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Distributions, relative abundances and reproductive biology of the deep-water crabs Hypothalassia acerba and Chaceon bicolor in southwestern Australia

Smith, Kimberley Dale (2006) Distributions, relative abundances and reproductive biology of the deep-water crabs Hypothalassia acerba and Chaceon bicolor in southwestern Australia. PhD thesis, Murdoch University.

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Three species of large crab are found in Western Australian waters, namely the champagne crab Hypothalassia acerba, the crystal crab Chaceon bicolor and the giant crab Pseudocarcinus gigas, all of which are fished commercially in these waters. This thesis reports the results of studies carried out on the biology of the first two species, for which there were previously very little information. The results increase our knowledge of the benthic fauna in deeper waters off the southwestern Australian coast and provide data that can be used by fisheries managers to develop plans for conserving the stocks of H. acerba and C. bicolor.

The champagne crab Hypothalassia acerba is found southwards of Kalbarri at ~ 27 degrees S, 114 degrees E on the west coast and eastwards to Eucla at ~ 32 degrees S, 129 degrees E on the south coast. There is a small commercial trap fishery for H. acerba on both the lower west and south coasts of Western Australia. However, on the west coast, H. acerba is managed as a single species fishery, whereas on the south coast it is a component of a multi-species fishery, which also includes the southern rock lobster Jasus edwardsii and P. gigas. On the west coast, the commercial catches of H. acerba increased sharply from ~ 1,500 kg in 1989 to reach maximum levels of 30-46,000 kg in 1997-99, reflecting a marked increase in fishing effort. However, it subsequently declined to essentially zero after 2000 due to effort shifting towards fishing for C. bicolor. Catches of H. acerba on the south coast peaked at 26-27,000 kg in 1997-98 but, in contrast to those on the west coast, remained relatively high in 2001 to 2003.

The crystal crab Chaceon bicolor occurs in water depths of ~ 450 to 1220 m around Australia and New Zealand. However the commercial fishery is almost entirely located between Carnarvon on the north-west coast at ~ 25 degrees S, 113 degrees E to approximately Windy Harbor at ~ 35 degrees S, 116 degrees E on the south coast. Commercial catches of C. bicolor in southwestern Australia, which came almost entirely from the lower west coast, rose from very low levels in 1997 to ~ 222,000 kg in 2001 and then remained close to this level in 2002 and 2003. These trends largely reflect an increase in fishing effort.

Hypothalassia acerba was sampled seasonally by setting traps at depths of 35, 90, 145, 200, 255, 310 and 365 m on the west and south coasts of Western Australia. Catch rates on the west and south coasts peaked sharply at depths of 200 and 145 m, respectively, but at similar temperatures of 16 - 17 degrees C. The catches on those coasts contained 69 and 84% males, respectively. The carapace length of H. acerba declined significantly by 4 mm for each 100 m increase in depth. Males attained a greater maximum carapace length than females on both the west coast, i.e. 135 vs 113 mm, and south coast, i.e. 138 vs 120 mm. Furthermore, after adjustment to a common depth of 200 m, the mean carapace length of males was greater than females on both the west coast (96.6 vs 94.6 mm) and south coast (101.5 and 91.4 mm) and the latter difference was significant (p < 0.001). These results thus show that, for H. acerba, (1) the distribution is related to depth and temperature, (2) body size is inversely related to water depth and (3) males grow to a larger size and are more prevalent in catches than females. There was also evidence that the distribution of H. acerba changed slightly with season and that there was spatial partitioning by this species and other large deep water invertebrate predators.

The trends exhibited by reproductive variables demonstrate that H. acerba reproduces seasonally on the lower west coast, with ovaries maturing progressively between July and December and oviposition occurring between January and March. The characteristics of H. acerba on the south coast differed in the following ways from those on the lower west coast. (i) No ovigerous females and only two females with egg remnants were caught. (ii) Ovaries did not develop late yolk granule oocytes until females had reached a larger size. (iii) Investment in gonadal development was less. These results strongly suggest that conditions on the south coast are not as conducive for ovarian development and reproduction and indicate that females migrate from the south to lower west coast for spawning. In contrast to H. acerba, C. bicolor reproduces throughout much or all of the year on the lower west coast, presumably reflecting its occupancy of far deeper waters where environmental conditions vary less during the year. Although the mean weights of ovigerous females of H. acerba and C. bicolor were not significantly different (p > 0.05), the mean fecundity of the former species (356,210) was significantly greater (p < 0.001) than that of the latter species (192,070). The relatively high fecundity of H. acerba may reflect adaptations by this species to optimise egg production during its relatively short breeding season.

The size at onset of sexual maturity (SOM) of the females of crustacean species, which is often used by fisheries managers for developing management plans for such species, is typically estimated using logistic regression analysis of the proportions of mature females in sequential size classes. The validity of this approach depends on the composition of the samples reflecting accurately that present in the environment. However, catches obtained by traps, a passive fishing method, typically contain disproportionately greater numbers of large crabs, whereas those obtained using active fishing methods, such as seine netting and otter trawling, will presumably represent far better the size composition of the population. Since H. acerba and C. bicolor could be caught in numbers only by using traps, comparisons between the influence of passive and active fishing methods were explored using the extensive data previously collected for Portunus pelagicus employing different sampling methods (de Lestang et al. 2003a,b). These data are analysed in order to demonstrate that the females of P. pelagicus caught by trapping were predominantly mature, whereas those obtained by seining and trawling contained numerous immature as well as mature females. The samples of females collected by trap are, therefore, clearly biased towards mature crabs. Consequently, for any size class, it would be predicted that the proportion of mature females in trap catches will be overestimated, thus shifting the logistic curve fitted to the proportions of mature crabs at each size to the left, and thereby yielding an underestimate of the SOM. This conclusion is substantiated by the fact that the carapace width of female P. pelagicus, at which 50% of individuals reach maturity (SOM50), was estimated to be markedly greater when using the proportion of mature females obtained by seine-netting and otter trawling collectively, i.e. 101.1 mm, than by trapping, i.e. 86.1 mm. From the above data for P. pelagicus, it is considered likely that, through a greater vulnerability of mature females of these species to capture by traps, the respective SOM50s derived for female H. acerba and C. bicolor from trap samples (i.e. carapace lengths of 69.7 and 90.5 mm) will represent considerable underestimates of the true SOM50s.

Many workers have assumed that the chelae of male crabs undergo a change in allometry at the pubertal moult and that this could thus be used as the basis for determining the size of those crabs at morphometric maturity. Since initial plots of the logarithms of propodus length and carapace width (CW) of the males of P. pelagicus and carapace length (CL) of the males of H. acerba and C. bicolor revealed no conspicuous change in allometry, the question of whether the chelae of these species undergo such an allometric change was explored statistically. The Akaike and Bayesian Information Criteria were thus used to ascertain whether a linear, quadratic, broken stick or overlapping-lines model best represented the above logarithmic size data. Since the broken stick model provided the best fit for P. pelagicus, the chelae of this species does undergo allometric change. This occurred at 80.0 mm CW, which is ~ 8 mm less than the CW at physiological maturity. In contrast, my analyses provided no evidence that the chelae of either H. acerba or C. bicolor exhibited an inflection and thus morphometric maturity could not be determined for these two species from chela length. Thus, mangers will have to use the SOM50 for physiological maturity, which was estimated to be 68.1 and 94.3 mm CL for H. acerba and C. bicolor, respectively.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Biological Sciences and Biotechnology
Supervisor: Potter, Ian
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