FISHERY BULLETIN: VOL. 80, NO. 3 



the phyllosoma larvae in the eastern Indian 

 Ocean. Consequently, in the following analyses a 

 single stock has been assumed, although latitudi- 

 nal differences in some population parameters 

 have necessitated a simple division of the fishery 

 into areas north of lat. 30°S and south of lat. 

 30°S. 



The Breeding Stock 



From the research logbook information, it was 

 evident that spawning rock lobsters were con- 

 centrated in the 20-30 fathom depth range with 

 an average of 89. 9% of the total catch of spawning 

 females each year being taken in this depth 

 range during the period 1966-80. Year-to-year 

 variation was small with the percentage ranging 

 from 86.4% in 1971 to 92.1% in 1967. In addition, 

 the majority of spawning female rock lobsters 

 (average of 81.4% for the period 1966-79) were 

 captured during January and February each 

 year with, again, little year-to-year variation. 

 Accordingly, the catch per pot lift of spawning 

 females taken in 20-30 fathoms in January and 

 February each year has been used as a basis for 

 the calculation of an index of abundance of 

 spawning females. No adjustment for soak time 

 was made since catch per pot lift has been shown 

 to be independent of soak time (Morgan 1977). 

 The use of catch per pot lift data as an index of 

 abundance assumes, of course, that catchability 

 remains constant from year to year. Morgan 

 (1974) has shown that catchability varies during 

 a year in response to molt condition, water tem- 

 perature, and water salinity. Year-to-year varia- 

 tion in catchability of spawning P. cygnus fe- 

 males for January and February is likely to be 

 small, since it would be expected that these ani- 

 mals would be in an intermolt condition, and 

 year-to-year temperature and salinity variation 

 on the spawning grounds in January and Febru- 

 ary is small (Morgan and Barker 1979). 



The average size of the spawning females var- 

 ies with locality, being larger in southern areas 

 (Morgan and Barker 1979). Average size infor- 

 mation on spawning females has been collected 

 on a regular basis since the 1971-72 season from 

 commercial vessels fishing out of the ports of 

 Dongara, Jurien Bay, Lancelin, and Fremantle 

 (Fig. 1). These data have been previously pre- 

 sented in a series of annual reports on the fishery 

 (e.g., Morgan and Barker 1979). Occasional col- 

 lections of size composition data of spawning fe- 

 males were made prior to the 1971-72 season, 



particularly at Jurien Bay and Lancelin (B. K. 

 Bowen, unpubl. data). These two sources of data 

 have been used to calculate the average size of 

 spawning female P. cygnus for various years. 

 The relationship between size and fecundity 

 (Morgan 1972) has then been used to calculate 

 the number of eggs produced by a spawning fe- 

 male rock lobster of this average size. Panulirus 

 cygnus spawns only once per year in most areas 

 in the wild (Morgan 1980b). 



Data on the catch rates of spawning female P. 

 cygnus taken in 20-30 fathoms in January and 

 February each year, the mean size of these 

 spawning females, and the resultant fecundity 

 are shown in Table 1, separated into two areas: 

 north of lat. 30°S and south of lat. 30°S. 



Index of Abundance of 

 Spawning Stock 



The most appropriate index of spawning suc- 

 cess in P. cygnus is the number of first stage phyl- 

 losoma larvae released during the hatching peri- 

 od. However, since it has not been possible to 

 measure phyllosoma abundance directly, an in- 

 direct measure, utilizing the abundance and 

 fecundity of the spawning females, is necessary. 

 Thus the total number of first stage phyllosoma 

 larvae released is approximately equal to (total 

 number of spawning females) X (their average 

 fecundity). 



The number of spawning females in each of the 

 two coastal areas (north and south of lat. 30°S) 

 may be estimated from their catch rate (a mea- 

 sure of density) multiplied by the area of the 

 spawning grounds. The area of the spawning 

 grounds for coastal localities north and south of 

 lat. 30°S is given in Table 2. Thus, for example, a 

 measure of the total number of spawning females 

 north of lat. 30°S in 1966 is given by 0.44 (from 

 Table 1) X 6,690 (from Table 2) = 2,943.6. It 

 should be noted that this measure gives a rela- 

 tive, not an absolute, figure for the numbers of 

 spawning females since a knowledge of the catch- 

 ability coefficient per unit area, q, would be 

 necessary to convert catch rate (c/g) into absolute 

 numbers (N) by using the relationship 



c/g 



qX N 



where A = area of the spawning grounds. 

 Catchability has been measured at the Abrol- 



478 



