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



also were very similar among the reefs at all 

 subsequent sampling times, suggesting that the 

 drastic reduction in the density of P. cygnus on 

 reef III, and the method of reconstituting the 

 population, did not have an evident effect on 

 these characteristics over the 1-yr period of the 

 experiment. 



2. The population sizes of ^3-yr-old P. cygnus juve- 

 niles, estimated from catches with baited traps, 

 were significantly lower on reef III than on reef 

 V in January 1981 and August 1981, but were 

 similar (i.e., not significantly different) in Janu- 

 ary 1982. However, these data, and the estimates 

 of mortality derived from them, are subject to 

 the usual problems associated with mark-recap- 

 ture techniques. Phillips (unpubl. data) has found 

 in more recent studies that the baited traps are 

 capable of attracting juveniles from over a wide 

 area and hence the population size and density 

 estimates applied to individual reefs probably are 

 inaccurate. However, the trends in the popula- 

 tion estimates from the mark-recapture data are 

 supported by the visual estimates of population 

 size, indicating that they reflect what was actual- 

 ly happening on these two reefs. 



3. Clearly, the visual estimates provide a more spe- 

 cific set of information about population levels 

 of juveniles living on the reef. The method also 

 allows direct estimates of numbers over short 

 time intervals and with minimal disturbance of 

 the western rock lobsters. However, although 

 there is no doubt about the drastic decline in the 

 numbers of tagged lobsters on reefs I and V after 

 the period of initial tagging, it is not possible to 

 determine if this was as a result of tagging mor- 

 tality, an emigration as a response to handling 

 and tagging or part of the normal behavior 

 pattern. 



Despite the attempts to select directly comparable 

 reefs for the experiment, it is possible that reefs III 

 and V do provide different environments for the 

 resident P. cygnus. The movement of 71 individuals 

 from reef V to reef III may be part of a typical move- 

 ment from shallower to deeper reefs. It seems 

 unlikely that water depth, per se, is the primary fac- 

 tor involved, because the difference in depth be- 

 tween the two reefs is no more than 2 m. The reefs 

 used by Chittleborough (1970) at Garden Island, on 

 which he found P. cygnus juveniles remained for 

 several years with little movement even from one 

 part of the reef to another, were similar in depth 

 to reef III. 



Observations during sampling indicated that on 

 reefs I and V, which are both located close to the 

 beach, relatively large amounts of plant detritus 

 build up around the base of the reef and under 

 ledges and that turbidity of the water is sometimes 

 quite high. These effects also occur on reef III, but 

 are less pronounced. This suggests that less favor- 

 able conditions on the shallower, inshore patch- 

 reefs may cause some P. cygnus to seek reefs 

 slightly farther offshore which have more suitable 

 conditions. It also may help to explain the higher 

 variability in numbers of juveniles from the visual 

 estimates made on reef V compared with reef III. 

 Some individuals may temporarily emigrate from 

 areas such as reef V during periods of adverse 

 conditions. 



Survival and mortality data, which were obtained 

 from both the mark-recapture estimates of popula- 

 tion size and the numbers of tagged animals recap- 

 tured on the same reef, indicate that survival of P. 

 cygnus juveniles was significantly higher on reef III 

 than on reef V over the 1-yr period of the experi- 

 ment. This was evident for individuals of all four age 

 groups. It suggests that the experimental reduction 

 in numbers of juveniles on reef III, resulting in 

 relatively low population densities (29-61% of the 

 original, natural number) during the 1-yr experi- 

 mental period, led to significantly higher survival 

 than on reef V, where P. cygnus juvenOes were pres- 

 ent at natural density levels. One explanation for 

 this is that reduced densities of juveniles on a reef 

 may lead to a corresponding reduction in predation 

 mortality and adverse interspecific effects of crowd- 

 ing. An alternative explanation for the very high 

 survival of P. cygnus juwenWes estimated on reef III, 

 relative to reefs I and V, is that the tagged juve- 

 niles on reef III remained for the entire year while 

 many of those tagged on reef I and V emigrated to 

 other reefs. 



These mortality data are subject to several sources 

 of error, including the basic problems associated 

 with mark-recapture sampling to obtain population 

 estimates (Bailey 1951). The age-specific estimates 

 of survival obtained from comparisons of numbers 

 of tagged individuals released on each reef in Janu- 

 ary 1981 and recaptured there in January or Feb- 

 ruary 1982 requires a major assumption. It is that 

 all of the tagged individuals not recaptured on a 

 particular reef after one year have died. This un- 

 doubtedly is not the case because some of those in- 

 dividuals probably moved to other reefs at Seven 

 Mile Beach after their release. To the extent that 

 this occurred, the survivorship estimates are low. 



785 



