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).