FORD ET AL.: POPI'LATION OF WESTERN ROCK LOBSTERS 



vivorship over the 1-}t period, based on assumptions 

 considered later in this paper. A striking feature of 

 the data for reef III is the very high percentage of 

 recaptures, ranging from 97.4 to 100% for 3-, 4-, 

 and ^5-yr-old individuals, and with a somewhat 

 lower value of 58.1% for 3-yr-old animals. Percent- 

 age recaptures for all age groups combined were 

 71.4% on reef III, as compared with much lower 



REEF 



JANUARY 1981 

 ALL TAGGED 463 



REEF I 



JANUARY 1982 

 TOTAL CAUGHT 503 

 TAG RECAPTURES 93 



LnunLnunLnLnLnirjLninirjLOLnLnLnioun 



O^ OOOJUD O'^OOCXJtDO'^OOCNJ (X)0 ^ 



CNjojcMnco^TrM-LriLncDCDUDr^i-^cooo 



CARAPACE LENGTH (mm) 



Figure 3.— Size-frequency distributions of juvenile Panulirus 

 cygnus on Reef I at Seven Mile Beach. Western Australia, in 

 January 1981 and 1983. 



values on reef V (14.7%) and reef I (20.1%). Sep- 

 arate comparisons employing chi-square tests of in- 

 dependence indicated that the number of juveniles 

 recaptured were significantly higher on reef III than 

 on reef V for each of the four age groups {P < 0.005). 



Visual Estimates of Population 

 Size 



Estimates of the mean total numbers of P. cygnus 

 juveniles on the three test reefs, based on visual 

 sampling by two divers, are summarized in Table 4. 



The data for reef I are incomplete but show a 

 dramatic decrease from 265 on 10 January 1981 to 

 66 on 19 January 1981. There was also an overall 

 reduction from 265 in January 1981 to 181 in Janu- 

 ary 1982 and 173 in January 1983. 



The data for reef III (Table 4) indicate that re- 

 moval of P. cygnus by trapping on 11-15 January 

 1981 reduced the number of juveniles from 705 pres- 

 ent on 10 January to about half of that number, with 

 395 (56%) observed on 19 January and 346 (49%) 

 on 3 February. Further removal by trapping on 4 

 and 5 February 1981 reduced the number of juve- 

 niles to approximately 29% of the original 10 Janu- 

 ary level (205 observed on 18 February). 



From March through June 1981 the mean esti- 

 mated numbers of juveniles on reef III varied from 

 189 to 260, representing approximately 27-48 (S 

 = 36%) of the original, natural population level ob- 

 served on 10 January 1981 (Table 4). By January 

 1982, the number of juveniles on reef III had in- 

 creased to 430, or approximately 61% of the level 

 observed in January 1981 and by January 1983 the 

 number had increased to 590 or approximately 84% 

 of the natural level two years before. 



The numbers of juveniles on reef V were rather 

 more variable, ranging from a low of 72 (25 March 

 1981) to a high of 780 (October 1981). Of special im- 



Table 3— Number and percentage of juvenile Panulirus cygnus tagged in each age class on three reefs at Seven Mile 

 Beach, Western Australia, in January 1981, and the numbers present and recaptured in January and February 1982. 



'In January 1981 



779 



