Country diary: Swimming in the sea through a roiling shoal of silver sprats

As we arrived at the beach, the setting sun blazed orange, backlighting the sail-shaped Spinnaker Tower, which stood proud on the horizon. It was the perfect evening for a swim, the calm, dark sea reflecting like quicksilver. As we slipped in, two swallows swooped low above our heads – a welcome distraction from the water’s gasp-inducing chill.

All was quiet as we struck out on a loop of the bathing area. On the return leg, a lone sandwich tern was startled by our flotilla of fluorescent tow floats and flushed from its perch atop a marker buoy. As I followed its flight path, I noticed a frenzied flock of herring gulls and black-headed gulls hover-plunging close to shore.

As we swam closer, the gulls’ screeching cries merged with the screams of a group of children engulfed by the pulsating bait ball of sprats that had drawn the birds’ attention. The term sprat is applied to various species of small forage fish, but these were true sprats (Sprattus sprattus). They’re easily mistaken for juvenile herring (Clupea harengus), but can be distinguished by the sharply toothed keel along the belly and grey, rather than blue, dorsal colouration.

As I waded through the shallows the baitfish began to surge around me, wave after wave of them washing up on the beach. There was a splash behind me and something large bumped against my leg. Then another splash. A spined dorsal fin momentarily broke the surface, as a European sea bass (Dicentrarchus labrax) burst through the shoal in high-speed chase. In late summer, sprats move into coastal waters to spawn and it’s not unusual for them to be herded into shore by these predatory night hunters.

By the time I navigated a path through the roiling sea of silver, there were already hundreds of beached fish. Most were flapping and gasping, but some lay still, smothered by their shoal mates. I scooped up handful after handful, their slick-scaled bodies slipping through my fingers as I returned them to the sea, but it was a sisyphean task – as soon as I released one batch, another wave of thrashing sprats was deposited on the sand.