The impression that sticks with you is of carnage — animals slaughered on a wholesale scale.

“We shoot until we run outta shells,” one camo-clad local drawls from the cockpit of his flat bottomed swamp boat.

But they’re shooting at “Rodents of Unusual Size,” the destructive “swamp rat” named nutria. So any cringing at all this shooting, trapping, skinning and lopping off the tails of untold hundreds of thousands is minimized, or at least dulled by repetition.

Here’s a nature documentary set in “Beasts of the Southern Wild” country, the coastal wetlands of Louisiana, fast disappearing due to oil and gas exploration, drilling, pumping and canal digging, by Missisippi River mismanagement on a vast scale, and due to the incursions of a 20 pound beaver-toothed rodent imported from South America, the bane of coastal marshlands across the planet.

A whimsical animated opening (narrated by Wendell Pierce) recounts “a tale crazier’n Hell,” about nutria — how a few folks, encouraged by the state, imported the Argentinian rodents in the 1930s to broaden Louisiana’s fur industry.

The most famous importer, and the one who gets most of the blame (not all of it deserved), is E.A. McIllhenny, scion of the Tabasco Sauce empire. Nobody messing with nutria knew what they were dealing with, and keeping them penned up while they bred like their fellow rodents — rats and rabbits — was a challenge.

Which the nutria entrepreneurs abandoned, loosing their stock into the state’s marshes “to aid the state’s fur industry.”

As the fur is quite soft and pretty, that worked out — sort of. A LOT of people trapped and sold nutria for their fur. Until “Fur is Murder”  sea change of the 1980s. The market collapsed, and battered wetlands, broken by river-dredging, river traffic and the canals dug by the state’s rampant oil and gas exploration, were beseiged from below. Nutria love marsh greenery. And they dig burrows that flood and collapse the landscape they’ve denuded.

Generations of water folk who had used nutria trapping as a winter source of income were broke, and losing the land literally under their feet. So the state went after nutria with a vengeance — a $5 bounty on every nutria tail.

As much as this fifth generation Cajun or umpteenth generation Native American waterman (and the occasional woman) declare that they were taught “never to kill something unless you make something with it,” that’s just what they’re doing.

Tails cut off, fur-covered corpses with big orange teeth and a meat not unlike rodents we eat (rabbit) tossed back into the swamp.

If the carnage of “nutria skinning contests” doesn’t turn you off, the sheer waste just might.

But filmmakers Chris Metzler, Jeff Springer (they did “Plagues and Pleasures of the Salton Sea”) and editor/co-director Quinn Costello turn this documentary about a necessary evil — Louisiana IS washing away, after all, and they’re NOT going after the oil and gas industries, so — into a serio-comic essay on the duality of man — redneck man, anyway.

But as wildlife officials lecture about the utter necessity of scaling back the nutria population (still in the millions), as a state employee who tries to trap nutria out of the canals of New Orleans itself (levees and bridge foundations are being undermined), as hunters efficiently take their .22s to their shoulders and pick off another quarry and old-timers tally their day’s count with the state tail-tally assessor, we sense their grudging admiration for the critters.

You’re not getting anybody in North America to eat the meat (again “rabbit”) without major rebranding (the filmmakers named their production company “Tilapia,” after a re-branded trash fish). But the fur has the “sustainable” cachet.

There’s a Fur Queen Beauty Pagent, the nutria is a newly-hip sports mascot down on the bayou and one hunter’s even taken a nutria as a cuddly “high maintenance” pet.

So whatever outsiders might think of the carnage, or the acceptable hatred of an “invasive species,” the locals seem to have reached their peace with the invader, even if they’re not keeping them as pets or adding them to their gumbo.

MPAA Rating: unrated, animal slaughter, mild profanity

Cast: Briefly narrated by Wendell Pierce.

Credits: Directed by Chris Metzler, Jeff Springer, Quinn Costello. A Tilapia Film?PBS release.

Running time: 1:11