Lionfish are an exotic fish now found throughout the Greater Caribbean and eastern Atlantic that have become incredibly abundant on many reefs, especially in the Bahamas and off North Carolina. Lionfish are a piscivore (a fish that eats other fish) and were introduced from the Indo-Pacific by the aquarium trade in the late 1990s off Florida. Mostly likely, someone got tired of their fish and released them purposefully.
One hypothesis explaining their great success is the absence of natural enemies; predators, parasites and competitors. This is probably compounded by the fact that few Caribbean reefs have any predators left that could eat them (thanks to overfishing).
Another – and I think much more likely explanation – is because there is so much to eat in the Caribbean! Not because there are more fish, but because it is so much easier to catch them. Unlike fish in the Inro-Pacific, native Caribbean fishes do not appear to recognize lionfish as a potential threat. So the lionfish gobble them up, grow faster, make more babies, spread to new islands, etc.
Case in point: My lab group was working in Belize last week on the lionfish invasion. One of the things we were doing was collecting the otoliths and gonads from lionfish that we speared on a number of reefs to compare their fitness across the Caribbean (e.g., on reefs with and without native predators, etc). We also looked at stomach contents and many of them had parrotfishes in their tummies or still in their throats! The photo above is of the eggs from one lionfish we caught near Glovers Reef Atoll and the partial contents of it’s stomach (a juvenile striped parrotfish)!
Lionfish appear to be little more than machines that convert parrotfishes to baby lionfishes. Which is pretty much the purpose of all animals (consuming others and transforming them into your own genotype and species). But jeez, couldn’t those aquarium hobbyists have released a herbivore that could be converting macroalgae to fish biomass? That would have been much more useful.