Aspects of the general biology and fishery of the mud crab Scylla serrata (forskal) in Moreton Bay, Queensland

Abstract

Growth of post-larval S. serrata in Moreton Bay occurs as a progression through 4 allometric growth phases (G.P.I-G.P.IV). These embrace crabs of approximate carapace width ranges 0.36-2.0 cm; 2.0-10.0 cm; 10.0-16.0 cm and 14.0-20.0 cm respectively. Apart from the relative size and form of the abdomen, sexual dimorphism is not overt during G.P.I and G.P.II, i.e. in crabs below approximately 10 cm carapace width. During G.P.III and G.P.IV however, a progressive divergence in the relative size and form of the limbs (the chelae in particular) develops between the 2 sexes. The functional significance of allometric growth in S. serrata is discussed as is the use of alternative weight and linear parameters as indices of size and form. Problems associated with the use of morphometric data for taxonomic purposes are also discussed.The transition from G.P.III to G.P.IV in female S. serrata is achieved during the pubertal moult. This moult, which represents the terminal moult of most (≈97%) female S. serrata in Moreton Bay, is accompanied by an increase in mean carapace width from 12.85±0.85 cm to 16.65±1.0 cm. The concept of a pubertal moult was found to be irrelevant in reference to male S. serrata. Full development of the male genital tract spans at least 2 successive instars and functional maturity, i.e. the ability to successfully inseminate receptive females, is sometimes possessed by penultimate instar males (mean carapace width 14.18±0.82 cm) as well as by G.P.IV terminal instar (mean carapace width 16.45±1.00 cm) males.Real growth, i.e. the accumulation of organic matter, in S. serrata is an essentially continuous process which is interrupted only briefly around times of moulting. Full mineralization of the integument occurs during the first 26% of the moult cycle. Accumulation of organic matter is most rapid during this period. Relatively large losses of organic matter and soluble mineral salts associated with ecdysis are offset by the crab's predilection to moult in seclusion and to consume its own exuvia.Feeding rate follows a sigmoidal relationship with temperature over the approximate range 16o-27oC. Food consumption falls below maintenance requirements at temperatures below approximately 20oC and to almost insignificant levels at temperatures below 16oC. Preferred feeding areas of S. serrata vary considerably with size/ age/growth phase, sex and moult condition of individual crabs.Of the 2 growth components, moult increment and intermoult duration, the latter is far more sensitive to temperature. Moult increments are significantly depressed by protracted exposure to low temperatures. Lowest moult increments occur in early spring rather than during the coldest winter months.Intermoult duration increases progressively with size. Under near ideal temperatures (27o±0.5oC), the relationship between carapace width and intermoult duration is adequately described by the equation Y = 2.996 + 2.747X + 0.254X2 where Y = intermoult duration (days) and X = carapace width (cm). Least squares linear regressions have been fitted to premoult vs. postmoult carapace width data spanning the entire size range of post-larval S. serrata from Moreton Bay.S. serrata from tropical regions generally appear to grow at a lower rate and to a smaller definitive size than counterparts from sub-tropical and warm temperate regions under comparable conditions. Two possible explanations of these phenomena based on alternative genetic and physiological considerations are presented.Mating S. serrata were captured in all months except July (mid-winter). Peaks in the mating activity of Moreton Bay S. serrata occur in mid-spring and late summer. Spawning begins in spring, reaches a peak in early summer and ends during autumn. Incubation period varies in accordance with the rate/temperature equation D = 17936(T - 4.8)-2.33 where D = incubation duration (days) and T = temperature (oC). The resistance of t developing ova to bacterial and/or fungal infection appears to be markedly reduced at temperatures below 20oC.S. serrata were successfully reared through all larval stages. Food density and temperature were found to greatly influence feeding efficiency and hence the ultimate survival of larvae, zoeal instars in particular.Post-larval recruitment in Moreton Bay begins in late spring or early summer and ends in mid- to late autumn. The exact pattern of recruitment may however vary considerably from year to year.The seasonal nature of reproduction and growth imposes a distinct year class structure on populations of S. serrata in Moreton Bay. The form and progressive development of successive 1 + year old and 2 + year old classes were clearly illustrated in analyses of commercial catch data. Underlying year class structures appear typical of most if not all populations of S. serrata. Evidence that few Moreton Bay S. serrata survive more than 12 months beyond the terminal moult, i.e. to more than 3-4 years of age, is presented.Findings of the present study are discussed in relation to the management of S. serrata stocks in Queensland.