Workers on both sides of the Oder river that runs along parts of the Polish-German border collected over 190 tons of dead fish, according to official estimates, one and a half weeks after the mass die-off was first detected. The Polish fire brigade recovered 158 tons of fish from the Oder and a smaller river so far, German news agency (dpa) reports. The smaller Ner river, where dead animals were also found floating on the water surface, rises south of Lodz and flows into the Warta. Meanwhile, at least 36 tons of dead fish have been recovered in the German state so far, according to earlier estimates by the Brandenburg state environment ministry. Authorities on both sides of the Oder river are still trying to find out what is behind the mass fish die-off, assuming there could be several causes. Scientists suspect the rapid proliferation of certain algae could play a role. Poland"s Environment Minister Anna Moskwa announced an intensive probe into how the microorganisms entered the river. A Brandenburg laboratory meanwhile detected excessive pesticide levels in the water. Samples taken between August 7-9 at the Frankfurt Oder monitoring station, some 100 kilometers east of Berlin, showed high concentrations of a pesticide containing the active ingredient 2,4-dichlorophenoxyacetic acid, the Brandenburg ministry said. It is more likely that several factors caused the environmental disaster, according to the ministry. The excessive concentration of the pesticide over several days certainly had an impact on animals, plants, and microorganisms, officials said. The ministry didn"t rule out the possibility that even higher concentrations of the pesticide were present upstream, meaning they had already been heavily diluted upon reaching Frankfurt Oder. — Agencies