A Navy man should be prepared for anything on the water, but Shawn Eisenberg took it to new lengths Sunday, boating a tilapia on the Chicago River.

“We were catching crappie,” Capt. Pat Harrison said. “He reached down and said, `Hey, this one has teeth.’ ”

That’s right, Eisenberg caught a 13-inch tilapia on the main stem of the Chicago River in downtown Chicago by a discharge.

Not what was expected on an outing, guided by Harrison, for Veterans R&R, a Wauconda-based not-for-profit helping veterans. Eisenberg is active duty Navy, as was Arthur Rodriguez. The third fisherman was , an Air Force veteran.

I expect many surprises from the Chicago River system, but I could do without these.

Listen, dreams of swimming the Chicago River aside, it is not a pristine trout stream in the Driftless Area. It is a freaking urban waterway that flows backward, a very unnatural setting.

Normally our winters wipe out such released fish. The problem is the warm-water areas, such as some discharges, on the Chicago Area Waterway System (CAWS), which includes the Calumet and the Chicago River systems. As the water quality keeps improving, non-native species have improved chances to survive our winters around the warm-water areas.

It irks me that every year or so I need to check in with Jason Duracka of Animal Island Pet Shop in Midlothian to confirm an ID on a non-native fish. Then we go over the same routine.

If your aquarium fish have grown too large or you don’t want them any more, don’t just release them, thinking that is more humane. Sheez, this is the 21st Century.

“I am one of the few shops who will take them,’’ Duracka said. “They can drop them off at the shop any time.’’

Animal Island Pet Shop is at 14411 Cicero Ave, Midlothian. Phone is (708) 293-0600.

It shouldn’t happen, but too many reports come too often of such oddities as pacus and piranhas in our waters.

Carl Vizzone, veteran Chicago fisherman and son of a bait-shop owner, said he knows of several tilapia caught from the Chicago River over the decades. Then he added it reminded him of the 1980s when oscars were the rage. They would grow too large, then end up in Chicago lagoons, ponds and lakes.

The North Shore Channel had a rash of unexpected non-native fish many years ago, which were suspected to have come from a closed pet shop.

But some cool unexpected surprises have also come from the Chicago River system. And I mean even beyond the occasional walleye, sauger and hybrid striped bass.

In the summer of 2014, a spotted gar showed up on an Asian carp survey by the Illinois Department of Natural Resources on the North Shore Channel. That was particularly notable because spotted gar require cleaner water.

I could go for more natives like that on the CAWS, and how about, say, none of the non-natives.

STRAY CAST: More and more Rudy Giuliani mimics a just-caught channel catfish, flopping around, flapping lips, croaking.