TEMPERATE FRESH-WATER FOULING ON FLOATING NET CAGES - METHOD OF EVALUATION, MODEL AND COMPOSITION

Biofouling is a major problem in floating net cage culture during summ er months especially at marine sites. The experiments were conducted i n two freshwater sites (Eastern France) in order to determine appropri ate variables to describe freshwater fouling and to provide a descript ion of its spatial variation. Different mesh size nylon nets were subm erged below the surface of the water, from 12 August 1991 to 9 Septemb er 1991 in Migenbach reservoir (28 days), and from 22 August 1991 to 2 9 October 1991 in Lindre pond (69 days), Nets (mesh 2-20 mm) were weig hed (wet and dry) before and after different submersion times. The rat e 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 mont hs 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 Mirge nbach, 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 x 5 x 3 m ranged from 24.4 to 48 kg, Biomass of dry fouling per unit of net surface (D n) ranged from 100 to 250 g m(-2). The value per unit of settlable thr ead 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 da ys submersion). The Mirgenbach Ds results suggested that fouling bioma ss increased as a bivariable logistic function of submersion time and net surface. This was the first step of colonization. Fouling was most ly composed of Bryozoan Plumatella repens and Cyanobacteria Oscillator ia sp. at Mirgenbach and Cyanobacteria Aphanizomenon sp. at Lindre. Bi ofouling development depends on depth, maximum values were observed at 0.4 and 1 m depth. After 4 weeks of submersion in Mirgenbach reservoi r, net meshes were obstructed by fouling, while this was not observed after 69 days of submersion in Lindre pond. Differences in fouling rat es 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 surf ace was a determining factor for biofouling settlement. Constraints ge nerated by fouling should be evaluated prior to selection of culture f acilities. Also, two complementary indexes are suggested: percentage c over and dry biomass fouling per unit of settlable thread surface.