Aquaculture of Fucus species in the Baltic Sea by means of vegetative reproduction
Species of the brown algal genus Fucus are important ecosystem engineers of northern hemisphere coasts. Their biomass is harvested commercially for the production of cosmetic extracts and food supplements, e.g., at the coasts of France, Ireland and Iceland. However, environmental change (e.g., warming, eutrophication) poses increasing risk on wild Fucus stocks and has already caused some population declines, for instance in the Baltic Sea, where harvesting of wild stocks is prohibited as consequence. Commercial cultivation of Fucus would therefore be a useful alternative to the harvesting of wild stocks especially in the light of an increasing interest in algal products. But so far, no cultivation method for the commercial production of Fucus species has been developed. Therefore, a theoretical vegetative cultivation approach has been described which includes the following steps: thallus pieces with apical meristems (“seedlings”) are cut from wild plants and reared unattached in baskets/ net cages deployed in the sea; after a season of growth, the biomass is harvested and new seedlings are cut from the harvest which are used as initial biomass for the next growth season. This process is then repeated after each growth season. The aim of this doctoral thesis was to test various aspects of this cultivation approach in culture experiments. The experiments were performed in an experimental farm located in the Kiel fjord, western Baltic Sea. Two local Fucus species (F. vesiculosus, F. serratus) were used. First, it was tested if Fucus thalli survive long-term culture with the described cultivation method and which role the formation of sexual organs (receptacles) plays in the cultivation process. It could be shown that principally thalli survive and grow continually in culture with the described method. However, the formation of receptacles and their natural degradation after gamete shedding reduced the growth potential significantly. Therefore, the question was raised how culture biomass with low fertility (i.e., a low percentage of thallus apices with receptacles) can be obtained. In order to answer this question, two approaches were adopted: first, it was tested if thalli from attached wild populations with naturally high fertility acclimatize towards lower fertility if they are cultivated unattached. Second, it was tested if thalli from an unattached wild population in Glücksburg (Flensburg fjord), which morphologically resembles F. vesiculosus and exhibits low fertility in the wild, retain their low fertility in culture. Thalli from attached high-fertility populations retained their high fertility in culture and did not acclimatize towards lower fertilities by unattached cultivation. On the other hand, thalli from the unattached population in Glücksburg retained their low fertility over one year of cultivation. Consequently, only thalli from unattached low-fertility populations are potentially useful for the envisaged vegetative cultivation method. Second, the question was raised if epizoan fouling, which was frequently encountered on the culture biomass, can be reduced by regular desiccation during the cultivation process. In order to answer this question, the effect of different desiccation regimes (different frequencies and intensities of desiccation) on weight and abundance of the most common epizoans (Electra pilosa (Bryozoa), Amphibalanus improvisus (Crustacea, Cirripedia), Mytilus sp. (Mollusca, Bivalvia)) as well as on the growth rate of the cultivated Fucus species was tested. Desiccation regimes affected weight and abundance of epizoans as well as Fucus growth rates at varying degrees. Frequent mild desiccations (F. vesiculosus: 3x week -1 to 80% of wet weight, F. serratus: 3x week -1 to 90% of wet weight) proved most effective as they reduced epizoan fouling significantly but had no effect on Fucus growth. By applying these regimes, the epizoan share of the final harvest could be reduced from 13.0 ± 4.8% in undesiccated controls to 1.8 ± 0.2% for F. vesiculosus and from 19.1 ± 2.7 to 1.0 ± 0.1% for F. serratus. In conclusion, Fucus aquaculture with the described vegetative cultivation approach is possible if the cultivated biomass can be kept at low fertility (which seems to be possible by using seedling material from unattached low-fertility populations) and if epizoan fouling is reduced by regular desiccation. Future studies should investigate the genetic control of fertility in Fucus and the influence of environmental factors on fertility since low fertility is a crucial factor for long-term cultivation success. Alternative cultivation techniques (e.g., in land-based tanks) might enable a cultivation without epizoan fouling and should also be subject of future research.
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