FISHERY BULLETIN: VOL. 80, NO. 3 



shown in Table 8 will therefore represent a 

 majority of 6-yr-old spawning females with rela- 

 tively minor contributions from older year 

 classes. The relative index (R.I.) of the spawning 

 stock may then be directly related to the pueru- 

 lus settlement 6 yr earlier. This relationship is 

 shown in Figure 4 where again, a Ricker (1958) 

 stock-recruitment relationship provides a good 

 description of the data. (Proportion of sum of 

 squares accounted for is 0.845.) Parameter esti- 

 mates and their standard errors (SE) are A = 

 21.17, SE =2.43and£ = 0.013, SE =0.0016. This 

 relationship however may be less apparent at 

 lower levels of fishing effort where the spawning 

 stock would not be so dominated by the younger 

 year classes. 



50 100 



PUERULUS SETTLEMENT 



150 



FIGURE 4.— The Ricker (1958) stock-recruitment model, to- 

 gether with 95% confidence limits. The model is fitted to data 

 on the index of the annual level of puerulus settlement and sub- 

 sequent relative abundance of the breeding stock (i.e., adjusted 

 to the same level of fishing effort as explained in the text) of 

 Panulirus cygmis. The year shown is the season of hatching of 

 the larvae. 



DISCUSSION 



During the period 1969-79, it has been possible 

 to measure abundance of the various life history 

 stages of the rock lobster P. cygnus using a vari- 

 ety of methods. The subsequent preliminary 

 analysis of the interrelationships, shown in Fig- 

 ures 2-4 and Table 7, has provided information 

 both on the regulatory strategies of the P. cygnus 

 population and on the effect of fishing pressure 

 on the stocks. 



A dome-shaped relationship is apparent in 



Figure 2 which is indicative of strong stock-de- 

 pendent effects (following the terminology of 

 Harris 1975) operating during the planktonic 

 larval stages of P. cygnus. This results in the 

 greatest abundance of puerulus being produced 

 by an initially small number of spawning fe- 

 males and a very much lower abundance of 

 puerulus being produced by a large number of 

 spawning females. Cushing (1971) and Cushing 

 and Harris (1973) have shown that such dome- 

 shaped relationships are characteristic of fish 

 with high fecundities, whereas those with low 

 fecundities have more asymptotic relationships. 

 In this respect, the western rock lobster is highly 

 fecund (Morgan 1972), a characteristic which 

 confers upon the species a greater capacity for 

 stabilization and resistance to environmental 

 perturbations. The mechanism by which this 

 stock-dependent mortality occurs is not known, 

 although Harris (1975) and Cushing and Hor- 

 wood (1977) have shown that such pronounced 

 dome-shaped stock-recruitment relationships 

 can only occur when the number of predators (or 

 cannibals) is explicitly related to the number of 

 eggs or larvae released. Chittleborough (1979) 

 suggested that low levels of zooplankton off the 

 west coast of Australia impose density-depen- 

 dent (stock-dependent in Harris terminology) 

 mortality upon P. cygnus larvae. 



Once the puerulus have settled in the coastal 

 reef systems, more asymptotic relationships are 

 apparent which are indicative of density-depen- 

 dent, rather than stock-dependent, processes 

 (Harris 1975). This results in more asymptotic 

 relationships between the level of puerulus settle- 

 ment and the subsequent abundance of recruits 

 to the fishery (Fig. 3) and to the breeding stock 

 (Fig. 4). The intermediate relationship between 

 the level of puerulus settlement and the abun- 

 dance of juveniles of various ages (Table 7) does 

 not appear to be as clear. This may be a result of 

 the juvenile abundance being measured on small 

 areas of reef which may, or may not, be represen- 

 tative of the abundance of juveniles over a wider 

 area. It is possible, at times of high puerulus set- 

 tlement, that reef areas underutilized or not 

 normally used by juveniles may be occupied. 

 Alternatively the area available to juveniles for 

 either shelter or food supply may vary over a 

 period of years because of changing reef siltation 

 or seagrass bed development. Dramatic changes 

 in reef occupancy by juvenile and adult Panu- 

 lirus homarus homarus (Linnaeus) caused by 

 siltation have been observed at Durban in South 



484 



