Decomposing sargassum causes massive fish kill in Quintana Roo, exceeding previous events

A massive fish kill has occurred at Punta Caracol in Mexico's Quintana Roo state, where thousands of fish were found floating in discolored waters choked with decomposing sargassum. The phenomenon, confirmed by the municipal government of Puerto Morelos, located between Cancún and Playa del Carmen, has drawn significant attention.

Rosa Rodríguez, a researcher at the Institute of Limnology and Marine Sciences (ILyCM) of the National Autonomous University of Mexico (UNAM) in Puerto Morelos, explained that the event is caused by a lack of oxygen in the water. This oxygen depletion results from the decomposition of the sargassum seaweed accumulating on the shore, combined with other seasonal conditions.

Rodríguez clarified that while such events are not new, with the first major fish kill linked to sargassum decomposition documented in 2018, the current episode is far more severe. "The magnitude of the event registered in Punta Caracol surpassed any precedent I have observed before," she stated, noting that photographs and videos from monitoring groups show thousands of dead fish.

The primary cause is the decrease in dissolved oxygen. In areas where the water becomes stagnant, such as bays and coastal points, decomposing sargassum consumes available oxygen, leading to hypoxic (low oxygen) or anoxic (no oxygen) conditions. During late July and August, periods known as "calmas chichas" (dead calms) occur, with minimal waves and reduced water circulation. Fish schools swimming near the coast can enter these low-oxygen zones, become disoriented, and die.

Rodríguez warned that the problem could worsen in the coming weeks due to the significant amount of sargassum arriving. The species most affected this time were thousands of sardines of the genus Harengula, commonly used as bait by fishermen.