Q: The Mediterranean accounts for less than 1% of the world's ocean surface, yet it is one of the richest areas in terms of biodiversity. What makes it so special when it comes to fish species?
A: Every sea has its own unique characteristics, but the Mediterranean Sea is one of the few semi-enclosed seas. Combined with its specific geological history and hydrographic features, this makes it a distinct biogeographical entity. Although most of its marine life, including fish species, is of Atlantic origin, its geographic separation, so to speak, from that system and its specific environmental conditions make it home to numerous endemic species and locally differentiated populations. At the same time, it is inhabited by many widely distributed, cosmopolitan, and rare species, as well as an increasing number of non-indigenous species entering the Mediterranean primarily through the Suez Canal, as well as via maritime transport and other factors.
Q: How many fish species are currently known to inhabit the Mediterranean, and which are the most significant or threatened?
A: According to the latest checklists, around 760 fish species have been recorded in the Mediterranean Sea. However, this number is constantly changing. Some species were previously misidentified and subsequently removed from the list due to a lack of reliable evidence, while new species are being recorded at a rapid pace. These additions include non-indigenous species entering through the Suez Canal or introduced via human vectors such as ballast water, aquaculture, and the aquarium trade, as well as species completely new to science.
Q: It is often mentioned that many Mediterranean species are endemic. Could you explain what this means and why their protection is so important?
A: Strictly speaking, endemic fish species make up about 12% of the Mediterranean's native fish fauna. Under a broader biogeographical definition, which includes species extending slightly into adjacent areas of the Eastern Atlantic or Black Sea, that figure approaches 20%. Protecting endemic species requires special attention because the Mediterranean is one of the world’s most vulnerable marine environments, facing pressures like overfishing, habitat degradation, pollution, climate change, and invasive species. Due to their restricted range and reliance on specific habitats, endemic species are inherently at higher risk of extinction than widely distributed ones.
Q: What shifts in fish populations have you observed in recent decades, and what are the main drivers behind them?
A: Ecosystem changes are a natural part of marine life, occurring both gradually and rapidly. Human activities, however, have undoubtedly driven unprecedented changes across global oceans and the Adriatic Sea alike.
Due to climate change and ocean warming, we are witnessing a northward migration of warm-water species, both native and non-indigenous. Indo-Pacific species entering via the Suez Canal are spreading north, establishing viable populations in the Adriatic. Some of these pose serious ecological or physical risks, such as the highly toxic silver-cheeked toadfish (Lagocephalus sceleratus) or the invasive lionfish (Pterois miles), which impacts native fish populations through heavy predation. Furthermore, intense fishing, pollution, and coastal habitat destruction are reshaping fish communities. Sensitive groups like cartilaginous fish (sharks and rays) are on the brink of collapse and urgently need targeted conservation measures. Finally, species relying on shallow coastal zones are severely impacted as natural shores are replaced by artificial concrete structures, destroying their primary habitats.
Q: We hear more and more about invasive species entering through the Suez Canal. How do they affect native fish communities?
A: Non-indigenous species impact ecosystems through mechanisms common to invasive fauna, such as competition for food and space, predation on native species, disruption of fisheries, and risks to human health. While examples exist across the Mediterranean, two Lessepsian species (species that entered the Mediterranean via the Suez Canal) currently pose the greatest potential threat in the Adriatic: the silver-cheeked toadfish (Lagocephalus sceleratus) and the lionfish (Pterois miles).
The toadfish is still relatively rare in the Adriatic, but it is extremely toxic and can be fatal if consumed. The lionfish, on the other hand, has established a well-rooted population, is highly abundant, and actively preys on native species, particularly juvenile fish. In the future, lionfish predation is likely to have a major impact on local ecosystems, leading to declines in native species populations and reduced juvenile recruitment in affected areas.
Q: How important are Posidonia oceanica meadows for the survival of fish populations, and what happens when they are lost?
A: Posidonia oceanica seagrass meadows serve as vital nursery grounds for numerous fish species. They provide shelter from predators, significantly improving survival rates during early life stages, while also protecting adult fish. For species like the dreamfish (Sarpa salpa), Posidonia is a primary food source, and several other species use these meadows as spawning sites.
However, this does not mean other coastal habitats are any less critical. All shallow coastal zones, including the intertidal zone, are essential for larval and juvenile fish development, as well as home to many cryptic species. Consequently, losing these habitats directly leads to declines and the loss of species that depend on them.
Q: Can responsible behaviour by boaters and tourists truly contribute to protecting fish species? What concrete steps would you recommend?
A: Absolutely. Responsible behaviour primarily means reducing our overall footprint on the marine environment. This starts with avoiding anchoring in sensitive and vital habitats such as Posidonia oceanica seagrass meadows, and eliminating marine litter. Equally important is preventing chemical pollution, whether from toxic antifouling paints, improper fuel and oil handling, or discharging wastewater and sewage directly into the sea.
Responsible boating also means respecting recreational and sports fishing regulations. Beyond holding a valid permit, boaters must familiarise themselves with local fishing rules. This includes knowing minimum legal catch sizes, recognising closed seasons, and identifying endangered, strictly protected, or toxic species.
Q: If you had to highlight one key message for everyone navigating the Mediterranean regarding its fish and biodiversity, what would it be?
A: The Mediterranean is a global biodiversity hotspot, but it is also an exceptionally fragile ecosystem threatened by human activity and climate change. The biggest misconception is believing that one person's individual impact is negligible. In reality, our collective actions steadily erode the sea’s resilience and ability to recover.
A single anchor dropped in a seagrass meadow, a piece of plastic thrown overboard, a chemical spill, or an undersized fish kept might seem minor on its own. However, multiplied by the millions of people living along and visiting the Mediterranean coast, these small acts accumulate into immense pressure on marine life.