Temperate freshwater fouling on floating net cages: method of evaluation, model and composition

Biofouling is a major problem in floating net cage culture during summer months especially at marine sites. The experiments were conducted in two freshwater sites (Eastern France) in order to determine appropriate variables to describe freshwater fouling and to provide a description of its spatial v...

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Veröffentlicht in:Aquaculture 1996-08, Vol.143 (3), p.303-318
Hauptverfasser: Dubost, Nathalie, Masson, Gérard, Moreteau, Jean-Claude
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Sprache:eng
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Zusammenfassung:Biofouling is a major problem in floating net cage culture during summer months especially at marine sites. The experiments were conducted in two freshwater sites (Eastern France) in order to determine appropriate variables to describe freshwater fouling and to provide a description of its spatial variation. Different mesh size nylon nets were submerged below the surface of the water, from 12 August 1991 to 9 September 1991 in Mirgenbach reservoir (28 days), and from 22 August 1991 to 29 October 1991 in Lindre pond (69 days). Nets (mesh 2–20 mm) were weighed (wet and dry) before and after different submersion times. The rate of dry weight gain of the nets (DG) in Mirgenbach reservoir reached 350% after one month of submersion and was less than 50% during 2 months of submersion in Lindre pond. Biomass of wet fouling did not exceed 110 g m −2 (69 days of submersion) at Lindre (3–5 mm mesh). In Mirgenbach, only 21 days were necessary to obtain 400–1400 g m −2 (2 to 20 mm mesh) and the estimated wet weight of a net bag 5 × 5 × 3 m ranged from 24.4 to 48 kg. Biomass of dry fouling per unit of net surface (Dn) ranged from 100 to 250 g m −2. The value per unit of settlable thread surface (Ds) ranged from 62 to 340 g m −2 after 21 or 28 days of submersion at Mirgenbach; Ds was less than 25 g m −2 at Lindre (69 days submersion). The Mirgenbach Ds results suggested that fouling biomass increased as a bivariable logistic function of submersion time and net surface. This was the first step of colonization. Fouling was mostly composed of Bryozoan Plumatella repens and Cyanobacteria Oscillatoria sp. at Mirgenbach and Cyanobacteria Aphanizomenon sp. at Lindre. Biofouling development depends on depth, maximum values were observed at 0.4 and 1 m depth. After 4 weeks of submersion in Mirgenbach reservoir, net meshes were obstructed by fouling, while this was not observed after 69 days of submersion in Lindre pond. Differences in fouling rates could be related to reservoir characteristics (temperature, current velocities). An increase in diameter and in distance between threads facilitated algae and animal settlement on nets. Settlable thread surface was a determining factor for biofouling settlement. Constraints generated by fouling should be evaluated prior to selection of culture facilities. Also, two complementary indexes are suggested: percentage cover and dry biomass fouling per unit of settlable thread surface.
ISSN:0044-8486
1873-5622
DOI:10.1016/0044-8486(96)01274-4