Clues to the cause of mass fish deaths

The red water streak that appeared in Quang Binh and Ha Tinh is not algae bloom or alluvium, but a layer of iron film - an important clue to finding a mobile poison nest on the seabed.

After the phenomenon of unusual fish deaths in some coastal areas of the Central region, the Vietnam Academy of Science and Technology and related ministries and sectors took samples and analyzed them. When the initial results were available, the Ministry of Science and Technology was assigned to chair the establishment of a National Science and Technology Council, with three research teams on groups of agents causing fish deaths including: chemistry, biology, hydrometeorology and marine dynamics...

Dr. Vu Duc Loi, Vice Chairman of the Scientific Council to find the cause of fish deaths, said that studies have ruled out earthquakes, tsunamis, oil spills, and epidemics and focused on two main groups: red algae and chemical toxins. However, remote sensing images of the sea area from Ha Tinh to Thua Thien - Hue did not detect signs of widespread algae blooms, so scientists believe that it cannot be the cause of mass fish deaths.

Comparison image between fish killed by phenol (left) and fish killed naturally.

To find toxins, scientists took a large number of samples from Vung Ang to Thua Thien Hue, especially the Formosa area with a high concentration at 27 points. Experts took 289 surface and bottom water samples at high and low tides from April 27 to 29; 97 sediment samples, 135 plankton samples (algae group), 34 benthic animal samples, 254 dead and live fish samples for analysis.

At this time, the group discovered many samples of fish that died in the wild had burns on the head and tail, especially the gills, and the body and tissue were congested.

The dead fish samples had heavy metal and arsenic levels lower than the Ministry of Health standards, but some samples contained cyanide ranging from 0.39 to 40 mg/kg, and phenol levels ranging from 5 to 340 mg/kg. Two other dead fish samples tested by Australia also showed high levels of phenol in muscle, liver and eggs.

"The fish samples all showed changes in tissue structure, many samples had stuck gills, some showed signs of burned heads and tails - typical signs of phenol poisoning. Sediment analysis in coastal areas of four central provinces also showed phenol," said Mr. Loi.

To be more certain, the group tested the toxicity of the extract of the dead fish sample. The fluid from the dead fish when decomposing into the water continues to kill other marine fish."If fish die due to algae, it cannot cause other living fish to die," Dr. Loi said, concluding that the cause of fish deaths is not nature but humans, specifically phenol and cyanide - two chemical agents that cause seafood deaths.

The cyanide found in fish could be due to fishing activities or waste from coking, but phenol only appears in coking wastewater, so the group of scientists "traced" Formosa to find the source of these two toxins.

A toxic layer of mucus covers fish and coral reefs, causing seafood to die..

Specifically, during the pipeline flushing process, Formosa uses a large amount of acid - an element that forms complexes with iron very well. The amount of wastewater containing citric acid during the flushing process is not treated separately but is led directly to the industrial water treatment plant.

In addition, the coke oven wastewater treatment process contains a large amount of iron, and the treated wastewater is also led to a centralized industrial water treatment plant. This plant only plays a filtering role, but cannot treat toxins such as phenol or cyanide.

How do toxins travel?

"Finding the source of the discharge and the cause of the pollution, scientists suspected that phenol or cyanide was in a free form that dissolved and would be diluted by seawater. If the fish died, it would only be in one spot and could not spread to Hue," Dr. Loi recalled.

Scientists predict that the mechanism of movement of these toxins along the ocean currents may originate from some unusual traces such as red and black water in Ha Tinh and Quang Binh. Mucus membranes covering dead fish on coral reefs, reddish brown covering sediment in Thua Thien Hue - the end point of the incident.

Based on the above assessment, the group tested the toxicity of water samples collected from the brick-red water stain in Quang Binh on May 4, Ha Tinh on May 5 and May 12, and found that the fish died at 80-100% within 3-30 minutes. At the same time, when analyzing the water samples, the iron content in the suspended solids was high (nearly 25%), iron hydroxide (nearly 50%) and contained phenol.

Hypothesis on the pathway of iron colloid and toxin transport.

The unusual water color is not the color of blooming algae or silt, but rather a form of iron colloid that absorbs toxins such as phenol and cyanide - products caused by human waste discharge. Analysis of the mucus membrane covering the fish on the coral reef collected in Thua Thien - Hue on April 24 also found high iron content and phenol.

From the above conclusion, scientists have found a link between the wastewater containing acid, iron from pipe cleaning and the biochemical wastewater containing FeSO4 (iron II sulfate) with phenol, cyanide. They form an iron colloid system that carries toxins when discharged into the sea, also known as a "mobile toxic nest". This mobile mucus layer follows the direction of the ocean current from Ha Tinh to Thua Thien - Hue, causing fish to die due to clogged gills or the effects of phenol and cyanide toxins.

During the migration, phenol and cyanide are gradually released and the colloidal form can settle to the bottom. When there are tides and waves, this colloidal layer is pushed to the surface of the water, creating unusual colored streaks as seen in Quang Binh and Ha Tinh.

When arriving in Hue, the mobile toxic nest encountered a whirlpool and had to stop, so Da Nang was not affected like the other localities.

"With independent and objective scientific evidence, the cause and culprit of the unusual seafood deaths have been clearly identified and Formosa has admitted it," said Mr. Loi.

According to VNE