Overbreeding of invasive lionfish threatens Mediterranean ecosystem

ISTANBUL

Climate change-driven longer warm periods, the absence of natural predators, and the lionfish’s high adaptability are enabling the invasive species to reproduce successfully over an extended spawning season and spread rapidly across the Mediterranean, scientists have warned.

A study conducted by researchers from Istanbul University off the coasts of Fethiye and Kas in southwestern Türkiye and published in the Journal of Fish Biology provides a detailed assessment of the lionfish’s reproductive biology and growth characteristics.

The analysis examined 605 specimens, including 277 males, 302 females, and 26 individuals of undetermined sex.

Sampling was conducted using small-scale trammel nets, with additional specimens collected by spear-fishing divers during the summer months. The fish were obtained from depths of 20-70 meters (65-230 feet), and lab analyses focused on seasonal variations in the gonadosomatic index, hepatosomatic index, and condition factor.

The findings showed that spawning activity among female lionfish is most intense between June and November, while reproductively active individuals exhibit spawning potential from March through November.

Researchers identified the species’ heightened reproductive capacity during warmer months as a key factor behind its rapid expansion across the Mediterranean.

Female lionfish reach sexual maturity at a length of 20.1 centimeters (7.9 inches). On average, a single female releases more than 20,000 eggs per spawning event, with the maximum number reaching as high as 66,000.

‘An asymmetrical competitive situation emerges’

Taner Yildiz from Istanbul University’s Faculty of Aquatic Sciences told Anadolu that the lionfish has a prolonged reproductive window lasting approximately eight to nine months.

Yildiz noted that for many native, economically important Mediterranean species — such as red mullet, pandora, gilthead seabream and grouper — the reproductive period is typically concentrated within a much narrower window of two to three months, or at most four to five months, generally from late spring to early summer.

While some pufferfish species also exhibit early maturation, Yildiz said lionfish follow a fast life-history strategy marked by a long spawning season and high egg production, increasing pressure on the eggs and juveniles of native species.

He said the lionfish’s ability to spawn repeatedly from spring warming through late autumn gives the tropical-origin species a clear advantage over native fish with shorter breeding seasons, creating an uneven competitive balance — a pattern also observed in other invasive species such as pufferfish.

Female lionfish can exceed 40 centimeters (15.7 inches) in length, and based on Mediterranean age-length data, first sexual maturity is estimated to occur at around two years of age.

Yildiz said the lionfish’s rapid spread in the Mediterranean is driven not by extremely early maturity but by a combination of high adaptability, an extended warm-season reproductive period, high egg production, favorable temperatures, low predator pressure, and climate change, supported by the availability of artificial reef-like habitats and a flexible reproductive strategy.

Measures remain insufficient

Yildiz said current measures to control lionfish populations across the Mediterranean basin remain insufficient.

He noted that reward programs implemented in Türkiye for pufferfish represent an important step in supporting small-scale fishers, adding that similar incentive mechanisms could be developed for lionfish.

Yildiz also underlined that the Agriculture and Forestry Ministry's planned establishment of an international-standard Invasive/Alien Species Monitoring Center in Antalya province would be a positive step for early detection, monitoring, and assessment of the ecological impacts of invasive species, contributing to better coordination between scientific research and fisheries management.

The complete eradication of lionfish is unrealistic based on scientific data, Yildiz said. Ecosystem pressure could be reduced through regular removal programs in sensitive areas, controlled legal exemptions for spearfishing and scuba-assisted harvesting limited to invasive species, promotion of safe consumption and marketability, and a holistic invasive species management approach alongside pufferfish, he added.

Well-designed, science-based, and continuously implemented control programs, supported by the planned national monitoring centers, could functionally reduce lionfish populations and ease pressure on the Mediterranean ecosystem, he noted.