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Chapter: Aquaculture Principles and Practices: Oysters and Mussels

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Seed stock of Mussels

There are several methods of collecting seed stock, but many of them are only local variations.

Seed stock

There are several methods of collecting seed stock, but many of them are only local variations. Bottom culture of mussels, as practised in the Netherlands, depends on seed stock which has settled on the intertidal and deeper waters, which is collected manually or by dredging from special mussel boats fitted with a number of dredges.

In areas where bouchet or stick culture is practised, the same substrate is also used for collection of seed stock. The poles or sticks are erected in rows near natural mussel beds where spat setting normally occurs regularly, a few months before the spat-fall season (from May to about July in France). During the period before the settling of mussel larvae, barnacles usually settle on the poles and hydroids like Tubularia settle in turn on the barnacle shells,and these constitute excellent substrates for the settling of mussel larvae. As the young mussels grow they leave the original filamentous support and attach themselves to the poles. Harvesting of the mussel seed is performed before the poles become overcrowded, by scraping with special tools.

 

Another means of collecting mussel seed in France is on layers of fibre ropes (mainly coir ropes) of about 1 cm thickness, stretched horizontally on supports of wooden beams built on poles driven in the deeper portions of the intertidal zone. The fibre surface is a very suitable substrate for mussel seed, and facilitates early settlement and quick growth. As the mussels grow fast, the ropes have to be transferred for on-growing before they become too heavy with grown mussels. In the Philippines, green mussel seed are collected on bamboo stakes and they grow on the same stakes until they reach market size. Seed are also sometimes collected on bivalve shells or coconut shells and then transferred to the stakes or rafts for on-growing.

For raft culture in Spain, seed mussels are collected on the same rafts used for grow-out. Small amounts are also gathered from sublittoral areas by raking. Traditionally, ropes made of esparto grass are used for seed collection. Other fibre ropes of sisal or nylon are also presently used. The ropes vary in length from 2.5 to 6m, according to the depth of water in the area. The ropes have wooden stays of about 1.5 cm diameter and about 24 cm length, at intervals of about 40 cm, to prevent the mussels sliding down the ropes during bad weather. Though larvae occur in the water throughout the year, the peak settlement is in spring or early summer, with a second peak in autumn. While the larvae of the spring spawn attach to the ropes and growing mussels on the rafts, the autumn larvae seem to attach only to rocks and boulders on the coasts. Seed of autumn spawn-ing show much higher growth rates, reaching a marketable size of 8–9 cm in 15 months after settling, while those of the spring spawning require over one and a half years to reach that size. So there is a greater demand for seed stock from the autumn spawning.

As can be seen from the above, seed for commercial mussel culture is obtained solely from natural reproduction. Methods of induced spawning have been developed, but only for experimental purposes. Sexually mature animals, when transferred to water at a high temperature after mechanical stimulation, will release gametes. Breaking off a chip from the valvular shell at the level of the visceral gan-glion and then transferring the animal to a higher temperature of 18–20°C will also induce spawning. Mussels transferred from winter temperatures of 0–1°C to a controlled environment of about 18°C have responded well and released gametes (Hrs-Brenko,1973).




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