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Philippines accuses China of poisoning disputed waters

April 13, 2026 5 min read International Relations GS Paper II (International Relations Bilateral/Regional Groupings) GS Paper III (Environment)
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What Happened

  • The Philippines' National Security Council (NSC) accused Chinese fishermen of pouring cyanide into waters around Second Thomas Shoal (Ayungin Shoal) in the Spratly Islands of the South China Sea, beginning in 2025.
  • Filipino troops reportedly seized 10 bottles of cyanide from sampan boats launched from Chinese fishing vessels during three separate incidents in February, July, and October 2025.
  • Philippine soldiers observed a Chinese sampan crew actively poisoning waters near the shoal in March 2026; subsequent water tests confirmed the presence of cyanide.
  • The NSC alleged the cyanide use constitutes "sabotage" — aimed at destroying the local fish population and depriving Philippine Navy personnel stationed aboard the grounded vessel BRP Sierra Madre of their food source.
  • The Philippines planned to submit a report to the Foreign Ministry as the basis for a formal diplomatic protest; the Navy and Coast Guard were ordered to intensify patrols.
  • The alleged mother ships of the sampan boats were described by officials as working for the Chinese Navy.

Static Topic Bridges

Second Thomas Shoal and the South China Sea Dispute

Second Thomas Shoal (Filipino: Ayungin Shoal; Chinese: Ren'ai Jiao) is a submerged reef in the Spratly Islands, located approximately 105 nautical miles from Palawan island in the Philippines — well within the Philippines' Exclusive Economic Zone (EEZ) as defined by UNCLOS. It is claimed by the Philippines, China, Taiwan, and Vietnam. The shoal gained strategic importance after the Philippines deliberately grounded the decommissioned warship BRP Sierra Madre on it in May 1999 to establish a permanent presence in the face of China's growing Spratly activities.

  • Location: approximately 9.4°N, 115.8°E; 105 nautical miles west of Palawan; within Philippines' 200 nautical mile EEZ.
  • Spratly Islands: an archipelago of reefs, shoals, and islands in the southern South China Sea; contested by China, Philippines, Vietnam, Malaysia, Taiwan, and Brunei.
  • BRP Sierra Madre: a WWII-era US-built tank landing ship, deliberately grounded in 1999; hosts a small rotating detachment of Philippine Marines.
  • China has demanded the Philippines remove the Sierra Madre since at least 2014; the Philippines has refused, periodically reinforcing and resupplying the garrison.
  • The 2016 South China Sea Arbitration Award (Permanent Court of Arbitration, The Hague) ruled that China's "nine-dash line" claims have no legal basis under UNCLOS; China rejected the ruling.

Connection to this news: The cyanide poisoning allegation is the latest escalation in an ongoing, multi-year confrontation at Second Thomas Shoal — moving from physical interdiction and water cannons to alleged environmental sabotage as China's tactics for pressuring the Philippine garrison.

Nine-Dash Line and China's South China Sea Claims

China claims sovereign rights over the vast majority of the South China Sea through what is commonly called the "nine-dash line" — a demarcation line encompassing approximately 90% of the South China Sea. This claim, which China first publicised in 1947 (then as an eleven-dash line), overlaps extensively with the EEZs of Vietnam, Malaysia, Brunei, Indonesia, and the Philippines. The 2016 UNCLOS arbitral award found the nine-dash line has no legal basis.

  • Nine-dash line origin: Based on a 1947 Republic of China map; adopted by the People's Republic of China; converted to a "ten-dash line" (including Taiwan) at times.
  • 2016 PCA Arbitration (Philippines vs China): Tribunal ruled that China's historic rights in the South China Sea are incompatible with UNCLOS; ruled China's activities in the Philippines' EEZ violated its sovereign rights. China rejected the award entirely.
  • China's "three Nos" on the Arbitration: China says the ruling is "null and void" and maintains its nine-dash line claims.
  • ASEAN's position: ASEAN states affected by the nine-dash line disagree with China's claims but the bloc has struggled to issue unified statements due to Chinese influence over some members (Cambodia, Laos).
  • India's position: India supports freedom of navigation and overflight in the South China Sea and has called for resolution of disputes under UNCLOS.

Connection to this news: China's alleged cyanide operation is part of a strategy of "grey zone" coercion — using deniable, non-kinetic tactics (maritime militia, water cannons, laser blinding, now possibly environmental sabotage) to exhaust the Philippines' will to maintain its South China Sea presence without triggering a formal military response.

UNCLOS, EEZ Rights, and Environmental Protection Obligations

Under UNCLOS, states have sovereign rights within their 200-nautical-mile Exclusive Economic Zone for the purpose of exploring, exploiting, conserving, and managing living and non-living resources. UNCLOS also places affirmative obligations on states to protect and preserve the marine environment (Part XII). Deliberately poisoning fish stocks in another state's EEZ would violate both the Philippines' EEZ resource rights and UNCLOS's marine environment protection provisions.

  • UNCLOS Article 56: Coastal states have sovereign rights in their EEZ for resource exploitation and conservation.
  • UNCLOS Article 192: States have the obligation to protect and preserve the marine environment.
  • UNCLOS Article 194: States must take measures to prevent, reduce, and control pollution of the marine environment.
  • Coral reefs are specifically mentioned in UNCLOS Article 194(5) as ecosystems requiring particular care.
  • Second Thomas Shoal's waters are reportedly rich in marine resources (fish) and the shoal sits above potentially significant mineral deposits.
  • The use of cyanide for blast fishing (and by extension, deliberate poisoning) is illegal under both international law and Philippine domestic law.

Connection to this news: The Philippines' planned diplomatic protest will likely invoke UNCLOS Articles 56, 192-194 — asserting that China's alleged cyanide use violates both the Philippines' EEZ resource rights and the international obligation to prevent marine environmental damage. This makes the case legally robust regardless of the underlying sovereignty dispute.

Key Facts & Data

  • Second Thomas Shoal: 105 nautical miles from Palawan; well within Philippines' UNCLOS EEZ of 200 nautical miles.
  • BRP Sierra Madre: intentionally grounded May 1999; hosts rotating detachment of Philippine Marines.
  • Cyanide seizures: 10 bottles from sampan boats in 3 incidents (February, July, October 2025).
  • 2016 PCA Arbitration: ruled China's nine-dash line has no legal basis under UNCLOS; China rejected the ruling.
  • South China Sea: approximately 3.5 million square kilometres; estimated 11 billion barrels of oil and 190 trillion cubic feet of natural gas reserves (US EIA estimates).
  • Spratly Islands: claimed by China, Philippines, Vietnam, Malaysia, Taiwan, and Brunei; subject of overlapping claims and physical construction activities.