748 



Fishery Bulletin 101(4) 



defined as the time (days) when > 5% of all ma- 

 ture females were ovigerous, and the proportions of 

 ovigerous females among all mature females in se- 

 quential 10-mm CW intervals during the spawning 

 period. The proportion of ovigerous females (O^) in 

 the^th size class during this period also represents 

 the average time a mature female in this size class 

 is ovigerous during that period and takes into ac- 

 count the fact that an ovigerous female spawns at 

 least once during a spawning period and that the 

 brood period (BP) of an ovigerous female is about 

 18 days at 20°C (Meagher. 1971). Thus, the mean 

 number of batches (NB ) produced by the mature 

 female crabs in thejth size class during a spawn- 

 ing period (average water temperature 20.4°C ) can 

 be estimated with the equation NB=O^SP/BP. 



The relationship between number of broods 

 and carapace width was described empirically 

 by fitting a modified logistic curve, NB=l+Nb^^J 

 1 1 -i-exp[-ln(l 9 )(CW-a)/(6-a)), ranging upwards 

 from a minimum of one batch to a maximum of 

 l+NB^^^ batches, where a and b are parameters. 

 The total fecundity of crabs at different carapace 

 widths was calculated as the product of batch 

 fecundity, BF, and the number of broods, NB, by 

 using the relationships between BF and CW and 

 NB and CW, as described above. 



Results 



o 



-•— Shark Bay 



-o — Cockburn Sound 



-i- Peel-Harvey Estuary 



-♦— Leschenault Estuary 



30 

 28 

 26 

 24 



a 22 



k 20 



E 



(D 



r 18 



0) 



14 

 12 

 10 



JFIylAMJJASOND 



Month 



Figure 2 



Mean monthly water temperatures for sampling sites in Shark Bay, 

 Cockburn Sound, Peel-Harvey Estuary, and Leschenault Estuary 

 (water temperatures in Koombana Bay were essentially the same as 

 those in Leschenault Estuary). Mean monthly water temperatures in 

 each water body were derived from data pooled for at least two years. 

 The black rectangles on the .v axis refer to summer and winter months 

 and open rectangles to autumn and spring months. 



Water temperature 



Mean monthly water temperatures at the bottom of the 

 water column in the Leschenault Estuary, Peel-Harvey 

 Estuary, Cockburn Sound, and Shark Bay followed the 

 same trends, with values rising to a maximum in mid to 

 late summer and declining to a minimum in mid-winter 

 (Fig. 2). Water temperatures in Koombana Bay were 

 essentially the same as those in Leschenault Estuary. 

 Although the mean monthly water temperatures in the 

 Leschenault and Peel-Harvey estuaries and Koombana 

 Bay in corresponding months were similar, they were 

 lower in these bodies of water than in Cockburn Sound in 

 eight of the twelve months of the year (Fig. 2). However, 

 the mean water temperatures in each month in Shark Bay 

 were greater than those in the corresponding months in 

 each of the above four more southern bodies of water. Thus, 

 for example, although the maximum mean monthly water 

 temperature was 28°C in Shark Bay, it never reached 

 25°C in any of the other bodies of water (Fig. 2). Likewise, 

 the minimum monthly water temperature was greater in 

 Shark Bay (19°C) than in either Cockburn Sound ( 16°C) or 

 the Leschenault and Peel-Harvey estuaries (12-13°C). 



Macroscopic and histological gonad staging 



Macroscopic examination of the gonads of a large number 

 of females and males oi P. pelagicus, covering a wide size 

 range, and, in the case of females, an histological examina- 



tion of the ovaries of a subset of these crabs, showed that the 

 ovaries and vas deferentia could be classified into four and 

 three developmental stages, respectively (Tables 1 and 2). 



Size at sexual maturity 



The minimum carapace widths of female crabs that had 

 undergone their pubertal molt ranged from 61 mm in 

 both the Peel-Harvey Estuary and Shark Bay to 84 mm 

 in the Leschenault Estuary. Although the CWg^'s derived 

 for females at maturity in Cockburn Sound (86.4 mm) 

 and Koombana Bay (86.9 mm) were not significantly dif- 

 ferent (P>0.05), both of these values were significantly 

 less (P<0.05) than the 92.0 mm for females in Shark Bay 

 (Fig. 3). The high CWg^'s for female crabs in the Peel- 

 Harvey (97.5 mm) and Leschenault estuaries (98.0 mm) 

 were not representative of females in their populations as 

 a whole (see "Discussion" section). 



The relationships between the dorsal length of the larg- 

 est cheliped and carapace width of male P. pelagicus in 

 each of the five bodies of water were described better by 

 using two log-log lines (Fig. 4A) rather than a single log-log 

 line. The CWjjg's of male crabs at morphometric maturity in 

 the four bodies of water on the lower west coast, esti- 

 mated with data obtained from an allometric approach 

 and employing the above log-log regressions, ranged only 

 from 86.2 mm in the Peel-Harvey Estuary and Cockburn 



