Sand blocking an estuary’s connection to the ocean, combined with reduced freshwater flow during a period of little rain, may explain the mass fish deaths at Playa Langosta, according to National University (UNA) biologist Teresita Ulate. Ulate said the interruption of water exchange between the mangrove and the sea could have reduced the oxygen available to aquatic animals. Her assessment offers a possible explanation, but the cause has not been confirmed.
The deaths occurred at the San Francisco Estuary in Playa Langosta. The Ministry of Environment and Energy (MINAE) opened an investigation after images showed dead fish, rays and other animals along the shoreline and in the water, including two bull sharks found dead in the estuary.
The ministry said the analysis, led by the National System of Conservation Areas (SINAC), would examine the blocked outlet, water quality and possible effects of El Niño, including warmer Pacific waters and changing coastal conditions. Authorities cautioned that assigning a cause before a technical evaluation would be premature.
Ulate explained that reduced rainfall meant less freshwater was reaching the mangrove while accumulated sand restricted its connection to the sea. She said El Niño had contributed to that drop in rainfall, part of the dry conditions now straining water supplies across Guanacaste, and that the episode could have been anticipated given the lower rain forecast.
Decomposing organic matter can add pressure because bacteria consume oxygen as they break it down, leaving less available for fish. Ulate said water testing she carried out in the San Francisco mangrove in June had already found low oxygen levels, which she described as a warning sign for the ecosystem. She added that daily monitoring would be needed to produce precise data.
The animals were reported dead on Nov. 4. Local lifeguards urged people not to collect or eat them because they were decomposing, and asked residents to stay away from the estuary, warning that the loss of fish could draw crocodiles closer to shore. The environment ministry also asked residents not to alter the estuary’s mouth without authorization.
Eduardo Vargas, director of the Tamarindo Integral Development Association, said strong seasonal tides had carried sand from other stretches of coast and sealed the opening. He said residents initially tried to clear it with shovels before bringing in heavy machinery, that the work had the necessary permits and that subsequent heavy rain helped widen the opening.
Vargas described the reopening as a stopgap rather than a fix, saying a lasting solution would require managing the mangrove from the surrounding watershed. Restoring that connection does not establish what killed the animals. Ulate cautioned that human activity could not yet be ruled out as a contributing factor and that SINAC’s analysis would be needed to better understand the episode.
