FISHERY BULLKTIN: VOL. 86, NO, 4 



The data for both reefs III and V were then 

 pooled. The data for 49 P. cygnus which were recap- 

 tured on reefs III and V were included in this set 

 (Fig. 7). Comparisons of these combined data were 

 made with the data for reef I, and the results in- 

 dicate that all of the significant differences in 

 growth described above were maintained except 

 that for 3-yr-old individuals in January 1981 -Janu- 

 ary 1982. The lack of a consistent significant differ- 

 ence between reefs at all ages and recapture inter- 

 vals suggests that the effect is not a simple response 

 to a superior environment, but rather that interac- 

 tions between reef and age and between reef and 

 recapture interval are present. 



DISCUSSION 



The density manipulation described in this paper 

 is an attempt to use an ecological field experiment 

 in a nonbenign, sub tidal habitat to study the popu- 

 lation processes of a spiny lobster. Practical limita- 

 tions within this environment related to wave effects 

 on drifting plant material and the need to enclose 

 enough area to adequately provide for the foraging 

 range of juvenile P. cygnus precluded the use of 

 large enclosures which would have effectively pre- 

 vented migration. Also, because of the potential 

 mobility of P. cygnus, it was not possible to increase 



and maintain the density of juveniles above natural 

 levels on a reef without such an enclosure. There- 

 fore, only an experimental reduction in density was 

 attempted: 



1. Data from the visual estimates show that we 

 were successful in reducing the number of juve- 

 nile P. cygnus on reef III to approximately 30% 

 of the original, natural level by removing animals 

 in January, February, and March 1981. Follow- 

 ing the last removal, the estimated numbers of 

 animals varied, but showed a slow increase over 

 the 1-yr period of the manipulation experiment, 

 with a mean of 36% of the original January level 

 during March through June 1981 and a mean of 

 55% of the original level during October 1981 

 through January 1982. However, the number of 

 juveniles on both reefs I and V, the control reefs, 

 also declined during this period, and the popula- 

 tion on reef V was estimated to be below that 

 of reef III on 18 February 1981. Therefore, it is 

 difficult to separate the effects of the systema- 

 tic removals from a general decline in numbers 

 indicated by what was observed on reefs I and V. 

 The size and age structures and the sex ratios 

 of P. cygnus juveniles on reefs I, III, and V in 

 January 1981 were very similar. Size and age 

 structures and the sex ratios of these juveniles 



0.14 r- 



JAN - MAY '81 MAY - SEPT '81 SEPT - JAN '82 JAN '81 - JAN '82 



0.02 - 



I 



I 



I 



I 



I 



I 



I 



3 *4 2 3 *4 



Age ( years ) 



3 ?4 



Figure 7.— Average relative growth rates (ARGRs) of juvenile Panulinis 

 cygnus on test reefs III and V (combined) at Seven Mile Beach, Western 

 Australia. 



784 



