4.2 Methods
4.2.1 Breeding stock surveys
Monthly breeding stock surveys to sample sexually mature (SCE: ≥ 93 mm CW; LE: ≥ 94 mm CW;
GB: ≥ 92 mm CW) and egg bearing female P. armatus were conducted over the peak period of the spawning season in southwest Western Australian crab fisheries (October and November) in each year of the SWRCP. As the shallow nature of LE prevented demersal otter trawling, research hourglass traps covered with 2” mesh were deployed to sample sexually mature female crabs in each of the three fisheries.
Three research hourglass traps were deployed for a 24-hour soak time at each of 15 sites within each fishery (Fig. 4.1 – 4.3, Table 1 in Appendix). Individual traps were baited with yellow-eye mullet (Aldrichetta forsteri) contained inside a perforated PVC bait tube measuring 55 mm diameter x 250 mm and sealed with end-caps joined using a length of shock cord. The traps were set 100 m apart along a transect at each site. Each trap was identifiable by a clearly labelled Department of Fisheries, Western Australia (DoF) float. The research traps measured approximately 115 cm diameter and 50 cm in height when extended (Fig. 4.4). The galvanised steel basal ring was connected to the pneumatic upper ring via two-inch mesh that tapered in towards the midline where entry gaps were located. Unlike commercial traps, there were no escape gaps so juvenile and sub-adult crabs were more likely to be retained along with adults. The traps were emptied into an ice-slurry for 60 seconds upon retrieval to subdue the crabs. Carapace width (CW), sex, and shell condition (hard or soft carapace) were recorded for all crabs captured, along with the state of maturity and egg development of female crabs. The catch was then returned to the water as close to the retrieval location as practicable.
Breeding stock sites sampled within the SCE and LE were identified after consultation with local commercial crab fishers and DoF staff. Within GB, thirteen of the fifteen sample sites were selected based on analysis of data collected by Bellchambers et al. (2006b), with the additional two sites chosen on advice from local DoF staff.
Figure 4.1. Breeding stock survey sites (●) within the Swan-Canning Estuary sampled using research hourglass traps during the SWRCP.
Figure 4.2. Breeding stock survey sites (●) within the Leschenault Estuary sampled using research hourglass traps during the SWRCP.
Figure 4.3. Breeding stock survey sites (●) within Geographe Bay sampled using research hourglass traps during the SWRCP.
Figure 4.4. (a) A baited research hourglass trap deployed underwater, illustrating the buoyant pneumatic ring extending the trap whilst submerged. (b) Research hourglass trap dimensions.
4.2.1.2. Statistical analysis
Catch rates of sexually mature female crabs by number and weight in the SCE, LE and GB were calculated at each sampling site for each sampling year and month. The probability of sexual maturity of each individual female crab, based on carapace width, was used to calculate the number of sexually mature female crabs in each sample:
50
1 1 exp
p L L
a
(1)
where L50 is the size at 50% maturity for each fishery (SCE: L50 = 93.8 mm; LE: L50 = 94.7 mm; GB:
L50 = 92.5 mm; unpublished data, de Lestang, 2016), and a is the corresponding slope parameter of the logistic function used to model size-at-maturity for each fishery (SCE: a = -3.58; LE: a = -4.32; GB: a
= -4.55; unpublished data, de Lestang, 2016). The distribution of catch rates of sexually mature female crabs, by number and weight, in a given year (over two sampling months and multiple sampling sites) was then used to produce a mean annual catch rate and its associated 95% confidence limits.
The breeding potential of female crab stocks in the SCE, LE and GB was determined by applying batch fecundity and batch frequency models in order to calculate catch rates of eggs per traplift as an index of egg production (EPI). These calculations took into account the size of captured sexually mature female crabs, as there is a direct relationship between size and spawning potential with larger females producing more eggs per batch, and more batches of eggs per spawning season (de Lestang, 2003).
Each female crab captured during October and November breeding stock surveys was assigned a total potential egg production based on a batch fecundity (E, number of eggs x 1000) to carapace width (L) relationship (unpublished data, Foster and Hesp, 2016):
2.837 2.837
1.017exp( 6.909) (1000) 1.016
E L L (2)
where 1.017 is the bias correction factor calculated as exp(2/ 2) (Beauchamp and Olson, 1973) to adjust for the mean estimate calculated in log space. A batch frequency (N) to carapace width (L)
b.
relationship (modified from de Lestang, 2003) was used to adjust the number of eggs by the number of expected batches to estimate the total fecundity for a crab of given carapace width:
1 1 exp ln(19)2 120.97 134.77 120.97
N L
(3)
The batch frequency-carapace width relationship was modified from that of de Lestang et. al. (2003) by adjusting for the change in size at maturity for crabs in Cockburn Sound from de Lestang (
50 86.4
L mm) which was previously used as a proxy for all crabs from southwest WA crab fisheries, to the revised 2016 analyses for each individual fishery (SCE: L50 = 93.8 mm; LE: L50 = 94.7 mm; GB:
L50 = 92.5 mm; unpublished data, de Lestang, 2016).
The probability of sexual maturity of each individual crab (See Eq (1) above) was used to calculate the number of sexually mature female crabs of given carapace width which had the potential to produce eggs. The probability of sexual maturity at length, batch fecundity at length and batch frequency at length relationships were used to calculate the number of eggs for each individual crab in the sample, which were then aggregated to calculate the number of eggs for each site. The number of eggs and the associated number of pots was then used to calculate the annual potential egg catch rates (eggs per potlift) at each site in a given sampling month and year for each of the three fisheries. The distribution of catch rates in a given year (over two sampling months and multiple sampling sites) was then used to produce a mean annual catch rate and its associated 95% confidence limits.