Production of Mussels

The mussels are bivalve molluscs found attached to the hard surfaces in the littoral and sublittoral zones. They attach themselves to the substrate by secreting long threads called byssus. Though they are considered sedentary, they may move from one area to another, if exposed to unfavourable environmental conditions. Mussels are regarded as one of the best candidates for aquaculture since they are filter feeders obtaining nourishment from the lowest level in the food chain; they feed on phytoplankton, detritus and associated microscopic flora and fauna. The two important species of mussels in India are the Green mussel, Perna viridis and the Brown mussel Perna indica.

Comparison between brown and green mussel

The green mussel has a wider distribution along the west and east coasts of India, including the Andaman Islands. In contrast, the brown mussel is restricted to the southwest coast of India. Now fishery for the green mussel exists in the region from Kollam to Kasargod and for brown mussel from Kollam to southwards along the Kerala Coast. Mussel provides animal protein of high nutritional value. Fast growth rate, adaptability to varying environmental conditions such as short periods of exposure to extreme temperatures, salinities, desiccation, relatively high levels of turbidity and simple culture technique makes it a candidate species for aquaculture in coastal waters.

Seed Production

A primary requisite in any farming operation is an abundant, reliable and inexpensive supply of seed. At present, most bivalve culture operations in the world are moving to hatchery produced seed rather than collecting seeds from natural sites. The natural seed is collected by keeping substrate or spat collection ropes in breeding areas to collect metamorphosing larvae, or the juveniles and transferred to growing areas for culture (grow-out) to marketable size. In other operations, juveniles are gathered from areas of natural abundance and are transported to growing fields that may be distant from the source of the seed. The alternative for the collection of the natural spat of bivalves is to produce seed in the hatchery. The uncertainty in the availability of natural spat in good quality and quantity has led to the stagnation of mussel farming in the last decade. And this has prompted ICAR-CMFRI to develop the hatchery technology for bivalves. The hatchery must be located close to the sea where pollution-free seawater of desired salinity is available throughout the year. Preferably an area where adult and mature mussel of the required size is available.

Procurement of broodstock

In mussels, the sexes are separate, and they attain sexual maturity within a year. The mature broodstock having a minimum size of 6-7 cm size is collected from the wild, quarantined and maintained primarily in the broodstock holding tank of l t capacity at a density of 3-4 g/l of its live weight. Before feeding in the morning, the water in the tank is replaced daily to avoid build-up of bacteria and metabolic waste and provided with Isochrysis sp. and Chaetoceros sp. cells @ 5-6 million/ml. Around 60-80 l of algal culture per tank is used to feed daily. If sufficiently mature brood-mussels are available, they can be directly used for spawning or kept under low-temperature re-circulation system for a long time.

Maturation

Maturation of broodstock is done in an FRP tank of 1 t capacity which has special provision for photoperiod adjustment and hot and cold water facilities. For gonadal maturation, adult male and female mussels are placed in the tank at a density of 3-4 g/l of the total live weight biomass by adjusting the photoperiod (12 hr light and 12 hr dark) and maintaining the water temperature between 20-26°C. It is fed with Isochrysis galbana and Chaetoceros sp @ 7 million cells/ml. Algal culture of 80-100 l per tank is used for daily feeding.

Spawning

The mussel can be easily stimulated to spawn in a hatchery, if they are fully mature with turgid gonad. In mussel, sexes are separate, and the reproductive condition of broodstock is determined by visual examination of the gonad which includes the assessment of the physical extent, fullness and colour of gonad and the degree to which it is filled with gametes. The testis is creamy white in colour while that of the ovary is orange or reddish. During spawning, mussel loses up to one- third of its body weight. Spawning of the mature brooders can be carried out in spawning tank of 200-500 l capacity or trays at a density of 3-10 numbers. A rise in 4-8⁰C above the ambient temperature induces spawning of green mussel. Millions of eggs are freely released by the female into the water, which are fertilised simultaneously by the sperms of males, and the eggs settle down. The fecundity of adult mussel is 5- 20 million, and the hatching rate from egg to larva is 95 %.

