The numbers of sprats varies greatly from year to year and this may result from the vulnerability of their floating eggs if the weather is cold and stormy during the spawning season. Herring eggs, by contrast, sink to the seabed, where they stick to stones. Safer from the elements, they are still liable to form a tasty meal for bottom-feeding fish such as haddock.

Throughout history, the oil-rich herring has been of huge commercial importance. Around the coasts of northern Europe, numerous towns and villages have sprung up wherever the shoals of herring came in close to the coast and could be landed. In Britain, rail links were built for transporting the fish from ports to big cities.

The large silvery scales of these fish are easily lost in battle. After a shoal has come under attack by predators, the water is filled with detached scales looking like tiny scraps of silver paper. To get their own meals, the fish take the relatively violence-free option of sieving for plankton as they swim.

A shoal of young herring or sprats swirling overhead is one of the most fabulous underwater sights, and it can be witnessed within metres of the shore. The sparkling, swooping maelstrom is punctuated by predatory fish darting up from below and sea-birds diving in from above to take their share. ItÕs the frenzy of the gathered birds that often gives away a shoalÕs presence.

Herring and their relatives form shoals of almost unimaginable size, containing up to several thousand tonnes of fish. They can turn an area of sea milky with their spawn or, by releasing gas bubbles from their swim-bladders as they rise towards the surface, make the water appear to boil.

The herring family includes shoaling fish such as the sprat, pilchard (called sardine when young) and anchovy, as well as the herring itself. They are all silvery and slender rapid open-water swimmers, so the different species are difficult to distinguish under water. The shoal of young fish in the photograph consists mainly of sprats, with a few herring. Such mixed shoals are collectively known as whitebait.