Tuesday, July 3, 2012
Lionfish are beautiful but venomous. Very recently these fish have become quite a large problem in the Caribbean, the fastest invasion documented for a marine fish. The Indo-Pacific lionfish (Pterois miles and P. volitans) are native to the reefs of the Indian and Pacific Oceans. They are also very popular aquarium fish. Whether through accidental or purposeful releases in the late 1970’s through the present, lionfish have made their way into the Caribbean. It started out as not-so-bad (as such things do) with only 5 or 6 individuals, but the problem has grown (as such things do) to a self-sustaining population that reaches over 1,000 lionfish per acre in some locations. P volitans seems to have taken up all of the reef real estate south of the Bahamas, while both species can be found north of Florida extending to Bermuda and out in the Sargasso Sea. But what makes lionfish such a problem? Well, mainly their appetite. Lionfish are generalist carnivores with voracious appetites, consuming more than 56 species of fish and many invertebrates. They have evolved to food availability in the Pacific, which may be patchy, and so they eat as much as they can whenever they can. Using this appetite, they have been known to reduce their fish prey by up to 90 percent. They are capable of permanently impacting native fish reef communities across multiple trophic levels. Another problem? They have a very high rate of reproduction. Lionfish become sexually mature at about 7 months to 1 year old and spawn in pairs. Females will release 30,000 eggs every spawning cycle, adding up to about 2 million per year in some cases. These eggs settle out as baby fish in about 30-40 days. Are you doing the math? Because that’s a lot of fish and a really big invasion problem. Today I’m going to take a look at two papers (out of an ever-growing number) that ask why lionfish are successful invaders and which habitats within their invasion zone they flourish.
The first paper looks at why lionfish are successful by comparing Kenyan and Bahamian lionfish populations. This is not an uncommon type of comparison. However, it is seldom studied in marine invaders as a whole, and since marine predatory vertebrate invaders are rare it is even less common in this realm. The idea is relatively simple: Invasive species are not a problem in their home ranges, they are kept in check by other components and members of their ecosystem. Comparing invasive species to their native counterparts can reveal shifts in ecology and behavior and can shed light on the factors contributing to a successful invasion and even some potential control methods. This study tested if lionfish on invaded Caribbean reefs have reached greater abundance than they normally reach on their native reefs, and they tested potential ecological differences by measuring lionfish body size and activity levels between the native and introduced fish. To see if lionfish are in greater abundance and/or size in introduced areas, the researchers conducted underwater visual surveys of lionfish in both their native (Kenya) and introduced (Bahamas) ranges, recording numbers of fish and total length of each fish. During these surveys, they also recorded lionfish behavior as active (i.e., either hunting, swimming, hovering in the water column or moving over the substratum) or inactive (i.e., resting motionless on the substratum).
This study found that invading Bahamian lionfish reached a higher abundance than their ecological equivalent in Kenya. However, it is important to note that when they combined the density of all five Kenyan lionfish species they were similar to Bahamian P. volitans, and that some Bahamian reefs had much greater densities than others. The Bahamian lionfish were also about 50 percent longer and had an overall biomass that was 13 times higher than Kenyan equivalents or the Kenyan lionfish species assemblage. There are several hypotheses as to why including lack of exploitation, low predation, low predator diversity and abundance, low fishing pressure, and a release from congeneric competitors. They are so numerous that they now make up a significant portion of the fish biomass on invaded reefs.
The second paper looks at the progression of lionfish into different habitats. In their native range, P. volitans and P. miles are predominately found on coral, rock, and sand substrates from <1 to 50 meters underwater. Their invasive range has been observed to be much broader, extending into habitats that include reefs, seagrass, mangroves, and in depths from 1 to >600 meters of water. Two previous studies that have looked at this habitat question have found that mangroves supported higher densities of smaller-sized individuals than nearby reefs and that lionfish in seagrass were smaller than those on reefs (both suggesting a nursery function). And while there is an international effort to document the spread of the lionfish, there has been less emphasis placed on how a new location becomes colonized. This study looks the invasion history as well as this colonization. The study area was located around South Caicos (a small island in the Turks and Caicos Islands) and Long Cay (on the eastern edge of the Caicos Bank). Five different marine habitat types were distinguished: mangrove, seagrass, sheltered shallow reef, exposed shallow reef, and deep reef. Using surveys consisting of timed swims, relative density of lionfish (number of individuals seen per observer and per unit effort) was calculated within these habitat types from 2007 to 2010. To look at size frequencies of lionfish within these habitats, individuals were caught and depth, habitat, type of shelter used, and total length were recorded. The age of individuals was estimated from total size.
They found that by the end of 2010, lionfish had been observed in all five habitats with relative densities consistently rising throughout the course of the study period. Back-calculation of settlement dates indicated that lionfish may have started settling there as early as 2004. Sightings during their surveys initially showed that the density of lionfish in seagrass was 20 times higher than on deep reefs, but as the study went on the relative densities became similar across the habitat types with the concluding year showing the deep reefs to have over an order of magnitude higher lionfish density than any other habitat. There was also a significant difference in the sizes of lionfish caught in different habitats. Lionfish in deep reef habitats were significantly larger than those in seagrass and sheltered reefs, but they found no size differences in individuals from shallow habitats. Individuals found in these shallow habitats were younger than those found on deep reefs. Most of the lionfish were found to shelter on, in, under, or around other structures (corals, rocks, seawalls, trash, etc.). Observations of exposed reef habitats found lionfish to be conspicuously absent until 2010. They were found preferentially (but not exclusively) to settle in shallow habitats (seagrass, sheltered reefs, mangroves) before moving to deeper water once they had grown larger. However, they would have had to pass through these exposed reefs on their way to the deep reefs. The few individuals found on exposed reefs may be a result of this movement combined with the turbulent conditions associated with this habitat type. This evidence supports the idea that seagrass, mangroves, and sheltered shallow reef areas may serve as nursery habitats and adult fish move to deeper reef habitats later.
From all I’ve gone through today, the story looks pretty bleak. And I’ll be the first to admit that it doesn’t look optimistic. But there is good news. There are several lionfish research programs and international efforts to control or even eradicate these fish from the Caribbean (see some links below). And another bonus? Apparently they taste great!
Emily S. Darling, Stephanie J. Green, Jennifer K. O’Leary, & Isabelle M. Coˆte (2011). Indo-Pacific lionfish are larger and more abundant on invaded reefs: a comparison of Kenyan and Bahamian lionfish populations Biological Invasions, 13 (9), 2045-2051.: 10.1007/s10530-011-0020-0
John Alexander Brightman Claydon, Marta Caterina Calosso, & Sarah Beth Traiger (2012). Progression of invasive lionfish in seagrass,mangrove and reef habitats Marine Ecology Progress Series, 448, 119-129.: 10.3354/meps09534
Here are a couple of websites to get you started looking in to this problem:
CCFHR: Invasive Lionfish
REEF Lionfish Program
Interview with Chris Flock from Ocean Support Foundation