An environmental group in Brazil has begun testing river water in areas around where a dam holding back mine waste collapsed.
SOS Mata Atlantica made the first of several tests on Thursday about 0.6 miles (1 kilometer) away from where a dam owned and operated by Vale collapsed.
Preliminary results in the first area tested along the Paraopeba River showed a low level of oxygen in the water and high levels of nitrate and phosphate.
Biologist Thiago Felix said only a few species of fish could survive such conditions.
Earlier this week, dead fish were seen floating along the river several miles from the dam breach.
The collapse on Friday led to an avalanche of mud. Authorities have recovered 99 bodies and say another 257 people remain missing.