Invasions of non-indigenous species (NIS) are widely regarded as a major threat to native ecosystems and to global biodiversity.
The Atlantic invasion of two lionfish species (Pterois volitans, P. miles) since the 1990s has been extraordinary. Lionfish have rapidly spread throughout the western North Atlantic, the Caribbean Sea and the Gulf of Mexico, reaching very high densities and inflicting important environmental and socio-economic impacts.
The native range of the red lionfish (P. volitans) is the tropical Pacific Ocean, while the closely-related common lionfish (P. miles) is restricted to the tropical Indian Ocean and the Red Sea, with a narrow overlap in the Indonesian region.
The presence of the lionfish in the Atlantic is likely due to human-mediated introduction through the ornamental fish aquarium trade, and it is acknowledged that a small founder population originated from aquaria in south Florida.
An online portal actively promoting the hunting of lionfish specimens outside their native range nails it perfectly when stating that “invasive lionfish are disastrously out-breeding, out-living, out-eating and out-competing every other native fish in the Western Atlantic Ocean, Gulf of Mexico and the Caribbean Sea. If left unchecked, lionfish will ultimately cause the destruction of the reefs, native fish stocks and the livelihoods of everyone that depend upon them.”
Such foresight from the western Atlantic is prompting marine biologists in the Mediterranean to raise the alarm.
The common lionfish appeared in the eastern Mediterranean Sea in 2012 and has quickly proliferated and spread, reaching the central Mediterranean Sea in recent years, with two individuals recorded in eastern Libya in December 2018, caught by a longline fisher at a depth of eight metres and by a spearfisher at a depth of 25 metres.
Lionfish constitute a public health hazard
Closer to home, the species has also been recently recorded in Tunisian waters. Reports of its sighting in Maltese waters have not yet been fully substantiated and hence are dubious. A recently-published study has shed light on the origins of the lionfish invasion of the Mediterranean, concluding that this is the result of a passive (not mediated directly by human activity) movement of individuals from the Red Sea, rather than from other means (ship ballast water or an aquarium release).
Lionfish constitute a public health hazard, as aptly elaborated in an online informative brochure on the species formulated by the Health Promotion and Disease Prevention Directorate. Quoting verbatim from the brochure, the species’ “sharp spines contain venom which causes severe local pain to humans that may spread to the rest of the affected limb. This pain tends to become more severe around one to two hours after an individual is stung and usually lasts for six to 12 hours but sometimes persists for weeks. The pain is often accompanied by redness and swelling in the affected area. Less frequently, skin blisters and vesicles can occur in the surrounding skin, while skin necrosis occurs rarely.
“Occasionally lionfish injuries can cause significant systemic effects (on the whole body) such as fever, chills, nausea, vomiting, dizziness, headaches, seating and muscle and joint aches, among others. Lionfish stings are usually not life-threatening but on rare occasions they can have serious effects, such as slowing or speeding up of the heartbeat, low blood pressure, chest pain, problems with breathing, fainting, severe allergic reactions and seizures.”
From an ecological point of view, lionfish species are classified as Invasive Alien Species (IAS), given their rapid rates of proliferation, their voracious appetite and the almost complete lack of biological control/predators for the same species. Lionfish need only one year to reach full sexual maturity, potentially releasing eggs every four days, for a maximum of two million eggs per year, with juveniles showing a very high rate of survival. The species is relatively long-lived, living up to 15 years in the wild.
Lionfish can eat prey over half the size of their own body as long as it will fit in their mouths. Their stomach can expand up to 30 times the normal volume, and a lionfish will fill up to capacity as soon as it is able. Lionfish are not picky eaters and feed indiscriminately. Through stomach content analysis, scientists have catalogued over 70 different species that lionfish will eat. Besides fish, they also eat invertebrates, with a number of lobster fisheries in the Caribbean collapsing as a result of the lionfish invasion.
The native species that invasive lionfish are feeding upon do not recognise lionfish as a predator and flee. In fact, the opposite has been shown to be true. Small fish will often congregate around a lionfish, most likely believing that the lionfish’s long spines, fin rays and feathery pectoral fins offer them shelter and protection from predators. However, when the lionfish is ready to eat, these fish are herded into alcoves, where they cannot escape. Lionfish use lightning-fast strikes and gulp down dozens of whole fish at a time. Science has demonstrated that a single lionfish can reduce native marine creatures by 80 per cent to 90 per cent in its range within just five weeks.
The Spot the Alien Fish campaign (www.aliensmalta.eu) has, since summer 2019, pledged to handsomely reward the first submission of a lionfish report from Maltese waters (by submitting a location verifiable photograph or the actual specimen itself). This foray was met with scepticism from some quarters, with dissenting voices claiming that such a graceful and appealing fish should not be eradicated.
Given the facts presented above, these two invasive fish species (which can be described as the ‘perfect invaders’) definitely merit our attention and early warning and detection systems, such as citizen science campaigns, are useful in alerting national competent environmental authorities (such as the ERA) so as to coordinate a rapid management response.