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Biology of the blue swimmer crab, Portunus pelagicus (Linnaeus), in Western Australia

de Lestang, Simon Nageon (2002) Biology of the blue swimmer crab, Portunus pelagicus (Linnaeus), in Western Australia. PhD thesis, Murdoch University.

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Portunus pelagians was collected by seine netting, otter trawling and crab trapping at regular intervals from two marine embayments (Koombana Bay and Cockburn Sound) and two estuaries (Leschenault and Peel-Harvey) on the lower west coast of Australia and from Shark Bay, nearly 1000 km further north. The data derived from these catches were used to determine the age and size compositions, growth rates, diets and aspects of the reproductive biology of this species in different environments. Attention was also focused on elucidating whether the biological characteristics of the assemblages in the Peel-Harvey Estuary had been influenced by the construction of an artificial channel joining that estuary to the ocean and whether marked changes in fishing pressure in that estuary and Cockburn Sound had led to changes in the age and size compositions and growth rates of this species in those systems.

The growth of each sex in Cockburn Sound and the Peel-Harvey Estuary were described using a seasonal von Bertalanffy growth curve fitted to the means of the carapace widths of the cohorts present in size-frequency data. The growth rates of P. pelagians did not differ significantly between females and males in either of these two systems or between the assemblages in these two water bodies. Growth was highly seasonal, with little or no increase in size occurring during the cold winter and early spring months. Size-frequency data strongly indicated that, in both of these systems, relatively few P.pelagians live beyond 18 months and that, particularly as a result of legal restrictions against retaining ovigerous crabs, heavy fishing mortality occurs at an earlier age amongst male than female crabs. Although a lack of small crabs in the samples inhibited the construction of growth curves for crabs in the Leschenault Estuary and Koombana Bay, the mean carapace widths attained by P. pelagians at the end of its firstthese pathogens. year of life in these two systems, i.e. 105 and 107 mm, respectively, were either the same or very similar to those in Cockburn Sound (105 mm) and the Peel-Harvey Estuary (109 mm), indicating that the growth rates of P. pelagicus in these four systems were also very similar. Morphometric data demonstrated that the second moult of mature females is accompanied by an increase in the relative width of the abdomen.

Unlike the situation in females, the abdominal flap becomes loosely attached to the cephalothorax in males at a prepubertal rather than a pubertal moult. Males become gonadally mature (spermatophores and seminal fluid present in the medial region of the vasa deferentia) at essentially the same carapace width (CW) as they achieve morphometric maturity, as reflected by a change in the relative size of their largest cheliped. This contrasts with the situation in crabs such as Chionoeceles opilio, in which gonadal maturity precedes morphometric maturity. The logistic curve derived from the prevalence of mature male P. pelagicus generally had a shallower slope when using morphometric rather than gonadal data, probably reflecting the lesser precision of the allometric method for determining whether a male P. pelagicus is mature. However, the very close correspondence between the CW50S obtained by both methods implies that either method can be used to calculate this variable for management purposes. Portunus pelagicus attains maturity at a significantly greater size in Shark Bay than in the four more southern water bodies, where water temperatures are lower and the densities of crabs and fishing pressure were greater. As a result of female P. pelagicus leaving estuaries at various times after attaining maturity, the CW50S derived using the prevalence of mature females in estuaries represent overestimates for those populations as a whole. The number of egg batches produced in a spawning season was estimated as ranging from one in small crabs to three in large crabs. These data, together with the batch fecundities of different-sized crabs, indicate that the estimated number of eggs produced by P. pelagicus during the spawning season ranges from ca 78 000 in small crabs (CW = 80 mm) to ca 1 000 000 in large crabs (CW= 180 mm).

Examination of the cardiac stomachs of Portunus pelagicus in the Peel-Harvey and Leschenault estuaries showed that this species does not feed just before or immediately after moulting and that significantly greater volumes of food were present in the stomachs of recently-moulted than intermoult crabs. Although the volumetric contribution made by calcareous material to the stomach contents was similarly high in all size classes of recently-moulted crabs, i.e. 47 to 55%, the volumetric contributions made by small bivalves decreased with body size, whereas the reverse occurred with shell fragments of large decapods and, to a lesser extent, polychaetes. The dietary compositions of intermoult crabs in both the Peel-Harvey and Leschenault estuaries were shown by classification and nonmetric multi-dimensional scaling ordination to differ markedly from those of recently moulted crabs and to undergo similar progressive ontogenetic changes. Thus, the contribution made by small benthic and epibenthic crustaceans, such as amphipods and tanaids, declined with increasing body size, whereas the reverse occurred with larger prey, such as nereid polychaetes, small decapods and teleosts. However, the dietary composition of P. pelagicus was influenced more by moult stage, i.e. recently moulted vs intermoult, than by body size. Although the dietary compositions of P. pelagicus in the two estuaries were not significantly different, they did differ from those recorded from coastal marine waters in the same region, reflecting differences in the potential prey in those two environments.

Comparisons with data obtained by Potter et al. (1983) provided overwhelming evidence that certain of the biological characteristics of Portunus pelagicus in the Peel- Harvey Estuary changed after a large artificial channel was opened into that system in 1994. Following the opening of the artificial channel, crabs entered earlier and remained longer in both the Harvey Estuary (into which the artificial channel opens) and a tributary river in which salinities likewise remained elevated for a longer period than previously. The higher salinities in the spring in the late 1990s probably account for both an earlier spawning and an apparently earlier emigration of ovigerous female crabs. Growth rates during the first year of life were greater in the late 1990s than early 1980s, possibly reflecting a reduction in the overall density of crabs within the estuary.

Commercial and recreational catches of P. pelagicus in Cockburn Sound have risen markedly over the last 20 - 30 years. However, since commercial fishers changed from using tangle nets to traps to catch crabs during this period, the annual catch per unit effort data are not directly comparable and cannot thus be used to elucidate whether these increased catches reflected an increase in crab density. Trawling was thus undertaken in the late 1990s to estimate crab densities in Cockburn Sound in this period and therefore facilitate direct comparisons with those estimated from trawl catch rates recorded in the early 1970s. The results demonstrate that, despite increases in commercial and recreational crab catches, the densities of P. pelagicus in Cockburn Sound have risen markedly between the two periods. This may be related to a decline in the abundances of large predators, such as pink snapper (Pagrus auratus), flathead (Platycephalus spp.), West Australian dhufish (Glaucosoma hebraicum), whiting (Sillago spp.), silver bream (Rhabdosargus sarba) and mulloway (Argyrosomus japonicus), through heavy fishing pressure, and possibly also to an increase in prey abundance. Size composition data demonstrate that appreciable numbers of crabs survived in Cockburn Sound until the end of their second year of life and even beyond during the early 1970s, whereas the vast majority of 1+ crabs were removed by heavy fishing pressure by the time they had reached 18 months old in the late 1990s. Growth during the first eleven months of life, i.e. in the period leading up to the age at which crabs reach the minimum legal size for retention, was significantly faster in the early 1970s than in the late 1990s when crab densities were far lower. This slower growth rate and reduced longevity in the latter period accounts for females becoming mature at a smaller size and for ovigerous females being represented by one rather than two substantial size cohorts, respectively. The essentially single size cohort in the late 1990s, and the first cohort in the early 1970s, correspond mainly to crabs in their first maturity instar, whereas the second cohort in the early 1970s predominantly represented crabs in their second maturity instar.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Potter, Ian
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