Aspects of reproductive and larval biology relevant to aquaculture-based enhancement of the Western School Prawn (Metapenaeus dalli)

This study is the first to describe aspects of reproductive and larval biology required for aquaculture-based enhancement of the Western School Prawn Metapenaeus dalli (Racek, 1957), in south-western Australia. Biological and environmental factors influencing the reproductive dynamics were investigated in a three year study of the population in the Swan-Canning Estuary. Estimates of total population fecundity were made for the first time, based on detailed histological studies of ovarian development. The morphology of different stages of larval development was described to inform subsequent larval rearing trials to investigate their physico-chemical and feeding requirements for large-scale production. Morphological analysis of ovarian development in M. dalli showed that maturation occurs in five stages i.e. immature, early maturing, late maturing, mature and post spawning. Histological methods developed in this study increased the precision of measuring oocyte size by 17 to 40%, for all cell types measured and provided the basis to developing a novel histological technique for quantifying oocyte composition. A study of the reproductive dynamics of female M. dalli in the Swan-Canning Estuary showed that reproduction occurred between November and March, when water temperatures were > 17 ˚C, salinities > 25 ‰ and stratification (bottom – surface salinity) < 3 ‰. Densities of gravid M. dalli were highly seasonal, peaking in November each year when 0+ females matured (19 mm CL), and were highest within the Lower Canning Estuary. Knowledge from the histological studies of oogenesis was used to estimate individual fecundity ranged from 34,000 (18.1 mm CL) to 132,000 ova (27.1 mm CL). This information, which combined with the data on size at maturity and macroscopic ovarian condition, was used to estimate the total egg production by M. dalli in the estuary. Egg production peaked in December/January and differed among the three years. Results suggest that closing fishing between November and December would protect the early breeding aggregations of M. dalli. Taxonomy of the six naupliar, three protozoea, three mysis and first post-larval forms of M. dalli was described from larvae cultured in the laboratory over 12 days at 26 ˚C. Morphological development in M. dalli broadly followed those recorded for other metapenaeids, albeit with differences in the number, location and composition of individual setae and other minor spinal development. Survival of larval M. dalli was highest in water temperatures from 22.6 to 29.4 ˚C and lower at 32.6 ˚C, when assessed at a salinity of 35 ‰. Under various water temperature regimes from 21.4 to 29.7 ˚C, larval survival was highest at salinities of 35 to 40 ‰ and significantly lower at 30 ‰. Larval development increased rapidly with temperate from 21.4 ˚ to 29.4 ˚C, but only minor increases occurred above 29.4 ˚C. Survival, development and dry weight of larval M. dalli was investigated using feeds of three microalgal species Chaetoceros muelleri, Chaetoceros calcitrans and Tetraselmis suecica. Overall growth performance, assessed by incorporating survival, development and dry weight of larval M. dalli into a normalized biomass development index, determined that both mono-specific and mixed diets containing C. muelleri and T. suecica were best for M. dalli larvae. These results increase the understanding of the reproductive and larval biology of M. dalli required for successfully implementing aquaculture-based enhancement (ABE) and provide useful information for managing the Swan-Canning Estuary recreational fishery.