FORD ET AL.: POPULATION OF WESTERN ROCK LOBSTERS 



moved in January 1981 at Seven Mile Beach are 

 given in Table 1 . Animals with carapace lengths (CL) 

 ^40 mm were marked with individually numbered 

 western rock lobster tags (Chittleborough 1974b). 

 Animals with CL <40 mm were marked with in- 

 dividually numbered Floy^ No. FD-68B spaghetti 

 tags (Davis 1978). Growth data were obtained from 

 tagged individuals recaptured on the three test reefs 

 during resampling in February, March, May, Aug- 

 ust, and September 1981, and January and February 

 1982. Growth of tagged P. cygnus between recap- 

 tures over the period January to May was used to 

 provide data on single molt increments. 



Size and Age Structure 



Age classes were identified from length-frequency 

 distributions, as described by Chittleborough (1970) 

 and Chittleborough and Phillips (1975). From anal- 

 ysis of the size structure present in January 1981, 

 juveniles up to 38.0 mm CL were judged to be 2 

 years of age at that time; 38.1-55.0 mm, 3 years 

 of age; 55.1-68.0 mm, 4 years of age; and those 

 >68.1 to be 5 years of age or older. Similarly for 



■•Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



January 1982, animals up to 42.5 mm CL were con- 

 sidered to be 2 years of age at that time; 42.6 

 mm-55.0 mm, 3 years of age; those 55.1-68.0 mm 

 to be 4 years of age; and those >68.1 to be 5 years 

 of age or older. 



Population Size, Density, and 

 Mortality Rates 



Estimates of the population size, density, and mor- 

 tality rates of P. cygnus juveniles on reef I have been 

 made at Seven Mile Beach since 1970, using recap- 

 tures from baited traps (Chittleborough 1970; Chit- 

 tleborough and Phillips 1975). These estimates for 

 reef I were continued during the period of this study 

 on reef I and, in addition, estimates were also under- 

 taken for reefs III and V, using the same traps and 

 mark-recapture methods. In the present study, 

 12-13 traps were set around the perimeter of each 

 test reef to ensure catches directly from the area 

 of the reef. At each sampling time an array of 12-13 

 traps was set simultaneously around reefs III and 

 V in an attempt to reduce attraction by traps of P. 

 cygnus from one reef to another. 



During the initial tagging in January 1981, trap- 

 ping was conducted on each reef for four consecu- 

 tive days, while during each subsequent recapture 



Table 1 .—Numbers of juvenile Panulirus cygnus examined on three reefs at Seven Mile Beach. 



Western Australia. 



'Additional to those recaptured in January 1982. 



2+ = not scored. 



775 



