Growth, reproduction and recruitment of the doughboy scallop, Mimachlamys asperrimus (Lamarck) in the D'Entrecasteaux Channel, Tasmania, Australia

The doughboy scallop, Chlamys (Mimachlamys) asperrimus (Lamarck, 1819) is an abundant benthic bivalve mollusc found throughout south-eastern Australia. Large populations of doughboys extend over wide areas in Bass Strait, and a commercial fishery for the species has operated irregularly in the D'Entrecasteaux Channel in southeastern Tasmania since the 1930's. This study describes the growth, reproduction and recruitment of the doughboy scallop in the D'Entrecasteaux Channel in southern Tasmania. Growth rates were observed from monitoring populations of scallops in natural beds, reseeded populations, suspended culture and individual tagging. Values of Loo and K from the von Bertalanffy model for a natural population were 94 mm and 0.578, and for the suspended culture population, 105 mm and 0.573 respectively. Aging was determined from external ring counts and von Bertalanffy growth curves. Observations of ring formation in M. asperrimus suggest that the major external rings are formed due to spawning stress. This is supported by the lack of a major ring in scallops < 40 mm that have grown through a winter season but not spawned. Histological studies of gonad development determined the various stages in reproductive development. Males matured and released sperm e.arlier than females. Gonads began early development in late March - early April and immature oogonia are evident by May. Maturation continues through the winter months with a major spawning event occurring in late September to mid October. A minor spawning may be observed in December, but the significant decrease in GSI at this time may have been a consequence of oocyte lysis and re-absorption. M. asperrimus has some reproductive development in year 1 (0+) but the gonad does not mature past stage 3. Maturity is reached in the second year (1 +).The smallest female observed with mature ova had a shell height of 57.5 mm. The collection and identification of two hermaphroditic individuals during this study had not been previously recorded for M. asperrimus. Natural mortality estimates for M. asperrimus range between 0.23 to 1.04 yr-1 for fully recruited year classes. Recruitment in M. asperrimus in the D'Entrecasteaux Channel region is both spatially and temporally erratic. For the six year period during 1987 to 1992, settlement of juveniles was only significant in two years (1988 and 1990), with the highest number of pre-recruits being observed in 1987. Stock and recruitment parameters are discussed in relation to the change in population dynamics of the M. asperrimus fishery in the D'Entrecasteaux Channel.