Thousands of dead crabs on Guanacaste beaches: here’s what we know

Esta publicación también está disponible en: Español

This week, thousands of mora crabs (Euphylax dovii) were found dead on the beaches of Conchal, Brasilito, Flamingo, and Potrero, located in the canton of Santa Cruz, covering the sand in a reddish hue.
The reasons for this phenomenon have not been confirmed, Andres Beita, a scientist with the organization MarViva, explained to La Voz, but there are hypotheses.

According to the expert, the crabs’ deaths may be related to abrupt changes in oceanographic conditions, particularly the warming of the sea associated with the El Niño climate phenomenon.

Warming waters can cause physiological stress in organisms and reduce oxygen availability. Changes in winds, currents, and waves can also contribute to the animals being washed ashore,” he said.

In fact, the marine species expert explained that the purple crab has a known association with periods of warm water and El Niño events, as this is when their populations tend to increase.

Is this a common occurrence on Guanacaste’s beaches?

According to MarViva’s records, it is not a common phenomenon on Guanacaste’s beaches—at least not on this scale.

However, there are records of deaths and strandings on beaches at various points along the Eastern Tropical Pacific, which stretches from the coasts of Mexico to Ecuador. On the Cocos Islands, there are records of deaths in 1938 and 2015, according to the organization.

What does this mass die-off say about the health of the ocean?

Andrés Beita explained that mass die-offs of species can serve as a signal that a significant change in environmental conditions has occurred.

In this case, ocean warming and possible changes in oxygen availability are factors that could be causing stress on the species,” he added.

The black crab is part of the ocean’s food chain. It is preyed upon by fish and other marine animals.

To determine the causes and possible large-scale environmental implications, monitoring must continue.

Are they safe for human consumption?

Black crabs are not a species of particular commercial interest nor a fishery resource, but just to be safe, the specialist recommends not consuming them in this context.

Until the cause of the mortality is known with certainty, it is not prudent to harvest them for consumption,” he said.

Is it safe to swim at these beaches?

The beaches where the dead crabs were found are in highly touristed areas, but for now, recreational activities at these sites are not recommended, Beita emphasized. At least not while authorities are conducting their assessment.

As a precaution, it is best to avoid areas where there are decomposing animals.
What should I do if I encounter a similar occurrence while walking on the beach?

Andrés Beita’s recommendations are as follows:

• Do not collect, handle, or consume the crabs.
• Prevent children and pets from playing with or coming into contact with the dead animals.
• Report the incident to the appropriate authorities.
• Take photographs and record the beach location, date, time, and scale of the incident—information that can be very useful to investigators.
• Avoid removing large numbers of the animals on your own before it is determined whether samples should be taken for research.
• Follow the recommendations of health and environmental authorities.

Beita also suggests that municipal authorities should coordinate the site assessment with environmental and health authorities. Once the appropriate handling of the animals has been determined, proceed with their removal and proper disposal, the specialist noted.