In the case of strip spawning, sperm suspension is added to egg suspension. The gamete suspension is then gently mixed, and a sample is examined to ensure a sufficient number of sperms. The fertilisation occurs almost instantly in tanks, upto 10-15 minutes after the females spawn. The first polar body can be observed under a microscope 15-20 minutes after fertilisation.

Incubation

After spawning, the adults are removed from the spawning tank, and the fertilised eggs are collected and rinsed by pouring through a 20 qm sieve held in a basin of filtered seawater to remove the excess sperm, unfertilised egg and metabolic waste. It is then incubated at a density up to 10-15 no./m1 in a glass tank (200 l) or FRP tank (500-1000 l) provided with gentle aeration. The fertilised egg starts cell division in 20 minutes, divides repeatedly and hatches-out into morula larva. After hatching, the embryos are passed through a 100-150 qm mesh screen suspended in the tank to remove larger debris. Optimum salinity and temperature are 25-35 ppt and 24-27⁰C.

The morula exhibits phototropism, swims and congregates at the surface. 5 hours after fertilisation, it gets transformed to blastula by the re-orientation of the cells. The cells then convolute in and form dermal layers and gastrula stage is formed within 6-7 hours after fertilisation. Gastrula stage transforms into trochophore within 7-8 hours by developing a long single flagellum and tuft of cilia at the apical side and the rear side and swim with the flagellum. The ectodermal cells of trochophore secrete embryonic shell material and assume a 'D' shaped veliger or straight hinge stage by 18-20 hours at 27^oC in which the flagellum and tufts of cilia disappear, and a new locomotory organ called velum develops. The early embryonic development of the larva is completed by veliger stage which measures 50-55 micrometer dorso-ventrally.

Rearing of larvae

The D-veliger larvae are transferred into an FRP larval-rearing tank (2-10 t capacity) for rearing till the settlement or transferred at eyespot stage to a downwelling system. Washing, grading, counting and measuring the larva is done in every alternate days. The veliger metamorphosed into umbo stage (130-260 micrometer) within 7-15 days in which shell valves are equal, and mantle folds develop. The umbo stage reaches the eyespot stage (260-367 micrometer) within 14-17^th day when the blackspot is seen at the base of foot bud with the development of ctenidial edges. The larvae are fed with mixed algal diet consisting of Isochrysis galbana, Nanochloropsis oculata and Pavlova sp. The daily requirement of algal cell up to eyespot stage is given in the Tabe below.

Feeding schedule

Settling of larvae and rearing of juveniles

Development of foot is observed on 17-19 days indicating the transformation of eyespot larva into pediveliger stage with the appearance of gill filaments and it is transferred into upweller microhatchery  unit. Once the foot becomes functional, the ciliated velum disappears, and the larva starts settling to the bottom, and byssal gland becomes active and secretes byssus threads for their attachment. The pediveliger stage is a transitional stage from swimming to crawling, and the larva has both velum (velar cilia) for swimming and foot for crawling. Plantigrade stage (390-470 micrometer) is reached within 20-21 days by the secretion of the adult shell with fast shell growth all along the margin, except umbo region. Labial palps, additional gill filaments and byssus threads also develop further at this stage. The transformation of plantigrade into spat (490-550 micrometer) starts within 21-28 days by the extension of anterior and posterior ears wherein left valve is slightly concave than the right. On the 42^nd day, the spats are harvested from the rearing tank.

Plantigrade stage & Spat (42^nd day)

The settled spat are stocked in the tank for culture until they reach 2 mm shell length. Larval-rearing can be done from the eyespot stage, and further rearing in nursery phase can be done in up-wellers, down-wellers or tray systems of varying configuration.

