British tourists headed for the popular Balkan country of Croatia have been urged to stay alert when visiting its beaches due to an invasive animal species that packs a painful and dangerous sting. While previously it only made sporadic appearances, the devil firefish (Pterois miles) has now firmly established itself in the Adriatic Sea as of 2024, according to marine experts from the Institute of Oceanography and Fisheries in Split.

According to Jutarnji list, the Institute has confirmed that the species, also known as the common lionfish, is now a permanent resident in the Balkan region. Between June 2024 and January 2025 a worrying 122 devil firefish sightings were recorded along the eastern coast, highlighting the extent of the species’s rapid spread.

The greatest number of observations were noted near the picturesque islands of Lastovo and Vis, particularly worrying given the fact that Vis Island prides itself on its countless postcard-perfect beaches, including Silver and Tepluš. Along the mainland shore, it was the region around Dubrovnik that recorded the most observations – an area that saw a total of 4.2 million overnight stays in 2024, and some 335,000 in April 2025 alone.

“It is particularly interesting that the devil firefish was spotted even during the winter months, including in shallow bays and at relatively low temperatures,” researchers noted. “Environmental conditions in the Adriatic, especially in the central and southern parts, clearly pose no obstacle to this species.”

Experts have emphasised the risks to human health, as the devil firefish’s sting can be dangerous if left untreated. 

The common lionfish is a species of ray-finned fish native to the western Indo-Pacific region. It can vary in colour from reddish to tan or grey and has numerous thick, dark, vertical bars on its head and body. The fish can grow up to 14 inches (35cm) in length, with a dorsal fin that has 13 long, strong spines and several soft rays.

The fin spines are highly venomous and have caused death to humans in some reported cases. While a fatal sting is rare, they can cause extreme pain, vomiting, convulsions, minor paralysis and breathing difficulties.

Therefore, immediate emergency medical attention is strongly recommended, even for healthy adults who have been stung. Some people are more sensitive to the venom than others after being stung, and symptoms and reactions to the venom vary in severity from person to person.

The experts also warned that the species could threaten native fish species and disrupt the balance of marine ecosystems. In response to the threat, scientists have included mitigation strategies in their study. These include potential removal of the species, especially from protected marine areas.