Advances in the seed production of Cobia in Vietnam

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Posted by SimonWilkinson on 4/9/2005 7:00:00 (175 reads)  
By Le Xan
Research Institute for Aquaculture No I, Vietnam.

Cobia Rachycentron canadum culture is expanding throughout the world, notably in China and Vietnam. Cobia have an extensive natural distribution, grow quickly, and can feed on artificial diets. Under culture conditions, Cobia can reach 3–4 kg in body weight in one year and 8–10 kg in two years. The products from Cobia are exported to the US, Taiwan Province of China and local markets. Market price of one-year farmed Cobia are around US$ 4–6 kg in Vietnam. Research on seed production and grow out culture of cobia in Vietnam began in 1997-1998.

Right: Cobia broodstock

Broodstock and spawning
Broodstock can be acquired by purchasing wild fish or by collecting dominant individuals from grow-out operations (selecting broodstock from different parents to avoid inbreeding). Most of the fish more than two years in age have fully developed ovaries, but it is best to collect three-year old broodstock if possible. In Vietnam, cobia spawn twice per year during April to May and September to October. Conditioning of broodstock usually starts some 3-4 months before anticipated spawning, by feeding with trash fish, squid and swimming crabs supplemented with mineral vitamins and 17a-methyltestosterone. The amount of trash fish fed is about 4 – 5%/body weight per day.

Mature fish are spawned in dedicated spawning tanks or sometimes in floating net cages. Spawning tanks are 60m3 in volume with a depth of 2.5m. Female broodstock are administered with an injection of LRH-e or LRH-a at a dosage of 20 mg/kg female, with males receiving half of this dose. There isn’t a need to injection all females but only one or two pairs. Spawning of cobia usually takes place at night, although it occasionally also happens during the day. After spawning, fertilized eggs are separated out and collected using seawater at 35–36‰. Sinking eggs should be discarded.

Eggs are stocked in the incubation tank at a density of 2000–3000 eggs/litre. The incubation tank is 500m3 in volume maintained with light aeration. Water exchange is carried out at 200-300% per day, using an input and overflow pipe system.

Larval rearing
Larvae of cobia are reared in cement ponds, composite tanks or earthen ponds. A suitable pond size is of 400-500m3 in volume with an average depth of 1–1.2 metres. Rearing ponds are fertilized to stimulate production of natural live feed before stocking with larvae. Live feed density needs to checked frequently, and if low, must be supplemented with correctly sized live feeds (rotifer or copepod) to suit the larvae as they grow. After 22 – 25 days, larvae can be fed with mixed food or artificial diets. However, there may be a need to transfer larvae to a larval rearing tank where they can be trained to accept the new food and receive proper care.

Right: Larval cobia.

A suitable size for larval rearing tanks is 3–10m3 in volume. The optimal temperature for rearing the larvae is in the range 24–30OC, with a salinity of 28–32‰,pH 7.5–8.5 and light intensity about 500 lux. Larvae of cobia that must be tamed can be reared in salinity of 20 – 22‰. The microalgae N. ocullata, Chlorella or I. Galabana should be supplied and maintained at a density of around 40,000–60,000 cells/ml in the rearing tanks. We have found that dark coloured larval rearing tanks (green or black) tend to give better larval survival.

Density
The optimal density for larvae in rearing tanks varies with their age as follows:
* 1–10 days larvae density at 70–80 individual/litre
* 11–20 days larvae density at 20–30 individuals/litre
* 21–30 days larvae density lower than 10 individuals/litre.

In the earthen ponds, stocking density are 1,500-2,000 individuals/m2.

Right: Cobia fingerlings.

Water exchange
Daily water exchange rates are as follows:
* Between days 0–10, 0–10% of tank water is exchanged.
* Between days 11–20, 30–50% of tank water is exchanged using natural flow.
* After day 20, 100–200% of tank water is exchanged daily. We use a simple biofilter, but the electricity cost can be quite high.

Grading
Grading is very important to reduce cannibalism. By day 25, larvae harvested from rearing tanks should be graded into small and large size groups, and maintained separately with their own rearing regimes.

Feeding larvae
First larval feeding is with rotifer B. plicatilis at a density of 15 individuals per ml until 12 days after hatching. Artemia nauplii can be given from 7–20 day old larvae. Artificial feeds can be introduced from day 17–18, but it typically takes around 3-4 days to train the larvae to accept them.

In feeding experiments using enriched rotifers and Artemia nauplii we found that the enriched live feeds give better results than unenriched feeds.
The composition of artificial diets we use are as follow:
* Fresh tunny meat minced: 47%
* Mixed fish meal (45% protein): 25%
* Soybean meal, rice bran meal: 15%
* Vitamins, mineral meal: 3%

All compositions are mixed; crushed and sieved to a size suitable for the larvae’s mouth. Artificial diets should be made daily.

Metamorphosis in cobia requires around 25 days to complete at a temperature of 26–28OC with adequate feed. After day 25, larvae can be weaned completely onto artificial diets.

In Vietnam, some hatcheries involved in rearing cobia larvae with the regime above achieve a survival rate of 15–20% (from day 0–day 25), and 40–50% from day 25 to 50, after which fry are typically around 7.5-8.5 cm in length