Invasive Lionfish Succumb to Disease - But They Won't Disappear
Populations of invasive lionfish in the western Atlantic have been dramatically culled by the spread of a new disease, but it won’t halt the invasion, according to recently published research from scientists at the University of Florida.
Lionfish, of which there are 12 species, are commonly encountered natives of Indo-Pacific waters but had were not present in the western Atlantic until recently - the first specimen was discovered at a Florida beach in 1985. How they were introduced to the region remains the subject of much discussion, however, the most commonly held theory is that they were discarded as unwanted tropical aquarium fish.
Since then, the red lionfish (Pterois volitans) and common lionfish (Pterois miles) have become well established in the Caribbean Sea and Gulf Mexico, with sightings along the eastern coast of the Americas reported as far north as North Carolina and as far south as Brazil. With no natural predators in the region, invasive lionfish have become a significant threat to local species of fish and the coral reefs in which they live. So vast are the numbers of lionfish, an entire industry has grown from their hunting, including an extensive restaurant trade, and even PADI have approved a 'Lionfish Hunter' distinctive specialty course created by an instructor in Honduras.
On 5 August 2017, a commercial fishing operation run by Josh Livingston of Dreadknot Charters reported that a large proportion of the fish that it had landed during a hunt near Destin, Florida, was found to have significant skin lesions. According to Alexander Fogg, one of the study's co-authors and marine resources coordinator for the Okaloosa, Florida, County Board of Commissioners, out of 503 lionfish that were landed that day, 201 had ulcerated skin. The diseased fish were brought to the attention of the study's lead author, Holden Harris, PhD candidate at the University of Florida, who works closely with the commercial fishers of the northern Gulf of Mexico.
Many lionfish landed in commercial catches and hunting tournaments displayed signs of the ulcerative skin condition (Photo: Holden Harris/University of Florida)
Harris began to search through the populations of lionfish for instances of the new disease, using ROV surveys to study deeper reefs and monitoring the catch of commercial fishing operations and lionfish hunting tournaments, competitions which have become popular throughout the Caribbean and around the Gulf of Mexico. The ulcerative skin condition was found to be present in large numbers during 2017 and 2018 with commercial fishing operations reporting a 52 per cent decline in their take, while tournaments were found to have been affected by 62 per cent decline in the number of fish caught at individual reefs.
During previous research during 2016 and early 2017, the population density of lionfish at artificial reefs around the northern Gulf of Mexico had been found to be as high as 31 individuals per square metre. ROV surveys conducted by Harris and his team found that the number of lionfish at some of the most densely populated artificial reefs declined by almost 80 per cent after the skin disease had taken hold.
At the time of the study’s publication in the online journal Scientific Reports, the root cause of the skin ulcers remains unclear, although it is thought to be pathogenic, rather than parasitic, in nature. The high population densities and lack of genetic diversity of the region's lionfish would make the species susceptible to disease, but ‘despite considerable efforts, [the scientists at the University of Florida who are working on diagnosing the condition] haven’t been able to identify a causative agent,’ Harris told DIVE. ‘However, the speed that the reports emerged throughout a wide region appear consistent with an epizootic outbreak.’
While the emergence of the ulcerative disease is significant and has likely caused a ‘precipitous’ decline in lionfish populations, the study stresses that other factors may be involved. The 2017 hurricanes of Harvey, Irma and Nate and 2018’s hurricane Michael – the strongest category 5 storm to ever make landfall in Florida – may all have had an impact on lionfish populations, redistributing larvae or the fish themselves to create an appearance of lower population density than would be otherwise the case.
Although the disease has had a severe impact on lionfish populations, Harris says that any hope that it will eliminate lionfish from the western Atlantic is misplaced. By way of comparison, the study references the introduction of myxomatosis to invasive rabbit populations in South Australia, which precipitated a 90 per cent decline in rabbit populations, which then recovered. Similarly, the prevalence of the ulcerative skin disease among western Atlantic lionfish populations has already declined, indicating that although the numbers remain low, the species is starting to recover.
‘I do not think the disease will eradicate them,’ said Harris. ‘Size composition data indicate that the proportion of juveniles has returned to levels we observed in 2014, indicating a potential rebound may be underway. Even if densities are down,' adds Harris, 'we still need people in the water removing lionfish!'
One of the most important questions raised as a result of the outbreak is whether or not lionfish remain safe for human consumption. 'To our knowledge, there have been no reported cases of human illness due to consuming ulcerated lionfish,' said Harris. 'However, if a lionfish has ulcers or appears unhealthy, we suggest you avoid direct contact and not eat the ulcerated lionfish. If you have touched an ulcerated fish, wash with soap and water.'
Otherwise, says Harris, 'Eat lionfish! It’s one of the best options to mitigate their impacts on native reef communities.'
The complete study is published under a Creative Commons license in the online journal Scientific Reports: www.nature.com/articles/s41598-020-58886-8