FISHERY Kl'LLETIN: VOL. 8H, NO. 4 



period trapping was conducted for two consecu- 

 tive days. The single mark-recapture estimate of 

 Bailey (1951) was used to calculate population 

 sizes. 



Visual Estimates of Population 

 Size 



Direct estimates of the total number of juveniles 

 present on each test reef were made by two divers 

 during the day. Initially the surface of each test reef 

 was mapped, and the map was transferred to ace- 

 tate writing sheets for use underwater, so that 

 major features such as crevices, holes, and sections 

 of ledges could be recognized and searched in a 

 uniform manner during each census. The two divers 

 moved slowly around and over the reef, counting 

 and recording juvenile P. cygnus. Underwater lights 

 were used to aid in this process. Repeated counts 

 were often necessary to obtain consistent results for 

 sections of the reef with large aggregations of juve- 

 niles. One diver followed approximately 2-3 m 

 behind the other, and after each section of the reef 

 was censused, the numbers of juvenile P. cygnus 

 recorded by the two divers were compared. Only 

 three observers conducted all of the visual censuses 

 and, after experience was gained initially, differ- 

 ences between total counts by any two divers on a 

 reef usually were less than 5%. Counts by the two 

 divers were compared for 12 of these censuses, 

 employing separate Wilcoxon signed-rank tests in 

 which the data recorded by each diver for a given 

 section of the reef were paired. The results for all 

 12 censuses indicated no significant differences in 

 counts between divers {P > 0.05). 



There are two primary sources of error in this 

 method. One is that few individuals <40 mm CL can 

 be seen on the surface of the reef or in holes or crev- 

 ices. The other results from reduced visibility caused 

 by turbidity and water turbulence. This second prob- 

 lem was largely avoided by only doing counts when 

 conditions of turbidity and water turbulence were 

 favorable. 



During January-June 1981 and October 1981- 

 February 1982, visual density estimates were ob- 

 tained monthly or bimonthly in reefs III and V. 

 However, storm conditions and poor visibility 

 during the remainder of 1981 precluded observa- 

 tions. Visual density estimates were conducted on 

 reef I during January 1981 and in January and 

 February 1982, while in January and February 

 1983 estimates were conducted on all three test 

 reefs. 



Density Manipulation Experiment 



All of the juvenile P. cygnus caught on reefs I and 

 V (the control reefs) in January 1981 were tagged, 

 measured, and released. On reef III (the treatment 

 reef) 1,202 P. cygnus were caught during four con- 

 secutive days in January 1981 and graded into size 

 categories (5 mm CL size intervals). This was done 

 by measuring the animals and holding them in water 

 in mesh bags suspended from the side of the boat 

 during the 2-3 hours required for processing. 



To reduce the population of lobsters on reef III 

 by approximately 75%, three out of each four ani- 

 mals in each size group were removed from the reef 

 and translocated to another locality out of the Seven 

 Mile Beach area. Selection was done by removing 

 the appropriate number of individuals from each size 

 category blindly to avoid bias. This helped to assure 

 that the groups of juveniles returned to reef III had 

 a size frequency and sex ratio similar to those of the 

 original population. The remaining 304 juveniles 

 caught from reef III were remeasured, tagged, and 

 released on that reef. During the next two sampling 

 periods in February and March 1981, any untagged 

 P. cygnus caught on reef III were removed (Table 

 1) to aid in maintaining the density at approximately 

 25% of its natural level. 



Analysis of the Growth Data 



Two types of growth data were examined: 1) the 

 single molt increments of animals with <4 legs miss- 

 ing (based on the growth of animals recaptured 

 within four months of a previous capture) (Chittle- 

 borough 1976), and 2) the average relative growth 

 rate (Sandland and McGilchrist 1979), which has 

 been shown to be appropriate for analyses of P. 

 cygnus growth (Phillips et al. 1983). The data were 

 classified by reef, sex, age class, duration, and time 

 of year at liberty and by the number of legs miss- 

 ing at the time of tagging. 



The average relative growth rate data of P. cygnus 

 were condensed into a three-factor nonorthogonal 

 experimental design with missing cells. The data 

 were analyzed by examining differences in growth 

 between test reefs for each age and recapture inter- 

 val, using the Wilcoxon rank sum test, and by ex- 

 amining age, recapture interval effects, and their 

 interaction in a two-factor nonorthogonal analysis 

 of variance. 



In addition to P. cygnus caught on the reef of 

 original tagging, 75 individuals (12%) were recap- 

 tured on both reefs III and V. The data for the 49 



776 



