Tropical fish appears in Adriatic, puzzling Croatian scientists

Monrovia doctorfish (Photo: Svistoon/CC BY-SA 4.0)

Scientists in Croatia have been left intrigued by the unexpected appearance of a tropical fish species in the northern Adriatic Sea, an area known for being among the coldest parts of the Mediterranean.

The discovery was made near the Kvarner region, where researchers are now investigating how the fish managed to reach the waters off the Croatian coast.

The Adriatic Sea is home to around 450 species of fish. However, increasing maritime traffic and rising sea temperatures are gradually introducing non-native species into the ecosystem.

Marine biologist Marcelo Kovačić from the Natural History Museum in Rijeka told HRT show More that several unfamiliar species have already been recorded in the Adriatic in recent years.

Among them is the lionfish, which has recently spread through parts of the Mediterranean. Some other newcomers are herbivorous species, including the Monrovia doctorfish (Acanthurus monroviae).

The appearance of a Monrovia doctorfish in the northern Adriatic during 2024 surprised scientists. Originally native to tropical Atlantic waters, the species typically inhabits much warmer seas.

What has puzzled researchers even more is a new recorded sighting in 2025, suggesting the fish may have survived the winter in the northern Adriatic.

If confirmed, this would be unusual as the Kvarner area is considered one of the coldest regions of the Mediterranean Sea.

Another possible explanation is that multiple individuals entered the Adriatic separately. However, scientists say this scenario is also unusual, particularly since sightings have all occurred within a relatively small area near the coastal municipality of Kostrena.

Scientists seek help from the public

To better understand how the species arrived and whether more individuals are present, researchers are calling on the public for assistance.

Fishermen, divers and citizens who believe they have seen a surgeonfish are encouraged to report their observations to scientists at the Rijeka museum.

One of the fish’s most distinctive features is a sharp blade-like spine located near the base of its tail, which it uses for defence. The spine can inflict cuts on other fish and even injure humans if handled carelessly.

One specimen, donated by an underwater spearfisher, has already become part of the museum’s scientific collection and is being analysed as part of a research study.

Three other mysterious species

Researchers say the Monrovia doctorfish is not the only unusual visitor to the northern Adriatic. Three other species have also appeared in the area seemingly “out of nowhere”.

Scientists have ruled out the possibility that individual fish swam such long distances on their own.

Possible explanations include accidental release from aquariums or transport via ships, although modern shipping regulations make the latter less likely.

While the Monrovia doctorfish sightings remain sporadic and currently pose no major threat to the Adriatic ecosystem, another introduced species is causing increasing concern.

The blue crab is one of 17 new crab species recorded in the Adriatic over the past 15 years.

Its impact has already been felt in some areas, particularly near the Neretva River estuary and around the waters of Pula.

Marine experts warn that the species is a highly aggressive predator with no natural enemies in the Adriatic and an exceptional reproductive capacity.

Although it currently appears only occasionally in the Rijeka area, scientists say a population increase could lead to significant damage to fisheries and native marine species.

Despite its powerful claws capable of easily breaking open shellfish, the blue crab does have one weakness, it is highly valued as food.

Because of this, scientists suggest that controlled fishing of invasive species such as the blue crab, lionfish and other newcomers could help protect marine biodiversity while also opening new opportunities in gastronomy.