[Joe 6-pack]

"lionfish are poisonous, no? Why aren’t the sharks affected?"

"Even though lionfish are venomous, lionfish are not immune to being harassed or even eaten by other fish. Large eels, frogfish and other scorpion fish are all predators of lionfish in the wild."

I think that as the lionfish are an invasive species, the sharks just don't recognize them as a food source.

[JoeProBono]

Yeah, and besides which it takes a lot to kill a shark.