By Luke Dutney and Paul Palmer
Queensland Aquaculture News, Issue 21, june 2008. Pp:6-7

Cobia (Rachycentron canadum) have been successfully cultured throughout the Asia-Pacific region since the early 1990s, but the species only recently grabbed the attention of Queensland producers. Desirable biological traits for culture and rapid growth rates (6 kg in the first year, post-hatch), coupled with their high market appeal, make cobia an ideal candidate for aquaculture.

The primary requirements to demonstrate the viability of cobia culture in Australia are the development of hatchery protocols and feeding systems suited to Australian aquaculture practices. This will provide the biological material and economic data required to assess the species' suitability for existing aquaculture facilities.

These are the guiding principles for the collaborative project that has been developed over the last 12 months. Bribie Island Aquaculture Research Centre (BIARC) was tasked with developing successful spawning and larval-rearing protocols, and Gold Coast Marine Aquaculture (GCMA) was supplied with larvae and weaned juveniles to facilitate commercial evaluation and market appraisal of the species. Further planned work would see CSIRO and Ridley Aqua-Feed play major roles in feeds development.
Wider dissemination of juveniles will be considered pending development of the project beyond the current preliminary stage-taking into account commercial sensitivities and the results of the biological and economic analyses.

Data generated on the biological aspects of cobia culture will be published, widely disseminated and promoted. Generic economic analysis of the profitability of cobia culture, while protecting commercially sensitive data, will also be developed for public release.

Broodstock collection

Shortly after the inception of the project, two adult cobia (captured in 2006) were supplied to BIARC by GCMA. Both fish were placed on a compressed phototherm regime inside a 40 000 L tank supplied with flow-through seawater. They provided some early hands-on experience with the species, allowing BIARC to examine the fishes' behaviour and feeding habits and to refine their handling techniques.

Large-scale broodstock collection attempts were initiated in October 2007, using the DPI&F's 14 m research vessel Tom Marshall. The vessel was ideally suited to the collection exercise with 2 x 1000 L seawater flow-through transport tanks mounted on the deck and capacity to safely carry up to 10 crew. A local professional fisher and an experienced recreational fisher from CSIRO assisted DPI&F staff. Two trips were conducted in the outer reaches of Moreton Bay and 17 cobia (10 of which were considered mature) were successfully line caught and returned in good health to BIARC.

Hormone induction

Upon capture, the reproductive status of each fish was examined with the expectation of immediate hormone induction. Female fish were mature; however, they were considered to be in the very early stages of vitellogenesis. No sperm was extracted from the males, so hormone induction was postponed. The fish were separated by size and placed into the broodstock facilities at BIARC where they were maintained under ambient environmental conditions to avoid interrupting their natural developmental cycle.

The fish settled into a regular feeding pattern quickly, and were monitored monthly to assess their general health, weight gain and gonad development. In early January 2008 hormone induction was initiated and, on the second attempt, viable cobia eggs were produced. The first successful spawn produced in excess of 4 million viable embryos, which demonstrated high hatch rates.

A second successful hormone induction was conducted in March 2008 with a further 4 million produced. Such high fecundity and egg quality suggests that the broodstock have settled in well and are very amenable to the system at BIARC. The larvae from this later spawn will be used to conduct further preliminary work towards alternative fingerling production strategies using green water larviculture in combination with outdoor fertilised nursery ponds.

Larval-rearing

The aim of the project in these preliminary stages has been to test the suitability of established protocols and existing infrastructure for cobia production. The goals were to identify any potential bottlenecks in production and to produce sufficient juveniles for preliminary feeding trials at CSIRO, future domesticated broodstock, and pilot scale farm trials at GCMA.

The green water larval-rearing methods that have been used at BIARC in the past were applied with elements adapted from Faulk and Holt's 2005 review of green water culture of cobia. Newly hatched larvae were also supplied to GCMA to test the suitability of the methodologies and infrastructures used in their highly successful penaeid hatchery. The larviculture methods used at BIARC were found to be particularly suitable for this species up until 20 days post-hatch. However, heavy mortalities were experienced from this time through to the completion of weaning. These later mortalities are reportedly not unlike experiences with this species overseas, and certainly represent an area where there is plenty of scope for improvements to be made.

These initial attempts at cobia production have successfully achieved our primary goal of producing several thousand weaned juveniles that are suitable for some preliminary growout studies. But importantly, a number of critical factors and stages involved in fingerling production have been identified. These include fish and live feed densities and food availabilities at different sizes, aggression and cannibalism as a consequence of management approaches, and weaning strategies that may need to be particularly tailored to this species. These are all seen as major constraints to overall survival of farmed cobia, and in need of further development.

Further research

In the first year we have found that cobia are amenable to our established broodstock management and larval-rearing protocols and have definite potential for aquaculture in Queensland. However, further examination is needed to refine techniques. Future cobia research at BIARC will examine different seed production methodologies to identify the most appropriate and cost-effective options to improve larval survival and intensify weaned fingerling production. DPI&F's overall aims in this work are to provide the support necessary to appraise this species for culture in Queensland, and to provide structured assistance to researchers and industry in the future.