Toxic Red Tide in Yucatán Suffocates Fish Along the Coast

Algae bloom shows 336-fold increase as dissolved oxygen levels approach fatal thresholds for marine life

A massive red tide threatening Yucatán’s western coastline has reached critical levels. Scientists have documented a 336-fold increase in harmful algae that’s pushing dissolved oxygen to near-fatal levels for marine species.

Dead fish and live octopuses have been washing ashore at Progreso since late July, raising questions about seafood safety. Dr. Jorge Herrera Silveira, who heads the Primary Production Laboratory, told Diario de Yucatán that determining whether the specimens are safe to eat requires proper examination.

The bloom stretches 20 kilometers (12.4 miles) offshore from Progreso and between Sisal and Celestún along the coast. Most concerning are dissolved oxygen levels, which have dropped to as low as 2.6 milligrams per liter in some areas — dangerously close to the 2.0 mg/L threshold that defines a dead zone.

The third scientific sampling of 2025, conducted August 1 by CINVESTAV-Mérida’s Primary Production Laboratory, detected up to 897,372 cells per liter of the diatom Rhizosolenia sp. — a dramatic spike from the 2,670 to 72,090 cells recorded off Dzilam de Bravo on June 12, 2024.

“Below 2 mg/l is fatal for most species because the ecosystem suffers anoxia,” according to Spain’s Complutense University of Madrid, which notes that aquatic systems need 5 to 6 mg/L dissolved oxygen for most species to survive.

The current oxygen readings indicate marine species are already experiencing hypoxia, which occurs when dissolved oxygen falls below 3 mg/L. Some areas of the red tide are approaching the dead zone threshold that devastated Yucatán’s coast during a similar event in 2022.

Water temperatures in the affected zone range from 20.8 to 29.8 degrees Celsius (69.4 to 85.6 degrees Fahrenheit), while chlorophyll-a concentrations span 0.9 to 13.9 micrograms per liter.

Rhizosolenia sp., classified as a brown algae, had not previously been identified as a harmful. But now that it dominates the seafloor with unprecedented abundance, the species poses ecological risks due to its massive population and the resulting oxygen depletion.

Eight specialists, led by Professor Daniela Guadalupe Medina Euán and Dr. Herrera Silveira, warn that the bloom reflects ongoing water quality degradation. Their report states, “These results underscore the urgent need to implement sanitation programs and reduce untreated wastewater discharges to improve coastal water quality in the region. “

The scientists emphasize that poor water quality “poses a latent risk to the tourism activities that are being promoted” along Yucatán’s coast. The state has aggressively marketed its beaches and coastal communities to international visitors.

This marks the third harmful algal bloom sampling conducted in Yucatán waters this year as part of the ongoing Harmful Algal Bloom Research and Monitoring Program. Previous sampling on July 8 off Telchac and Dzilam detected dissolved oxygen levels between 3.2 and 6.3 mg/L, showing the rapid deterioration in conditions.

The July sampling found Chaetoceros sp. and Rhizosolenia sp. diatoms as the dominant species, but by August, Rhizosolenia sp. had completely taken over at the seafloor level.

The source of the problem

Scientists attribute the worsening conditions to untreated wastewater discharges and nutrient pollution flowing from inland sources into coastal waters. These excess nutrients fuel explosive algae growth, which subsequently consumes oxygen as the organisms die and decompose.

Previous red tide events in 2022 created actual dead zones along Yucatán’s coast, leading to temporary fishing bans and seafood safety concerns. The current bloom appears to be following a similar trajectory.

Red tides occur naturally but have become more frequent and severe due to human activities. Dead zones worldwide have nearly doubled every decade since 1960, with scientists linking the increase to coastal development, agricultural runoff, and climate change.

The timing coincides with peak tourist season in Yucatán, when thousands of visitors flock to coastal communities like Progreso, Sisal, and Celestún for beaches and seafood. Local fishing communities are monitoring the situation closely as they face potential restrictions on catches from affected waters.

Authorities continue sampling to track the bloom’s progression and determine when conditions might improve enough for normal fishing and tourism activities to resume safely.