Piles of rubbish could be seen drifting in the lake near thousands of decomposing fish, with swarms of flies spreading near the reservoir.
“This phenomenon appeared on the shore of the lake several days ago,” said Ahmad Askar, a local activist. “The fish started floating up, and in abnormal quantities … it’s unacceptable.”
Mr Askar and local fishermen described the sheer amount of fish that washed up was unprecedented.
They called on the Litani river authority to find the cause and go after anyone dumping wastewater into the lake.
Earlier this week the river authority said the fish were toxic and carried a virus and people to avoid fishing all along the Litani due to “an aggravated disaster that threatens public health”.
The pollution prompted a ban since 2018 on fishing in the reservoir, which was created in 1959 with a large dam to collect water for hydropower and irrigation.
Last month, volunteers removed clumps of sticky tar from some beaches along the Lebanese coast after an oil spill which environmentalists warned would harm marine life.
Ecological disasters are the last thing Lebanon needs as it suffers through an alarming financial collapse and the aftermath of a huge explosion that devastated Beirut port last August.
Additional reporting by Reuters