I hear them before I see them: a wailing, or something more like ululation. High-pitched, repeated vocalisations that are neither man nor cat, but somewhere in between. I throw open the cabin door, unsure what to look for, and for a moment see nothing new: only the rocky strand and the sweep of dark wrack high above the shoreline.

The strange, warbling sound comes again. Ai-ai-ai-ai. Not quite screaming, not quite howling, not quite trilling, but all of those things at once.

A flash of movement in the water, perhaps 50 metres out. A small, dark body leaps from the surf, followed smartly by another. My eyes struggle to understand what I’m looking at. Tiny figures catch a millisecond of clear air, then slice back through the surface, like dolphins or porpoises. But the silhouette is all wrong. And there, again – two small heads pop up, turn to one another, and seem to wrestle.

Dusk on the Isle of Lismore. Photograph: Cal Flyn

Ai-ai-ai-ai. Are they playing? Fighting? Their forms are slick and dark. When one launches again from the water, I see it clearly: four limbs, and a long tail – two otters, locked in battle. Over a fish perhaps, or one of the crabs we were watching earlier from the rowing boat as we drifted on the breeze, hands trailing through saline, watching the tiny creatures on the seabed below like tourists from a cable car.

They tussle and twist. Then one breaks free and makes a dash for the shoreline. The other shrieks and follows, both now skimming over the surface like stones.

What drama! You would hear them a mile away. The otters disappear behind a boulder, and for a moment all is silent but for the wash of the waves.

It has gone 8pm. The sun is lowering in the sky, ready for bed. The wind ruffles the heads of the trees on the island in the bay. A pair of little egrets pick their way through the shallows, pause to examine their surroundings, then move on, feet lifted high and deliberate. Then the voices come again. Ai-ai-ai-ai. The soap opera continues unseen.