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Fisheries > Culture Fisheries > Mussels

Hatchery Production of Spat

The basic technology for the production of spat of P. viridis has been developed by CMFRI at Madras and for P. indica at Vizhinjam. The criteria to be followed for site selection and the inputs required for the establishment of the mussel hatchery are given below.

Site selection

Selection of suitable site is the most important factor for successful running of a hatchery. The hatchery should be located near sea front with uninterrupted supply of good quality seawater, free from pollutants and suspended particles like silt. Areas close to the river mouths and discharge points of the industrial effluents and sewage are to be avoided. A salinity range of the seawater between 30-36 ppt is considered as optimum for year round seed production. It is of advantage to draw the seawater to the hatchery from a region where the substratum is rocky or coralline so that clear seawater with minimum turbidity becomes available. Freshwater supply, power and good approach road to the hatchery site are also necessary.

Spawning, fertilisation and development

Gonadal egg development begins when the water temperature falls below 21oC. Mussels in spawning condition are ‘fat’ and can be recognized by eye at this stage. During spawning, a mussel may produce up to 8 million eggs, each of which is 70 mm (0.07 mm) in diameter. Depending upon environmental factors, such as temperature, larvae may spend from three weeks to three months living as plankton prior to settlement. Water temperatures of 14oC induce spawning.

Male and female mussels are sexually mature within the first year; however, it is not until they are in their second year that they are at their optimum fecundity. Blue mussels are ‘broadcast spawners’, releasing eggs and sperm simultaneously into the water with fertilisation taking place in open water.

The sexes are separate although hermaphrodites occur occasionally. The fertilised eggs are planktonic, averaging from 0.07 mm in diameter and develop into unshelled larvae within a day. Blue mussel larvae are free swimming meaning their distribution is dependent on the tides and current. The larvae settle 2 to 4 weeks after spawning and are called ‘pediveligers’ at this stage. The pediveliger periodically settles to the sea floor to test for suitable substrate. If a suitable substrate is not found, the pediveliger moves back into the water column. The settlement substrate is very important, as it will determine the density of spat. Pediveligers are approximately 0.2 mm long when they attach themselves to a substrate by the ‘byssus’, a thread-like structure. Once the pediveliger begins to undergo metamorphosis they are called ‘spat’. Throughout life the blue mussel can move by releasing from the substrate and re-attaching to a new site by secreting new byssus threads.

In P. indica a temperature jump of 4°C above the ambient resulted in induced spawning. The eggs are brick red in colour and measure 45-50 mm. The various developmental stages are as follows:

Stage

Size mm

Hours/days

Morula

58-60

4 hrs

Trochophore

65-70

7 hrs

Early straight-hinge

70-76

17-20 hrs

Early umbo

120-140

7th hrs

Eyed larva

208-260

13th and 14th day

Pediveliger

280-320

16th day onwards

Heavy spat settlement takes place from 20th day onwards and continuous for 5-9 days. The spat set early attains a maximum length of 2.7 mm by 32nd day. Among various collectors tried, bunches of polyethylene monofilament gave good results. Also the sides and bottom of the spat rearing tanks provided favourable substratum for spat settlement.

Seed collection

The spawning season of the green mussel is between July and September and the spats are found carpeting the intertidal and submerged rocks. At present they are collected manually and during the peak season an individual would be able to collect 10-12 kg of seed in one hour. The seeds can also be collected using spat collectors such as roof tiles, coir ropes and nylon ropes. Even though Central Marine Fisheries Research Institute, Kochi, has perfected the hatchery technique for commercial mussel spat production there is no commercial hatchery at present in India. As such the culture operations have to depend on the availability of natural seed.

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