by Audra Burchfield,
Marine conservation student
Imagine being a reef fish around twenty years ago. You are swimming around when all of the sudden a new fish you have never seen before is on the reef. Over the next few years more of these strange fish are present and pretty soon they are common place. The reef has been invaded by an alien.
Alien invaders, better known as invasive species, can be a major problem in an ecosystem. These organisms can have extensive negative economical impacts and can threaten biological diversity. Predatory invaders are one credible cause of the decline and extinctions of native species all over the world (Vitousek et al. 1997).
Lionfish are a predatory, venomous, subtropical and tropical species native to the South Pacific, Red Sea, and Indian Ocean. They are popular in aquarium trade and this may have assisted in their settlement in the Atlantic Ocean (Whitfield et al. 2002, Semmens et al. 2004, Ruiz-Carus et al. 2006). Two species of lionfish Pterois volitans and P. miles have become established along the Southeast U.S. coast of the Atlantic, Bermuda, the Bahamas, and are expanding their range further into the Caribbean.
Lionfish are believed to be opportunistic predators, such as groupers and snappers which feed on other fish shrimp, and crabs (Hiatt & Stasburg 1960, Harmelin-Vivien & Bouchon 1976, Sano et al. 1984). Within the native range of lionfish, lionfish are found at a depth up to 50 m (164ft) and reach a total length (TL) of 38cm (15in) (Randall et al. 1990). Here, reports suggest several organisms prey on them (Bernadsky & Goulet 1991). In contrast, in their invaded range, lionfish are being found at a depth up to 304.8m (1000ft) (R. Gilmore unpubl.) and reach lengths of 47.6cm (18.7in) TL (J. Morris unpubl.). In their invaded range, lionfish are not known to have any predators, but recent reports have found lionfish in the stomachs of piscivorous (fish eating) groupers (Maljkovic et al. 2008). A study completed by Grubich and colleagues in 2009 reported lionfish in a much higher density of 5 to 177 times greater in the invaded range compared to their natural habitat.
A recent study by Muñoz et al. (2011) set out to determine the short-term and long-term diets of lionfish. To accomplish this, they sampled lionfish from June to August during 2004 and the same period in 2006, 226 lionfish were collected from 18 separate sites in Onslow Bay off North Carolina’s coast. These collections were done during the daylight hours, using spears, at depth ranges from 30 to 45m (98 to 148ft) on SCUBA using decompression methods on ascent. During the 2006 collections, a fifteen minute visual survey of the benthic fishes was completed to estimate the potential prey abundance of lionfish. Each survey was completed along a 25m (82ft) transect. The width of each transect was limited to 2m (6.5ft) and the focus of the survey was on juvenile prey fish less than 10cm (4in) TL. These juvenile fishes were located on or just above the bottom. After spearing the lionfish at depth, each fish was placed into a plastic bag to prevent loss of regurgitated stomach contents during assent (Muñoz et al. 2011).
Diets of the collected lionfish were determined by analyzing stomach contents and performing stable isotope analysis on lionfish tissues. The stomach content analysis permits scientists to see what the lionfish has eaten in the last few hours or days, while the stable isotope ratios allow scientists to understand what the lionfish diet has been for the past few weeks to months (Cocheret de la Moriniére et al. 2003). Stable isotopes, like carbon-13 and carbon-12, contain a combination of protons and neutrons that prevent the isotopes from decaying over time. On the other hand unstable isotopes, like carbon-14, will decay to a stable form. The trophic level of an organism can be estimated by testing several different isotope ratios, such as carbon and nitrogen isotope ratios, in sampled tissues. From this Muñoz et al. predicted from which trophic level lionfish are feeding (Fogel et al. 1999).
The stable isotope analysis is shown as a change in the ratio of carbon-13 to carbon-12 in lionfish tissues and this is directly related to the number of prey items consumed by the lionfish that feed on phytoplankton (microalgae) and bottom dwelling algae. These are primary producers, or the base of a food chain. In 2004, the lionfish were analyzed for a change in carbon-13 and nitrogen. The results were that carbon-13 changed very little, but nitrogen had changed. Seventy seven percent of the carbon-13 in lionfish diet is made up from microalgae and other primary producers, rather than other piscivorous. An isotope analysis was not done for the 2006 sample group
Results indicated that the lionfish collected were found on nearly all habitats sampled, including low, medium and high relief (build up) natural hard bottoms, artificial reef wrecks and algal sand plain sites. Of the stomachs collected, 81% contained food, and of those 96% had eaten prey fish from 19 different groups. However, 45% by volume and weight were unidentifiable. This is likely due to advanced digestion. Analysis of the stomach contents reveled that lionfish do appear to be generalist carnivores. Decapods crustaceans, cephalopods, bivalve mollusks, and echinoderms made up only 3.8% of the contents by both volume and count. The prey fish from 16 different families were consumed in much greater abundance. A difference was seen in the importance of prey species between the two sampling year which may have been due to prey availability or size differences between the sampled lionfish. Those lionfish collected in 2004 were significantly larger than the samples collected in 2006. Crustaceans were consumed considerably more by the smaller rather than the larger lionfish. It seems that the diet of a lionfish changes during ontogeny, consuming bigger prey as predator grows. The visual surveys, only conducted in 2006, did show a larger frequency of grunts in the environment and this is proportional to what was found in the stomachs of the 2006 sampled lionfish.
In summary, after analyzing the stomach contents of sampled lionfish collected off of North Carolina, Muñoz et al. (2011), establish that they display a generalist feeding strategy. Prey categories that were important in the diet of the 2004 sample were not as important in the 2006 sample. It is unlikely that these differences were due to the size of lionfish alone, as prey availability may be an additional factor involved. A visual survey in 2004 would have assisted in determining whether the change in prey consumption was due to ontogeny or prey availability. With lionfish seeming to be generalist predators that feeds primarily on benthic fish, the diet and habitat use appears to overlaps with native fish (such as grouper), where competition may occur. Some of these native fish are fighting back a bit. Lionfish have been found in the stomachs of Nassau Groupers (Maljkovic 2008) so there may yet be a silver lining to the lionfish invasion.
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