POSSIBLE APPLICATION OF INVASIVE DREISSENIDS AS PROTEIN SOURCE ALTERNATIVE IN INTENSIVE FRESHWATER FISH FARMING PROJECTS: GROWING DYNAMICS AND NUTRIENT VALUE OF MUSSELS IN SITU AND FEEDING WITH ALGAE
Invasive Dreissena species (D. polymorpha, D. r. bugensis) are dominant freshwater filter feeders playing intermedier role in the food chain. Their biomass (dry mass without shell) reaches around 88 t/year in Lake Balaton the largest shallow lake in Central Europe, where dreissenids are main food sources for carp, bream and roach. We found that the protein content of the soft tissue was high (55-60% dw ) enough to count this animal as a potential protein source alternative provided by freshwater e cosystem service for feeding fry in intensive fish farming.
In the present study, we investigated the type and quantity of algae that yields intensive growing, higher weight and therefore more energy source biomacromolecules of dreissenids, which is beneficial in fish bait composition. M onocultures of t hose algae were used, which can be found with high number in the natural environment of dreissenids and can be easily cultured in laboratory conditions.
First, using flow cytometry, we decided the optimal algae concentration that dreissenids consume daily, when we measured the growth of the animals in a mid-term (3 months) microcosmos study. Results were evaluated in the view of data obtained from ani mals living in the western eutrophic and eastern oligotrophic bays of Lake Balaton.
The o ptimal algae concentration for mussel growth was 1-3x107 algae cell/L/day. Results showed that length increment, wet weight, protein and carbohydrate content of the mussels were highly correlated. P rotein levels were similar in mussels getting lake water in l aboratory and in situ (Fig. 1) . Absolute values of all energy source materials measured were the highest in case of feeding with Scenedesmus rubescens and Neochloris conjuncta, and the algae-mix containing equal concentration of the algae species used. These results were comparable with those originated from the eutrophic lake water. Carbohydrate data related to wet weight showed that the most effective algae was Scenedesmus , whereas Chlorella minutissima proved the best evoking the relative fatty acid level . In conclusion, considering natural conditions, amount of biomacromolecules could be deliberately enriched in dreissenids fed on selective algae monocultures.
