Prawn farmers rely on wild stocks of Penaeus monodon for the production of larvae. Hatcheries preferentially source females that have a nearly fully developed ovary, ie a gravid female, which are in an immediate pre-spawning state to meet required production schedules. However, fluctuations in the availability of wild broodstock, on a day to day basis as well as a seasonal one, coupled with variable spawning performance, make hatchery operations the weakest link in the production cycle. Often post-larvae from hatcheries cannot be reliably supplied to farms in the numbers required for grow-out production schedules. For this reason some hatcheries stockpile wild female broodstock with undeveloped ovaries, known as ‘blanks’, which are induced to spawn at a later date, as insurance to match larval production with grow-out cycles. The supply of wild broodstock to satisfactorily meet the need of the prawn farming sector is a precarious state and is set to deteriorate further if, as is predicted, Australian production expands. Even if additional trawling grounds can be found from which to source the extra wild broodstock required, the continued reliance on natural populations will ensure that the capability for hatcheries to meet production schedules will remain in a fragile state.

Epizootics during the grow-out period have had a profound impact on farm production. Recent evidence indicates that wild broodstock are carriers of potential pathogenic organisms, such as viruses, and that the incidence of infection varies seasonally. At various times of the year over 95% of wild broodstock trawled from Australian waters can harbour viruses that may be pathogenic (Walker et al. 1998). The vertical transmission of these viruses from mother to larvae is a continuous threat to production. In Australia several groups of researchers are developing diagnostic tools to assist in the identification of pathogenic organisms within broodstock. Success overseas in the production of specific pathogen free (SPF) broodstock has resulted in managerial regimes that have curtailed the frequency of epizootics. For example, in Taiwan the screening of wild broodstock and the production of captive reared broodstock, which are certified SPF with regards to White spot syndrome virus (WSSV), has resulted in grow-out cycles free of WSSV epizootics (Lo et al. 1998). Although the use of SPF broodstock does not result in disease resistant, or even disease tolerant stock, it is proving to be one effective managerial control measure which minimises the likelihood of epizootics due to an identifiable pathogenic organism which is responsible for disease and production losses. The variability in the supply of wild broodstock and the potential to introduce disease onto the farm is placing considerable interest on the production of closed-life cycle broodstock from farm stock.

Although closing the life cycle of P. monodon will add to production costs, the industry would have complete control over the captive reared broodstock and they would have a known life health history. The use of captive reared broodstock may be the most ecological and economical approach to ensure a sustainable industry. However, before the transition will occur the industry requires evidence that the spawning performance of captive reared broodstock is comparable to that of wild broodstock.

Commercial hatcheries in Australia report periods when wild spawner performance is unexpectedly poor. Inferior performance has been observed in association with conditions such as:

• Seasonal variability – broodstock caught on near-shore spawning grounds in north Queensland in August, which are thought to be the first broodstock to develop mature eggs after a dormant period during winter, often perform poorly in hatcheries.
• Location of trawling – broodstock captured from off-shore grounds are sometimes reported as inferior to those from near-shore grounds.
• Multiple spawnings – hatch rates can decrease with progressive spawnings after eyestalk ablation.
• Resident time in hatchery – if females molt in the hatchery before spawning hatch rates can be poor.
• Pond reared broodstock – performance of pond reared broodstock have been reported to be inferior to that of wild broodstock.
• Partial spawnings – females may only ovulate part of the egg mass, resulting in poor egg output per spawning
• Stress spawnings – females may spawn without normal spawning behaviour, such that the egg mass is shed in one spot as a gelatinous pile, and these generally fail to hatch.

Most hatcheries use hatch rates as a key performance indicator of broodstock. Hatch rate, however, is influenced by several factors. For example, male performance is dependent on sperm quantity per spermatophore and fertilisation capability of the sperm, ie ‘sperm quality’. Similarly, female performance is dependent on such parameters as total eggs spawned per spawning, egg quality, and hatchability. During egg formation in the female all of the nutritional and other requirements for successful hatching are ‘packaged’ in the egg at this time. In peaneid prawns, functional mouthparts do not occur until the fourth naupliar stage, even though the larvae do not actually become self-feeding until zoea stage 1. Therefore, the larvae are totally dependent on their accumulated yolk for nutrition during the first 36 to 48 hours after hatching. All of the nutritional and other compounds necessary for successful embryogenesis and early larvae development have all been previously stored during egg maturation within the mother as her ovaries matured. The yolk and its associated compounds are a major determinant of egg quality.

This manual has been compiled for hatchery operators to assist in the identification of some broodstock performance measures. Hatchery operators regularly report poor hatch rates from females that have average to above average egg production. A critical first step in isolating the reason for poor spawner performance is to identify the problem so that a treatment can be specified. This manual describes one approach to accurately determine whether poor hatches are due to the lack of fertilisation, and hence probably due to poor male quality, rather than poor female performance and egg quality. If fertilisation is high and hatch rates are poor, then either the female should be replaced or holding conditions of the female stock need improvement.

This manual is divided into three sections. Section I describes the general breeding biology of P. monodon. Although this information is not required to determine fertility rates it is included to assist in the potential trouble shooting of broodstock performance issues. Although much of the information may already be known to some hatchery personnel, the manual should provide a useful reference document, particularly when training new staff. Section II details the developmental changes that can be observed in egg development from the time of spawning and post-fertilisation to hatching. Section III describes the determination of fertility rates and how to distinguish between fertile and non-fertile eggs.

We are indebted to Don Booth, Matthew Salmon and a host of others who have either worked or volunteered in the Maturation and Hatchery Unit at AIMS. We acknowledge the assistance of the staff of the Anton Breinl Centre (Department of Public Health and Tropical Medicine), Townsville, for access to and assistance with flow cytometry equipment. The photographs in Figures 1.3b, d, f, h are credited to the PhD thesis work of Carol Fraser and the CRC Aquaculture Ltd. The video of early embryogenesis was produced by Marc Leutjens while on a visiting research program at AIMS. We thank Kate Wilson for critically reading and making comments on earlier drafts. The Science Communication section at AIMS kindly assisted in the final moulding of the text and illustrations. Barry Tobin produced the necessary transformations for publication on the web. The Fisheries Research and Development Corporation and AIMS funded the work that made this manual possible.

Lo C-F, Chang Y-S, Cheng C-T, and Kou G-H (1998) PCR Monitoring of Cultured Shrimp for White Spot Syndrome Virus (WSSV) Infection in Growout Ponds. Pp. 281-286. In "Advances in Shrimp Biotechnology", Flegel T W (ed). BIOTEC, Thailand.

Walker P J, Cowley J A, Spann K M, and Dimmock C M (1998) The Emergence of Yellow Head-Related Viruses in Australia. Pp. 263-265. In "Advances in Shrimp Biotechnology", Flegel T W (ed). BIOTEC, Thailand.