After settlement, the larvae are fed with mixed algal diet consisting of Chaetoceros spp. and Isochrysis spp. at algae cell density of 25,000 no./ml along with Nanochloropsis marina at a cell density of 50,000 no./ml. The daily requirement of algal cell to feed the bivalves from the pediveliger stage to spat is given in the Tabe below.

Feeding schedule

Algal culture

The success of a bivalve hatchery depends on the production of algae. Large quantities of high-quality algae must be available when needed. Since algae are used in all phases of production, the facility should be located centrally and conveniently. Space required for algal culture depends partly on levels of production, methods of culture and whether algae will be raised entirely inside the hatchery with artificial illumination, or if it will be raised outside under natural light, or a combination of the two. A well-ventilated greenhouse is required if algae are grown in natural light, and this structure needs to be placed so as to obtain the maximum amount of sunlight. Shading may be needed to protect younger, less dense cultures from strong sunlight.

Primarily algal culture unit consists of stock culture unit, carboy culture unit and indoor mass culture unit, or outdoor mass culture unit. The stock culture unit is a small air-conditioned insulated room where the isolated algae from the source water are cultured in various containers placed in shelves with fluorescent lights and aeration. Test tubes with algal slants and small flasks with the stock culture that are monospecific and axenic are kept in this room often in a refrigerated, illuminated incubator.

In carboy culture units, the algae are grown in carboy bottle of 20 l using the algae grown from stock culture unit as inoculums which can be made as a part of the stock culture facility or as a separate unit outside the stock culture area. Here also illumination with fluorescent lamps, aeration and carbon dioxide supply are required to maintain the healthy development of algae. Stock and carboy culture of algae are the two major units which have to be maintained with utmost care. For maintaining the healthy growth of the algae, the water used for preparing the culture medium must be sterile and free from any contamination. Sterilization of the water is achieved by ozone treatment of the water before preparing the medium. Indoor mass culture of algae is done in FRP tank of 1 t capacity which is housed in a building provided with intermittent opaque and transparent portions for subdued light penetration for the development of desirable species of the algae with a continuous aeration supply. Outdoor mass culture of algae is done in tanks of 2-5 t capacity. Indoor/outdoor raceways can also be used.

Water Intake and Treatment system

Water is pumped directly from the sea through in-situ filters which is first filtered using slow sand filters that filter out most particulate material greater than 20 pm. A slow sand filter consists of a tank inside of which lies a bed of sand supported by gravel. Water is allowed to flow through this layer of sand with particles of varying sizes and depth. The layer is not dense but contains a number of channels and holes created between the particles that constitute filter medium. When water passes through the filter medium, particles larger than a specific size will be trapped in the medium and get filtered.

Water Intake, Treatment and Distribution System

Water filtered through the slow sand filter is collected in a water storage sump and treated with chlorine to remove the microbes and after dechlorination, again filtered through rapid sand filter to remove minute particles and stored in an over-head-tank so that the effect of gravity maintains a sufficient water flow through various units of the hatchery. Before utilizing the water for various hatchery purposes, final sterilization is achieved by UV irradiation. Sea water intended for stock culture of algae is further sterilized by ozone treatment to achieve 100% disinfection, which is highly essential for maintaining the pure culture.

Harvest and transportation

Mussel seed can be harvested from micro meshed cages or nursey silos by hand and transported safely in wet gunny bags upto 12 hours.

Micron-mesh cage & Spat in mesh cage

Farming in Backwaters

Site selection

Coastal waters free from navigation are suitable for mussel farming. Fluctuation in salinity during monsoon season is one of the main constraint in estuarine mussel farming. Usually, the culture period is from November to May in Kerala.

Water quality parameters

Water current : 17-35 cm/s

Temperature : 25-33°C

Salinity : 22-33 ppt

Farming structure

Rack culture is ideal for estuarine conditions where the water depth is between 1.5-3 m. The ideal size of fixed rack culture is 25 m2 (5x5 m) which is fabricated by placing bamboo/casuarina poles and tying with nylon ropes. Nine poles having length more than the water depth during maximum high tide is driven into the bottom and spaced at a distance of 2.5 m apart, and it is connected to each other in both directions by horizontally placed six poles of more than 5 m length. The horizontal poles should be above the water level at high tide, and the seeded ropes are suspended from it. In shallow areas of below 1.5 m depth, both ends of the seeded ropes are horizontally tied on to poles.

