JACKSONVILLE, Fla. -- Last week, a diver discovered what he calls the largest concentration of lionfish he has seen in years off the First Coast.

"So we've been diving off shore Jacksonville several years now," Joe Kistel said. He leads the marine advocacy organization called TSIRI in Jacksonville.

And for several years, Kistel has spread the word about how detrimental lionfish can be to the native environment.

But recently, "we started hearing reports of bigger concentrations of lionfish being found offshore Jacksonville and St. Augustine."

So last Friday he went diving 25 miles southeast of Jacksonville, and he found "the highest concentration of lionfish we have ever seen to date offshore our coast here in Jacksonville."

Kistel said the video couldn't capture all of the lionfish there.

'In the frame, you will see an easy 20 lionfish," he noted. "But that's not counting the lionfish under the ledge in the shadows in that spot which is just an 8 foot section of that ledge! The ledge goes on for hundreds of yards."

Kistel said typically, lionfish populations are more dense in the Gulf Of Mexico. Last month he shot video in the Gulf with another diver.

"It was unreal," he shook his head. "Every square inch had lionfish in it! [The diver] said he literally eradicated that site a few months before, and it was already congregated with lionfish again.'

And so when Kistel dove off Jacksonville and saw nearly the same amount of lionfish, "it actually started looking like what we're seeing in the northern Gulf -- right here off Jax."

And that's not good because non-native lionfish are bad for this area.

"These fish are like ecological beasts," Kistel noted. "They're directly compete with fish of economical and environmental concerns. So if you like to eat grouper and snapper at your restaurant, guess what... the lionfish are competing with those fish. For one, lionfish eating them when they're small enough and two -- they're eating their food sources."

Lionfish are not from here. They're from the Indo-Pacific... but they're amazing looking -- and popular aquarium fish.

It's believed that some were let out into the wild, and now their population has skyrocketed.

According to NOAA, lionfish have been reported along the southeastern United States coast from Florida to North Carolina and they are capable of causing extinct ions of native plants and animals.

So why is the lionfish population seemingly expanding?

They taste good. However, Kistel explained only a few restaurants and stores sell them... so the demand for lionfish is not great enough yet.

Secondly, Kistel said the animals are so biologically strong in the Atlantic and Gulf, that the population boom just continues.