FORD ET AL.: POPULATION OF WESTERN ROCK LOBSTERS 



of these which showed growth between tagging and 

 recapture were initially analyzed separately. 



Chittleborough (1970) found that loss of more 

 than three legs depressed growth. This was con- 

 firmed for our data by an analysis comparing the 

 average relative growth rates of P. cygnus in the 

 two leg-loss classes (<3 legs missing and >i legs 

 missing), using a f-test. There was a significant dif- 

 ference in growth rates between the two leg-loss 

 classes {P > 0.05). Although this difference is con- 

 founded with differences in the other factors, the 

 result was considered as sufficient evidence, when 

 combined with Chittleborough's findings, to exclude 

 from further analysis data for P. cygnus with >4 legs 

 missing. 



A further analysis was performed to determine 

 if the growth rates of males and females differed. 

 These analyses were done separately for each 

 reef, age class, and recapture interval, using the 

 Wilcoxon rank sum test. The results revealed no 

 significant differences in average relative growth 

 rates between the sexes (P > 0.05), so the data for 

 males and females of the same age group were 

 pooled. 



RESULTS 



Of the 304 tagged P. cygnus on reef III, 186 

 (61.2%; 87 males and 99 females) were never recap- 

 tured, while of the 1,141 animals tagged on reef V, 

 636 (55.7%; 324 males and 312 females) were never 

 recaptured. Similarly, of the 463 P. cygnus tagged 

 in January 1981 on reef I, 307 (66.3%; 162 males 

 and 145 females) were never recaptured. Compari- 

 son of the size ranges and size-frequency distribu- 

 tions of P. cygnus tagged on the three reefs in 

 January 1981 with those not recaptured (Fig. 2) 

 indicated that the "losses" were distributed equal- 

 ly over the full size range and therefore may be 

 assumed to be random. Ratios of males and fe- 

 males in these "losses" did not differ significantly 

 from the malerfemale ratios in the original tagged 

 population (chi-square test of independence, P > 

 0.05). 



None of the tagged P. cygnus on reef I were recap- 

 tured on reef III or V. However, 75 (12%) of the 

 tagged P. cygnus either on reef III or V were subse- 

 quently recaptured on the other reef, and some were 

 caught several times on reefs III and V. However, 

 only four of the animals originally tagged on reef 

 III were ever recaptured on reef V, suggesting a 

 general movement of P. cygnus from reef V to reef 

 III, i.e., inshore to offshore. 



Size and Age Structure 



The size-frequency distributions of P. q/g^ntts juve- 

 niles on reef I in January 1981 and January 1982 

 and that of the juveniles recaptured in January 1982 

 are shown in Figure 3. Similarly, the size-frequency 

 distributions of juveniles on reefs III and V in Jan- 

 uary, May, and September 1981 and in January 1982 

 are shown in Figure 4. There was change in the com- 

 position of the population on reef I between Janu- 

 ary 1981 and 1982, indicating immigration of 2- and 

 3-yr-old animals into the population of this reef. 

 Between January and September 1981 there was 

 an indication of immigration of untagged animals 

 to both reefs III and V, principally of animals ^3 

 years of age, while in January 1982 there was also 

 an obvious immigration of 2-yr-old animals to both 

 reefs. 



Population Size, Density, and 

 Mortality Rates 



The estimates of population size, density, and mor- 

 tality rates for P. cygnus juveniles >3 years of age 

 on reefs I, HI, and V are given in Table 2. The 

 estimate made for the population on reef III in Feb- 

 ruary 1981 assumes that the 304 tagged P. cygnus 

 released in January were still within the population 

 in February. 



The population densities of >3-yr-old juveniles on 

 reef I estimated each January from 1970 to 1982 

 are shown in Figure 5. The density in January 1981, 

 at the time the study began, was the highest ever 

 recorded. The density in January 1982 was also very 

 high and only exceeded by the levels in January 1974 

 and January 1981. 



The annual mortality coefficient for juveniles aged 



Table 2— Population size, population density and mortality-rate 

 estimates for juvenile Panulirus cygnus >3 years of age on three 

 test reefs at Seven Mile Beach, Western Australia. 



'Annual instantaneous mortality coefficient for those >3 years in 1981 

 1.655 



^Annual Instantaneous mortality coefficient for tfiose >3 years in 1981 

 1.302. 



^Annual instantaneous mortality coefficient for tfiose >3 years in 1981 

 1.825. 



777 



