Cobia aquaculture

Cobia (Rachycentron canadum) are large marine finfish that are well suited for aquaculture.

Market and industry

The main market is for whole fish produced in the first growing season. Cobia products are positively received by the public and are well known in the high-end food-service sector.

Cobia’s cooked flesh is white and firm with tight muscle bundles and is ideal for value-streamed (e.g. smoked) products. Hot-smoked cobia has sensory characteristics comparable to other smoked fish.

Cobia has strong retail potential as an option for diversification for aquaculture producers in southern and northern Queensland.

Specific advantages for aquaculture investment include:

fast growth of up to 6kg in the first year of life.
suitable for a range of production systems
superior flavour and texture compared to Yellowtail Kingfish and equivalent to Atlantic Salmon
strong market visibility and acceptance.

Cobia farming also faces challenges, including access to high quality water supplies, feed costs and susceptibility to disease.

Culture and production systems

Cobia can be farmed using:

recirculating tanks
coastal marine ponds
sea cages
inland areas fed by amended groundwaters.

Breeding

Cobia reach sexual maturity within 2 years of age under aquaculture settings. Mature broodfish spawn multiple times, from mid-spring to mid-autumn. Photothermal manipulation (27–28°C; 14-hour photoperiod) maximises ovarian development.

Hormone therapy using pelletised implants of luteinising hormone-releasing hormone analogue (LHRHa) can be used to induce final oocyte maturation, ovulation, and spawning in broodfish.

Groups of selected spawners are placed in a separate tank and spawn 37–40 hours after implantation.

Chemicals used in aquaculture, such as LHRHa, require veterinary advice and prescriptions before use.

Egg production

Females typically release more than 2 million eggs per group spawning event. Fertilised eggs:

are spherical and buoyant
hatch 22–24 hours after fertilisation, at 27–28°C
can be taken to hatcheries at the gastrula stage (approximately 12 hours after fertilisation)
should be transported in oxygen-pressurised aquarium bags to maximise hatching success.

A day after hatching, larvae are about 4mm (standard length) and exhibit first feeding by 3 days post-hatch (at 4.5mm to 4.8mm standard length).

Fingerling production

To produce fingerlings, larvae need to be:

stocked at up to 50 larva per litre in intensive rearing tanks that contain live microalgae
fed with successions of enriched rotifers (microscopic animals) and Artemia (also known as brine shrimp) until 12–14 days post hatch when they are 8–10mm standard length, before transitioning to manufactured weaning diets
graded (sorted by size) regularly to avoid cannibalism.

You may need to use several larval rearing and nursery phases to produce cobia fingerlings.

Alternatively, you can transfer larvae to extensive pond systems to feed on zooplankton (e.g. copepods) which are stimulated to bloom using organic and inorganic fertilisers.

At 25 days post–hatch, the metamorphosis is almost complete. Fingerlings can be moved or harvested at 35–40 days post hatch when they are 70–85mm standard length into indoor, recirculating systems or secondary, nursery phase outdoor ponds.

Feeding and growing

Cobia have high nutritional requirements. Deficiencies or excesses can lead to disease and reduced growth performance, particularly deficiencies in taurine. Dietary formulas with high levels of taurine vastly improve growth, performance and survival.

Cobia are voracious carnivores and readily accept manufactured floating pellet or kibble diets. The size of the pellet or kibble you use should vary with the size of the fish. Cobia also respond positively to feed ingredients that enhance digestibility.

Feeding frequency

Optimising feeding frequency, particularly for harvest–sized cobia (>2 kg) can be a practical strategy to reduce feed input without affecting growth performance. Feed conversion ratios (kilogram of feed to produce 1kg of cobia) range from 1.5:1–2.5:1 and depend on:

feed composition
fish age
stocking density
production system.

Grow–out

When cobia juveniles reach 20–30g, they can be stocked into grow-out ponds at 4000–7000 fish per ha. It's possible to produce 10–30 t/ha, depending on the geographic region.

Warmer temperatures in tropical North Queensland lead to higher growth performance. In southern regions, we recommend over–wintering under controlled conditions before stocking ponds to mitigate suppressed feeding and immunity that result from cooler waters(<20°C).

Stocking density impacts growth performance and needs to be managed throughout the growth cycle. Stock cobia in cages or compartments within ponds to better control health management, sorting, and harvesting.

Manage water quality carefully, using aeration and water exchange. This is necessary to handle Cobia's high levels of oxygen consumption and ammonia excretion. Exposure to extended periods of low salinity (< 8 ppt) can also pose health risks.

Managing disease

Taurine deficiency

Taurine deficiency is associated with:

green liver
kidney inflammation
bacterial infection by Photobacterium and Vibrio species.

Elevated dietary lipids can cause pancreatitis and increased visceral fat.

Malformations

Within 2 days of hatching, malformations such as notochordal axis syndromes (malformation of the vertebrae and nervous system) and pericardial oedema (fluid buildup around the heart) may develop. Malformed fish do not survive and malformations contribute to high mortality rates.

Bacterial disease

Epitheliocystis is a skin and gill disease in fish caused by pathogenic intracellular bacteria. Epitheliocystis are usually detected 18–35 days post hatch. Early detection and treatment with medicated feeds improves survival rates.

To prevent outbreaks and respiratory distress:

avoid rearing cobia larvae in water that has low salinity or is high in fine suspended solids
rigorously monitor and maintain on-farm water quality
ensure you have a health-monitoring program in place.

Parasites

Parasitic dinoflagellate (single-celled organisms) like Amyloodinium ocellatum and monogenean parasites (flatworms) such as Neobenedenia girellae can affect juvenile cobia, harvestable-sized fish, and broodfish. Initial signs include slowed movement and changes to feeding behaviour.

Some types of parasite eggs resist treatment, and repeat treatments for re-infestations may be necessary in semi-closed systems, like recirculating tanks or ponds.

Intersex

Cobia were the first reported gonochoristic fish species in Australia to exhibit intersex gonads. This is likely due to endocrine disruptors in intake waters affecting the reproductive system.

Intersex fish can severely impact sexual productivity. Female cobia grow dimorphically, which means a female cobia grows much faster than a male after reaching 2kg. Intersex gonads suppress this dimorphic growth.

Affected fish should be removed, and humanely killed.