Mystery continues to surround the cause behind a “catastrophic” mass die-off of fish in the Oder River, after Polish scientists said laboratory tests found elevated salt levels but no other toxic substances in the central European waterway. German municipalities have banned bathing and fishing in the Oder after thousands of dead fish were found floating in the 520 mile (840km) river, which runs from the Czech Republic to the Baltic Sea along the border between Germany and Poland. Conservationists expressed fears that the mass die-off could wreak havoc on the entire ecosystem of the Oder. “We have to see how the bird population develops and what will happen to the racoons and otters,” Karina Dörk, a district administrator of Germany’s Uckermark region, told the Tagesspiegel newspaper. “It is a catastrophe that will stay with us for years.” According to the interior ministry in Warsaw, 2,000 police officers, 300 firefighters and 200 soldiers have been deployed to help fish animal carcasses out of the water over the past few days. On Monday morning Polish and German environmental ministers announced a plan to use floating oil barriers to stop floating dead fish spreading further across the Szczecin Lagoon, from where the Oder flows into the Baltic Sea’s Bay of Pomerania. Samples taken on Saturday suggested contaminated waters had not yet reached the western half of the lagoon. Both the German and Polish governments have said they assume the die-off was caused by toxic chemical spillage from industrial production, and Poland has offered a reward of 1m złoty or €210,000 (£180,000) for anyone who can “help find those responsible for this environmental disaster”. But the search for the exact cause has been inconclusive, and at times frustrated by poor communication between the Polish and German sides. Last week, officials in Germany reported scientists had discovered a high concentration of mercury in water samples – a claim rejected by Polish officials over the weekend. “The state veterinary institute has completed the testing of fish for the presence of heavy metals,” said Polish environment minister Anna Moskwa on Twitter. “He ruled out heavy metals as the cause of the fish deaths.” Poland’s head of government Mateusz Morawiecki on Friday fired the CEO of Polish Waters, the state-owned company in charge of water management in Poland, and the head of the environmental protection inspectorate in response to their handling of the Oder pollution. Polish anglers had reported an unusually high number of dead fish near Wrocław on 26 July, but German officials said they were not informed of the problem downstream until the start of the second week of August.