This week I spent two afternoons at Back Bay National Wildlife Refuge. The first visit was cold and windy with heavy, low-hanging clouds. My photos from that day are grainy and blurred, including several photos of a rather large mammal crossing one of the many open areas of water.

At first I thought the creature was an otter. It was too big to be a muskrat, and the habitat was wrong for beaver.

But, what about the shape of its head? Doesn’t look like an otter’s head…

Which leaves me with nutria. (Please comment if you can correct or confirm my guess!)

I returned the next day, lured by warmer temperatures, clearing skies, and continuing curiosity. The animal wasn’t there when I arrived, so I walked the other trails for a few hours and circled back at sunset for one more try. By then the light was even worse than the previous day, so I almost missed the familiar form. Forms, because there were two.

I took a few photos, though I knew it was too dark for my camera’s lens, and I was on the point of leaving (the refuge closes at dusk) when smaller versions of my mystery mammals appeared.

The waning light defeated my camera, so all I have to share are shadows and silhouettes. My photos don’t show how the young animals played in the water, how they chased each other in widening ripples. How they ventured into open water, then hurried back to the safety of their parents.

I watched, enthralled, until the sun’s light disappeared completely. The scene was charming. Baby animals are always charming.

Except, in the case of nutria, charm quickly fades.

I have mixed feelings about eradication programs aimed at invasive species. Nutria undoubtedly wreak havoc on marsh ecosystems, but what are the chances they can be eradicated permanently? And what is the cost? The bottom line is that all ecosystems change. Coastal ecosystems, in particular, are under immense pressure. Can we hold back the tide? Should we? I’m not proposing that we do nothing, but I suspect eradication is not a sustainable goal.