In on-bottom culture, mussel seeds are relayed on the bottom of a water body leaving them to grow until the harvest, and this is generally practised in open waters or pens which can also be practised in shrimp or fish pond at a low stocking density. In this case, the mussel seeds will form clumps within a week and grow.

Raft culture is ideal, if the water depth is more than 3 m, where the ropes are suspended from a floating raft of 25 m² (5x5 m) at the surface of the water. The raft is made of bamboo poles placed parallel and across and tied with synthetic rope, and it is held afloat by tying with four airtight barrels of 200 l capacity at the corners and moored with concrete block. Protected bay and harbour are ideal for this.

Seeding of green mussel

Farming of mussel is mostly dependent on wild-collected spat which is collected manually during low tide from the natural bed available in the intertidal and sub-tidal waters. At first, the collected spat is thoroughly cleaned to remove epifauna and other organisms. The length of the seeded rope ranges between 1-2 m depending on the water depth. At first, a mosquito net of 20-25 cm width and required size is cut and spread on a smooth and flat surface in a shady place. At the middle of these pre-arranged netting, a rope of 18-22 mm diameter is placed length-wise. The spat of 15-25 mm @ 600-1000 g/m is spread uniformly in the netting and over the rope and thereafter wrapped inside the netting by keeping the rope at the centre and stitched tightly to get the spat cover around the rope. For avoiding slippage of mussels, knots are made or 10-15 cm length bamboo peg is inserted horizontally in between the twists of the seeded rope at regular interval of 25 cm.

After seeding, the seeded rope is suspended immediately from the farming structures. Generally, 60-120 no. of seeded strings with a length of 1-2 m are suspended 0.5-1 m apart. Within 2-3 days, the cloth starts to disintegrate, and the seed gets attached to the culture rope using byssus thread.

Care & Monitoring

The growth of the mussel depends on tidal flow and primary production. When the mussel is continuously submerged in water having good phytoplankton productivity and adequate particulate organic matter comprising of detritus, it grows rapidly. The seeded rope should be regularly examined and cleaned gently with a brush made of natural fibre to remove mud, silt and any fouling organisms. The major predators of mussel are crab, lobster and starfish.

Harvesting

Typically, harvesting of the mussel is done during April to June along the west coast of India and farmers are forced to sell their crop before the onset of monsoon to avoid mass mortality due to freshwater influx depending on the distance from bar mouth. Under culture conditions, green mussel and brown mussel attain a size of 80-88 mm (36-40 g) and 60-65 mm (25-40 g) respectively and yield production @ 5-10 kg/m of rope over 6-7 months. The farmed mussels give a better meat yield compared to those from the natural bed. As a filter feeder, it harbours microorganisms and contaminants present in the surrounding waters. Hence, a cleaning process called depuration is necessary to render the animal free of bacterial load and contaminants. When blooms of dinoflagellates occur, the harvest of mussel should be suspended as consumption of mussel from the affected area may cause gastro- intestinal disorders to the consumer.

Mussel Farming in the Sea

Mussels can be farmed in the sea using rafts or long lines; protected areas like bays are preferred compared to open waters. The long line culture is ideal for marine conditions at a depth of 5-20 m. Seeding and other management procedures are the same except that it has to be appropriately moored using heavy anchors or gabion boxes loaded with rocks. The long line is made of a 50-150 m long and 16-22 mm diameter synthetic rope which is held afloat with barrels or large floats and moored with anchors. The seeded ropes are suspended from the mainline. Sea farming of mussel is vulnerable to poaching, unpredictable climatic conditions and predation.

Source : Department of Fisheries, Government of Kerala