Hypoxia likely killed thousands of fish on Anakapalli coast, says CMFRI study

The large-scale fish mortality reported along the Boyapadu coastline, which is primarily affecting the Leiognathus species, has coincided with the presence of a near-shore algal bloom and signs suggestive of oxygen stress in the affected fish.

While the local fishermen suspect the effect of industrial effluents, observations and laboratory findings are insufficient to establish a definitive cause. However, the water samples have shown high densities of Trichodesmium cells.

Comprehensive water quality, toxicological, and phytoplankton investigations are required to determine the exact mechanism responsible for the mortality event for a continuous period.

The investigation, conducted by the Visakhapatnam Regional Centre of the ICAR-Central Marine Fisheries Research Institute (CMFRI), focused on the Boyapadu Fish Landing Centre in Nakkapalli mandal of Anakapalli District.

Following reports of the mortality on June 20, 2026, experts visited the site the next day and observed a massive aggregation of dead fish along the shoreline, stretching approximately 1,500 metres in length and 5 metres in width. Approximately 94% of the dead were identified as pony fishes (Leiognathus), commonly found in shallow coastal waters

The estimated biomass of the dead fish, based on the findings, is valued at approximately ₹4,96,889. This figure is calculated based on an approximate total weight of 11.04 tonnes of fish at a price of 45 per kg.

Preliminary investigation report

The ‘Preliminary investigation report on fish mortality incident, Nakkapalli’ was submitted by Dr. Joe K. Kizhakudan, Principal Scientist and Head of the ICAR-CMFRI Visakhapatnam Regional Centre, to the Chairman of the Andhra Pradesh Pollution Control Board (APPCB) in Vijayawada.

The report was prepared following an inquiry initiated by Shankar, Joint Chief Environmental Engineer (JCEE) at the APPCB Zonal Office, and subsequent discussions held during a video conference between the CMFRI team and the APPCB Chairman.

Observations

Laboratory examinations of the collected specimens revealed no external lesions or signs of disease or pathogenicity. However, a notable observation was the ‘open-mouth’ condition exhibited by the majority of the fish, a physiological symptom frequently associated with respiratory stress and oxygen-deficient conditions. Furthermore, researchers noted the complete absence of seabirds and scavenging birds along the affected stretch, despite the abundance of biomass.

Structured interviews were conducted with 10 local fishermen who witnessed the incident on June 20 at approximately 2.30 pm during the high tide.

Eyewitnesses reported that schools of fish were washed ashore alive before dying on the beach. A majority of the local community pointed towards potential contamination from nearby bulk drug industries. Fishermen suggested that industrial wastewater discharge pipelines, which extend about 2 km offshore, may have created zones that affected the fish before they were pushed toward the shore.

Algal bloom

Environmental studies highlighted an extensive algal bloom in the near-shore waters, characterised by a dense discolouration. Analysis revealed a dominance of Trichodesmium—a type of cyanobacteria—at a concentration of 380 nos/ml.

Experts noted that Trichodesmium blooms have been linked to ‘fish kill’ events through mechanisms such as hypoxia. The team also analysed temperature and nutrient levels, noting that intermittent rains may have created stratified zones, where a sudden rise in temperature triggered the bloom, subsequently leading to oxygen depletion at the bottom when the biomass decayed.

Preliminary assessment

The ICAR-CMFRI team’s preliminary assessment suggests that the mortality was likely caused by hypoxic conditions, evidenced by the open-mouth state of the fish and the presence of the Trichodesmium bloom.

The report indicates that when anoxic water is flushed towards the shore during high tide, it traps benthic fishes in a traumatised state, leaving them unable to swim. While the findings point towards eutrophication and oxygen dead zones, the institute emphasised that further investigation into the discharge points of local industries and ETPs is necessary to fully understand the impact.

Inquiry initiated

Reports from the Anakapalli district fisheries officials say that fishermen had specifically alleged pollution caused by the Hetero Complex in Nakkapalli. Consequently, an inquiry has been ordered, with government officials and experts collaborating to assess the full scope of the environmental impact on the region’s marine wealth.