EXPERIMENTAL MANIPULATION OF POPULATION DENSITY AND 

 ITS EFFECTS ON GROWTH AND MORTALITY OF JUVENILE 

 WESTERN ROCK LOBSTERS, PANULIRUS CYGNUS GEORGE 



Richard F. Ford/ Bruce F. Phillips, ^ and Lindsay M. Joll^ 



ABSTRACT 



A density manipulation experiment was conducted at Seven Mile Beach, Western Australia, to compare 

 growth and mortality for different density groups of juvenile western rock (spiny) lobsters, Panulirus 

 cygniis, inhabiting limestone patch-reefs. Juveniles on a control reef were left at their natural, high den- 

 sities while those on a treatment reef were reduced to approximately 25% of the original, natural density 

 by trapping and reintroduction, which maintained the original size-frequency distribution. Mark-recapture 

 studies were conducted on each reef at three monthly intervals for a year to estimate size-specific growth 

 rates, population densities, and mortality rates. Direct counts of individuals were made by divers to 

 estimate total numbers of juveniles on each reef. There were no statistically significant differences in 

 growth rates for any age category between the control and treatment reefs, but there were significant 

 differences in size-specific mortality rates between the treatment and control groups, with much lower 

 mortality on the treatment reef. Our results suggest that markedly reduced densities of juveniles on 

 a reef may lead to a corresponding reduction in mortality, but no effect on growth was evident. However, 

 part of the apparently higher mortality on the control reefs may instead have been due to emigration 

 of tagged individuals to other reefs. The difficulties of conducting manipulation experiments in the field 

 on a highly mobile species are discussed. 



The western rock (spiny) lobster, Panulirus cygnus, 

 occurs on the coast of Western Australia from North 

 West Cape (lat. 22 °S) to Cape Naturaliste (lat. 

 34° S). Juveniles (2-5 years old) inhabit the coastal 

 limestone reefs, primarily at depths of 1-10 m (Chit- 

 tleborough and Phillips 1975). They remain on the 

 reefs for several years, apparently with little move- 

 ment from one area to another (Chittleborough 

 1974a). Joll and Phillips (1984) found that the juve- 

 niles feed on a variety of animals and plants asso- 

 ciated with the seagrass beds surrounding the reefs. 

 Following the spring molt, larger animals (ages 4-6) 

 move to the adult habitats in depths of 30-150 m 

 (Morgan et al. 1982). 



Chittleborough (1970) observed that, near the 

 center of the geographical range, recruitment of 

 small juveniles appeared to exceed the holding capa- 

 city of the reef system. He concluded that density- 

 dependent mortality of the juveniles at such sites 

 limits their recruitment to the adult stock. He also 

 observed reduced growth rates of animals at these 



'Department of Biology, San Diego State University, San Diego, 

 CA 92182. 



^Division of Fisheries Research, CSIRO Marine Laboratories, 

 P.O. Box 20, North Beach, 6020, Australia. 



^Fisheries Department, W. A. Marine Research Laboratories, 

 P.O. Box 20. North Beach, 6020, Australia. 



sites and considered that the available food re- 

 sources may be inadequate for the maintenance of 

 optimum growth at such high densities. 



Although there have been many field studies on 

 the ecology of spiny lobsters (see review by Kanciruk 

 1980), none have attempted experimental manipu- 

 lation to elucidate the effects of population density 

 on growth and survival. This paper considers the 

 growth and survival of juvenile P. cygnus inhabiting 

 experimental and control patch-reefs at Seven Mile 

 Beach, following a manipulation designed to reduce 

 the density of juveniles of the experimental reef. The 

 hypothesis to be tested was that high population 

 densities of juvenile lobsters limit the growth and 

 survival of the western rock lobsters. Despite its in- 

 herent practical problems, a manipulation approach 

 was adopted as the one most likely to yield direct 

 evidence to evaluate the hypothesis. 



Manipulation experiments are best done using rep- 

 licated experimental areas and both increases and 

 decreases in the density of the species under con- 

 sideration (Connell 1974; Underwood 1979). How- 

 ever, where species (such as lobsters) cannot be 

 transplanted or enclosed effectively, the only prac- 

 tical option is to simply reduce densities (Connell 

 1983). In the case of the study described here, there 

 are also practical limitations in finding sufficiently 



Manuscript accepted June 1988. 



FISHERY BULLETIN: VOL. 86, NO. 4, 1988. 



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