De Lestang et a\ : Reproductive biology of Portunus pe/agicus 



755 



molt and thus, unlike the supposition of Sumpton et al. 

 (1994), does not coincide with the attainment of maturity. 

 The situation in males thus contrasts with that in female 

 P. pelaglcus, in which the abdominal flap becomes loose as 

 an outcome of the pubertal molt (Fielder and Eales, 1972; 

 Campbell, 1984; Potter and de Lestang, 2000; Smith^). 



The very close similarity between the corresponding 

 CWjq's derived for male P. pelagicus in each of the five 

 bodies of water by using morphometric and gonadal data 

 demonstrates that morphological and gonadal maturity 

 are attained by this species at essentially the same cara- 

 pace width. However, the question of whether a male crab 

 of about the size of maturity has become morphometrically 

 mature depends on determining whether the relative 

 length of one of its appendages is closer to the regression 

 line which relates the length of that appendage to the cara- 

 pace width in either juvenile or adult crabs. Because the 

 overall relationship between cheUped length and carapace 

 width of P. pelagicus does not undergo a marked shift at 

 around the attainment of maturity, the use of the allome- 

 tric method never enabled us to determine with absolute 

 certainty whether, in the region of size overlap, a male 

 was morphometrically immature or mature. The lack of 

 precision, when determining maturity with morphometric 

 data, could account for the slopes of the logistic curves for 

 the prevalence of "mature" individuals of P. pelagicus de- 

 rived from these morphometric data in the four southern 

 bodies of water being shallower than those obtained from 

 gonadal data. 



From the above, it follows that there would be an advan- 

 tage in determining the CW^q's for male P. pelagicus at ma- 

 turity by using data on gonadal state obtained by the simple 

 and direct procedure of examining the vas deferentia, rather 

 than relying on data obtained by an allometric method that 

 is indirect and relies on a careful measurement of the ap- 

 pendage lengths and carapace dimensions of a considerable 

 number of individuals. However, the remarkable similari- 

 ties between the CWg^'s derived by using morphometric and 

 gonadal data show that, if it is desirable to avoid damaging 

 the crabs, the data obtained from allometric analysis does 

 yield a close approximation of this important measure for 

 P. pelagicus. Thus, the CW^q derived from either gonadal 

 or morphometric data for male P. pelagicus can be used for 

 developing management plans for this species. 



The very close correspondence between the size at which 

 gonadal and morphometric maturity are attained by the 

 males of P. pelagicus contrasts with the situation recorded 

 by Comeau and Conan (1992) and Sainte-Marie et al. 

 ( 1997) for the males of the snow crab Chionoecetes opilio. 

 In this latter species, the males attain gonadal maturity at 

 a smaller body size than that at which morphometric ma- 

 turity is attained following the terminal molt. The males 

 of C. opilio with large cheliped and large body size are at a 

 competitive advantage over smaller males when courting 

 (Comeau and Conan, 1992; Sainte-Marie et al., 1997). Be- 



•^ Smith, H. 1982. Blue crabs in South Australia — their status, 

 potential and biology. Report 6, p. 33-51. South Australian 

 Fisheries Industry Council Grenfell Centre Level 14, 25 Grenfell 

 Street Adelaide 5000, Adelaide, Australia. 



cause the aquaria studies of Campbell (1984) have shown 

 that the large males of P pelagicus also have a similar com- 

 petitive advantage during courting, a male of this species 

 with mature gonads and differentiated chelipeds may not 

 be able to compete for females successfully if larger males 

 are present. 



Influence of migration on estimates of CW50 for female crabs 



The mean monthly gonad weights recorded for post- 

 pubertal individuals were less for females in estuaries 

 than in marine embayments, strongly indicating that 

 females often tend to emigrate from the estuaries to their 

 spawning grounds before their gonads are fully developed 

 (Van Engel, 1958; Potter and de Lestang, 2000). Such an 

 emigration from estuaries by mature female P. pelagicus 

 reduces the proportion of mature individuals within each 

 carapace width interval, thereby increasing the proportion 

 of immature females in these class intervals. This shifts the 

 logistic curve to the right and consequently increases the 

 CW5Q, which accounts for the significantly greater CWgg's 

 derived for females in estuaries than in marine environ- 

 ments on the lower west coast of Australia. For this reason, 

 subsequent comparisons of the CW^q's for female crabs in 

 the different bodies of water will focus on those derived for 

 assemblages in the three marine embayments. 



In contrast to the CWgg's for females, the CWgg's for 

 males at maturity in the two estuaries and the two marine 

 embayments on the lower west coast of Australia were not 

 significantly different. This presumably reflects the fact 

 that, unlike mature females, the large males of P. pelagicus 

 tend to remain in estuaries during the spawning period 

 (Potter and de Lestang, 2000). 



Influence of temperature on reproductive biology 



The CWjq's derived for males at "maturity" in each of the 

 five bodies of water never differed by more than 2.2 mm, 

 irrespective of whether gonadal or morphometric data 

 were used. However, the maximum CWgg's determined for 

 males in the two estuaries and two embayments by using 

 gonadal and morphometric data, i.e. 88.4 mm for Cockburn 

 Sound and 87.2 mm for the Leschenault Estuary, respec- 

 tively, were 8.6 and 8.8 mm less than the corresponding 

 CWgg's determined for males in Shark Bay Furthermore, 

 the CWgg's for female P. pelagicus in Koombana Bay and 

 Cockburn Sound were 5.6 and 5.1 mm, respectively, less 

 than that of females in Shark Bay. 



The greater CWgg's for P. pelagicus in Shark Bay than in 

 the other four bodies of water, which are located approxi- 

 mately 800 km farther south, runs counter to the general- 

 ization that the CWgg's for decapods tend to be inversely 

 related to water temperature (e.g. Campbell and Robinson, 

 1983; Jones and Simons, 1983; Dugan et al., 1991). Howev- 

 er, the opposite situation has sometimes been recorded and, 

 in those cases, has been attributed to differences among 

 populations of one or more of the following: density, preda- 

 tion pressure, and food availability (Hines, 1989; Polovina, 

 1989; Pollock, 1995; McGarvey et al., 1999). It thus appears 

 relevant that the mean density of P. pelagicus was far lower 



