 

 George whiting slows after mat- 

 uration, whereas that of snapper 

 and dhufish continues appreciably 

 after they have reached maturity. 



Spawning location, period, and 

 mode 



125 r 



100 



•= 75 



50 



25 



Glaucosoma hebraicum with gonads 

 at stages VI (spawning) and VII 

 (spent) were caught in waters rang- 

 ing from 10 to 150 m in depth and 

 at distances of 3 to 50 km from the 

 shore and between latitudes 28°55' 

 and 32°45'S. Thus, the spawning of 

 dhufish is not apparently restricted 

 to any particular water depth or 

 region along the coast. However, 

 because G. hebraicum greater than 

 300-320 mm in length (their size at 

 first maturity) were almost invari- 

 ably caught only around limestone 

 or coral reef formations, this species 

 apparently spawns in the vicinity of reefs. 



Because hydrated eggs and postovulatory follicles were 

 found in at least some of the ovaries of large females in 

 each month between November and April, it is evident 

 that G. hebraicum spawn between the end of spring and 

 middle of autumn. Although some fish commenced spawn- 

 ing in November, the mean GSI of female fish in that 

 month was still well below its maximum. This indication 

 that only a small amount of spawning occurs in Novem- 

 ber is consistent with the fact that many of the ovaries of 

 large fish were still at stages III and IV. Although most 

 of the ovaries of large females caught in May 1997 con- 

 tained some vitellogenic oocytes, these oocytes were usu- 

 ally undergoing atresia and the ovaries of other large fish 

 in that month were either spent or resting. Furthermore, 

 none of the ovaries of large G. hebraicum caught in May 

 contained hydrated oocytes. This finding provides strong 

 evidence that the spawning period does not extend into 

 May. There is also strong evidence that spawning peaks in 

 January and February. For example, by January, the ova- 

 ries and testes of most large fish were at stages V or VI, 

 i.e. prespawning or spawning, and, for the first time, some 

 were spent or even recovering spent (stages VII and VIII). 

 The maintenance of the GSIs of females at their maxima 

 in both January and February is attributable to the fact 

 that, because G. hebraicum is a multiple spawner, new 

 batches of hydrated oocytes were continually being devel- 

 oped in the ovary during these two months. However, the 

 GSIs of females and males both declined precipitously in 

 March, which demonstrates that, in the case of ovaries, 

 the release of eggs during spawning was not being com- 

 pensated for by a comparable production of new batches of 

 mature eggs. As spawning activity peaked in January and 

 February, it was appropriate to use 1 February as the birth 

 date of G. hebraicum when assigning an age to each fish. 



0.4 



03 2 



0.2 :i 



- 1 tB 



J 



n-m-;^^ 



n=^ 



4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 

 Age (years) 



Figure 9 



Age composition of Glaucosoiiia hebraicum. that were caught at lengths > the mini- 

 mum legal length of .500 mm, and maximum likelihood estimates (±95% confidence 

 limits) for the instantaneous coefficients of total mortality/year, Z, determined from 

 subsets of these data selected by using different initial ages. 



The fact that, during the spawning season, mature ova- 

 ries of G. hebraicum often contained yolk vesicle, yolk gran- 

 ule, and hydrated oocytes and, in some cases, also post- 

 ovulatory follicles, implies that this species is a multiple 

 spawner sensu deVlaming (1983), i.e. individual females 

 release eggs on more than one occasion in a spawning sea- 

 son. The oocytes of individual female G. hebraicum during 

 the spawning period ranged widely in size and, in many 

 cases, their diameters formed relatively discrete modes in 

 oocyte diameter-frequency distributions. The ovaries of G. 

 hebraicum thus contain hatches of oocytes that are pre- 

 sumably released at different times. Multiple-batch spawn- 

 ing over a protracted period enables a greater total number 

 of eggs to be produced and released during a spawning pe- 

 riod and results in eggs becoming discharged at different 

 times (McEvoy and McEvoy, 1992), which would increase 

 the overall chance of recruitment success. 



Implications of the biology of Glaucosoma 

 hebraicum for fisheries management 



The age composition data for dhufish older than 9 years and 

 larger than the MLL reflect an average level of the instan- 

 taneous coefficient of total mortality. Z. of 0.21/vear This 

 value is consistent with the estimate obtained from the 

 observed maximum age, taking into account the sample size 

 of 620 fish (Hoenig. 1983). The much lower value obtained 

 for Z, with Hoenigs ( 1983) regression equation for fish, i.e. 

 0.10/year, does not take into account sample size. 



The estimate of the instantaneous coefficient of natural 

 mortality, M, of 0.25/year, that was calculated with Ralston's 

 (1987) equation, exceeds the average value of 0.2L/year for 

 the instantaneous coefficient of total mortality, which was 

 estimated from the catch curve. Examination of the residu- 

 als from the regression line fitted by Ralston (Fig. 8.1 in 



