Sorry to see this happening to these waters! We have a similar problem in the eastcoast of Sweden! Here it has to do with a lack of oxygen in the water and an euthrophication due to the release of human waste and overfertilization from farming that follow with rainwater. Danube is passing many countries before it reaches the coast. It is a responsability for every country in the end this is an international problem like waste of plastic in the ocean! I wish your fishingwaters will be clean again! I feel sad for your situation!