R. KAAS & R. PEREZ
IFREMER Center de Nantes
Rue de I'lle d'Yeu, Boite Postale 10492
44037 Nantes, France.
In 1971, the brown seaweed Undaria pinnatifida appeared in the Thau lake on the French Mediterranean coast. It was probably introduced with oysters imported from Japan and spread slowly from the lake to the open shore. So, it can now be found from Sete to Port-Vendres.
This seaweed being edible, it was established that there is in Europe and in North America a potential market for Undaria and a constant demand from Asia. So, trials have been made to develop its cultivation in France. However, it would be difficult to achieve good results in the Thau lake and along the Mediterranean coast because the temperature of the seawater and the light intensity increase too rapidly in early spring; consequently, these phenomena squeeze into too short a period the time during which Undaria is of good commercial interest. So the idea to try to cultivate it in less warm water led us to make some experiments on the Atlantic coast of Brittany. The goal was to get two crops a year.
The cultivation technique
The methods involves cultivation in a tank of large quantities of young plants fixed on a string rolled around a square plastic frame. String and frame constitute the collector for spores or gametophytes. When the plants become visible to the naked eye, the well-covered string is unrolled from the plastic frame, put out to the sea and wrapped round a long line rope, one meter beneath the surface, parallel to the prevailing waves. The methods includes three steps:
the production of seeds,
the pre-culture in tanks,
the cultivation in the open sea.
Production of the seeds
Korean, Japanese and Chinese seaweed-farmers seed the collectors directly with spores released by ripe Undaria sp. between May and June. Then the frames are hung in a shaded tank of seawater for a period of four months before being transferred outdoors. The water must be changed each week and every two or three days. The collectors must be cleared from mussels, diatoms, Ectocarpus, Cyanophyta and other intruders by cleaning with a soft brush. The technique is very costly because it needs much time and many people.
Furthermore, it allows only one seeding period and consequently only one crop a year.
IFREMER applied another seeding process by using a vegetative gametophyte technique in flasks, known as the free-living technique. This method is able to provide large amounts of male and female gametophytes issued from only several selected spores.
As a matter of fact, there is a critical point at 21–22 °C under 40 umol m-2 s-1 in continuous daylight where the Undaria gametophytes lose their capacity for gametogenesis but continue to grow quickly until they are broken by the motion of the water; each part of them grows again and again if cut, and so on, and because of increasing density of gametophytes, the flask darkens more and more. New flasks can be seeded from the darkened one; they will undergo the same process if the critical conditions are maintained. The free-living method permits one to have gametophyte-seeds all through the year on condition that competitors, such as cyanophyta and diatoms, cannot develop in the flasks. For this purpose germanium dioxide at 2.5 ppm is used against diatoms and a specific antibiotic, kanamycin, at 50 ppm inhibits blue green algal growth.
When the time comes to seed the collectors, the flasks are first put in special conditions: by decreasing the temperature slowly to 17 °C and increasing the light intensity to 55 umol m-2 s-1, it is possible to make them become fertile.
Then, the black gametophyte solution is spread on both sides of the collectors. To obtain a good hold of the gametophytes, it is necessary to use a well-dried fluffy string which quickly sucks up the beads, jams the gametophytes between the fibres of the string which quickly sucks up the beads, jams the gametophytes between the fibres of the string and holds them in spite of the water motion.
The collectors are hung under a bank of daylight fluorescent tubes in tanks containing 0.2 um filtered water, to avoid any kind of competition. We use tanks 3 m long, 1.25 m broad and 1 m high able to receive 60–80 string rolled frames at a time. The filtered water change occurs every three days. It needs three weeks until the young plants can be seen with the naked eye. Their size is then around 1 mm and it is possible to bring them to the sea.
Cultivation in the open sea
The outdoor experiments are carried out on the coast of the Ouessant Island in the North-East of Brittany with the help of the fishermen cooperative.
The well-covered strings are unrolled from the frame and rolled round parallel horizontal ropes, 100 m long, stretched at one meter depth under the surface. We seeded 50 ropes a hectare. The distance between two ropes was 2 m.
The trials show that it is better to transfer the young plants to the sea before they reach 2 mm, otherwise, many of them are uprooted due to a weak attachment of their holdfasts or to diseases which appear rapidly after a three-week indoor culture. We do not know why. Three hypothesis can be advanced:
deficiency of an unknown nutrient needed at this size,
bacterial attack, which could be attached to the weakness of the plant from lack of a given nutrient;
lack of water motion.
We observe highly reduced losses after transfer to the open sea if, during the last week indoors, a strong water motion is induced in the pre-culture tank by an immersed pump; the motion certainly increases holdfast attachment. Furthermore, if the filtered seawater in the tank becomes running, or is changed each two days, the diseases do not appear. Research has been focused on these points to define the best transfer method.
In the sea, the plants grew rapidly, mainly when the temperature was between 9 and 12 °C, from December to April. They reached on average 5 cm 10 days after the transfer, 20 cm the 30th day, 90 cm the 60th day, 260 cm and more after the 4th month.
Two crops of 40 tons/hectare were harvested, the first at the beginning of February following a germling transfer in September, the second at the end of May following a transfer in February. We have tried starting a new outdoor culture in May but the plants stopped growing when they reached 20 cm long and developed rapidly a sporophyllum; it seems that this early maturation is induced by the long daylight. This last crop yielded less than the other ones: 20 tons/hectare.
Conclusion
Following our experiments of 1988 and 1989, French fishermen started to cultivate Undaria. The seeding by free-living gametophytes makes them free from the natural biological cycle of the seaweed. The production of young plants can occur as they want and need only three weeks instead of four months as in Asia.
Two crops a year would make them more competitive, in front of Asian farmers. The quality of the seaweed along the Brittany coast is of high commercial value.
In our experiments the output was only about 7 kg/m. But this low value was due to the fact that the quantity of Undaria growing outdoors was so high that the buoys were not powerful enough to hold all the ropes at the desirable level; some ropes plunged to the bottom and lost their plants, eaten by many animals.
It seems that the yield can be increased by (a) rolling two strings round each sea rope instead of one, (b) bringing the ropes closer than 2 m each from each other, and (c) by increasing the number and the size of buoys to maintain strongly the ropes at the optimal growing level.
