TAIPEI (Taiwan News) — A large number of dead fish were found floating in Tainan Canal early Friday (Nov. 1), likely due to low oxygen levels caused by heavy rain from Typhoon Kong-rey.

Personnel were sent to remove the dead fish and garbage from the canal. Since the canal was dredged and connected to Anping Harbor, water quality has improved noticeably, according to Yahoo news.

Populations of tilapia, milkfish, and grey mullet have spawned. However, heavy rains can stir up sediment in the canal, disturbing the water and harming water quality.

Heavy rain can also wash nutrients, sediment, and organic matter into waterways. This leads to chemical and biological imbalances, according to Rock River Coalition.

As bacteria decompose excess organic matter, they consume significant amounts of oxygen. If the waterbody experiences widespread low oxygen levels, fish may not be able to find areas with sufficient oxygen, leading to fish die-offs.

Water tests from samples collected showed normal pH levels of 7.5. A critically low dissolved oxygen level of 0.68 mg/L was measured, per CNA.

When the dissolved oxygen level in a river drops below 3.0 mg/L, it becomes detrimental or even fatal to most fish species. Only pollution-tolerant fish such as tilapia and mosquito fish can thrive in such conditions.

If the number falls below 2.0 mg/L, the majority of fish species cannot survive. To maintain a healthy aquatic environment for fish, the dissolved oxygen level should be above 5 mg/L.

Earlier this year, Keelung’s Dawulun River experienced a fish kill, with approximately 630 dead fish discovered. Low oxygen levels were also identified as the primary cause of death.

Water samples showed critically low dissolved oxygen levels of 0.76 mg/L.