Shad belong to the herring family. They look similar to (but much larger than) the familiar tinned sprat and pilchard, but are their less common relatives; residents of coastal waters, they move upriver to spawn.
There are two species of shad in northern European waters, the allis shad and the twaite shad. The larger of the two, at a maximum of 60cm (2ft) long, is the allis, an exceptionally rare, protected species.
The twaite shad – generally a bit smaller (up to 50cm/20in long) – is more common, particularly on the west coast and in the Rivers Severn, Wye, Usk and Tywi (as well as some Irish rivers). Nevertheless the twaite is still a rare fish, and much less common than it was in previous centuries.
Both species have suffered from the con- struction of weirs in the major rivers which hinder upstream passage, and from pollution in the lower reaches.
It is illegal to attempt to catch the protected allis shad so it’s important to be able to identify one from the other. (If you catch an allis by accident, release it immediately.) Both are silvery, herring-like fish with a sharp-edged belly, ridges on the gill cover and a deep notch in the mid-line of the upper jaw.
The twaite shad has a series of six to nine dark, round blotches on its sides just behind the head. The allis shad usually has only a single dark blotch.
The surest way to distinguish the two fish is to count the number of gill rakers on the first gill arch. The twaite shad has 40-60 rakers and the allis shad 80-130 rakers. Unfortunately, it’s very hard to count them without killing the fish.
Best of both worlds
Both species are migratory, inhabiting coastal waters and turning up in rivers for breeding. At sea and in estuaries they feed on small fish – young herring, sprats and sandeels go down particularly well – but they also feed heavily on crustaceans.
When in the river they don’t feed, although sometimes plant fragments are found in the stomach of fish caught. These are probably just bits of vegetation snapped at in passing (or in irritation).
Move on up
Shad mature at three to four years old. In spring adults enter rivers and migrate upstream. The allis shad is first to go – usually in May – and travels far up rivers to find spawning areas where the river bed is gravelly and the current swift. This fish is so rare that there is no known spawning population in Britain.
The twaite shad arrives in the river estuary later and migrates upstream to spawn on stones, rocks and coarse sand just above the area of tidal influence.
After spawning shad return to the sea at once. Allis shad rarely spawn again in their life, but twaite shad may make a second spawning run and sometimes even a third-one. Young twaite shad, while in the river, eat small near-surface-living crustaceans and some insects and their larvae; in estuaries they eat large numbers of crustaceans which live near the surface. As they grow they begin to feed on young fish such as herring, and sprat (species which use estuaries as nursery grounds).