Scientific alert in the Mediterranean: a toxic microalgae is detected in fish for the first time in the Peninsula

The discovery occurred during routine phytoplankton sampling carried out by the UA-Dénia Marine Laboratory. In them, the researchers analyzed samples collected in March and September 2023 at 12 stations on the coast of Dénia and Xàbia, both near the coast and at a greater distance.

The results found in these phytoplankton sampling were conclusive:

• In March, Gambierdiscus australes appeared in 75% of the samples.
• In September, its presence reached 100%.
• Concentrations ranged between 20 and 140 cells per liter.

These are not alarming figures, but they are historical data: it is the first time that this toxic microalgae has been detected in the waters of the peninsular Mediterranean. Until now, its presence had only been documented in the Balearic Islands in 2017.

The principal investigator of the study, César Bordehore, summarizes it clearly: “The consumer can rest assured. The concentrations found are not alarming and this species, within the genus Gambierdiscus, is not one of the most toxic.”

Even so, the discovery has attracted the attention of the international scientific community. For example, the specialized magazine Harmful Algae News, published by the Intergovernmental Oceanographic Commission of UNESCO, has already published about this discovery.

Ciguatera is a food poisoning caused by the consumption of fish that have accumulated ciguatoxins in their tissues. These toxins are not produced by the fish itself, but by microalgae such as Gambierdiscuswhich enter the marine food chain.

Small fish feed on contaminated algae, medium-sized fish eat the small ones, and large predators eat them all. Thus, the toxin becomes progressively concentrated, especially in large fish.

The symptoms of ciguatera can be very bothersome:

• Abdominal pain, nausea and vomiting.
• Severe diarrhea and dehydration.
• Cardiovascular problems.
• Neurological alterations, such as tingling or loss of sensitivity.

The most worrying thing is that these toxins are thermostable: they are not eliminated by cooking or freezing the fish. In addition, they are colorless, odorless and tasteless, making them impossible to detect with the naked eye.

However, the answer from scientists is clear: yes, it is safe to eat Mediterranean fish. Bordehore insists that there are very strict control protocols: “We know how to avoid possible food poisoning. There are prevention measures to prevent fish with high levels of toxin from entering the market, based on previous analyses.”

This means that the fish that arrives at the markets and fishmongers go through controls that guarantee their safety. Furthermore, the detected concentrations of the microalgae on the peninsular coast are low and do not pose an immediate risk to public health.

Where the emphasis is placed is on constant surveillance. Because although the situation is under control today, the expansion of these species requires being prepared for possible future changes.

In addition, the health authorities, as indicated by Health, of the Government of Spain⁽¹⁾recommend that:

• You must consume fish from authorized establishments, never from unauthorized ones or fish from poached fishing.
• If you do sport fishing, it is recommended that you analyze and avoid eating the largest ones. Its commercialization is prohibited.
• In areas where there is risk, avoid eating parts of the fish such as viscera, head, skin and eggs, especially from large fish, since this is where toxins accumulate the most.

One of the key questions is why Gambierdiscus australes It now appears on the peninsular Mediterranean coast. The answer points directly to climate change.

The origin of this genus of microalgae is in hot tropical waters and the increase in Mediterranean temperature, due to climate change, facilitates its geographical expansion. In other words, the Mediterranean Sea is becoming increasingly similar, in thermal terms, to the ecosystems where these species have always lived. This opens the door for them to colonize new areas that were previously too cold for them.

The researcher illustrates this with a very graphic phrase: “For a marine species, the increase of one degree of temperature means a world. It can be the difference necessary to colonize areas that were previously impossible.”

This phenomenon is not exclusive to microalgae. More and more tropical species are appearing in the Mediterranean: fish, invertebrates and algae that now find favorable conditions to survive.

Finally, the researcher points out that “we must be attentive to this monitoring, our job is to inform the competent administrations, as we have already done, and take the necessary measures at all times to maintain food safety.”

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