These aren’t salamanders, they only have two limbs and don’t have fingers on those limbs.
They aren’t sirens because they don’t have external gills.
I don’t think they’re amphiumas because those limbs look like fins, not tiny arms.
Electric eels (not actually eels) are lighter and have pitted skin.
I don’t think they’re lungfish, most lungfish have four limbs and all are solitary. You’d be hard-pressed to find that many lungfish in one section of river. I’m also not sure whether lungfish are quite that vigorous about getting to food.
So, yes, I’d say they’re eels! I’m surprised they didn’t try to eat the duck, freshwater eels can take down impressively large prey.