There is a reef off the West Sussex coast where, every spring, the males arrive first – and arrive transformed. For most of the year, a black seabream is a modest, silvery-slate fish. But a breeding male turns smouldering and dark, his flanks crossed by bold vertical bars and lit, where the light catches him, with flares of iridescent electric blue, brightest in a band across his brow. In this courtship dress, he fans the gravel from the bedrock into a neat saucer of a nest, then guards it – seeing off a crab, a curious diver, anything that drifts too close to the eggs he fans with fresh oxygen. It is one of the quietest dramas in British waters, and for a long time, hardly anyone above the surface knew it was happening.

That guarding male has a secret. Most black bream begin life as females. Only as they grow larger and older do some turn male: a quiet sex change that makes the hefty fish on the nest almost certainly a reformed female. As a result, the biggest black bream in any population are its males, and its future males are swimming around as the females in today’s waters. It is a delicately balanced way to run a species: fish too hard for the largest individuals and you strip out the males faster than the population can replace them. This is biology that rewards getting the management right.