Twaite shad are a silver, saltwater fish of the herring family. They have a blueish silvery tinge with between 3 and 10 distinctive dark spots along its flanks, starting darker at the gill casing and fading towards the middle of its back. Adult fish are typically around 30cms long when they migrate up rivers to spawn. Around 35% of shad in the River Severn are recorded to have hybridised at some point in the past with the slightly larger Allis Shad, Alosa alosa. Hybrids are fertile so the genetic mixing persists.
Shad feed on a variety of invertebrates when they are at sea and they also filter-feed on plankton. It was widely believed that Shad stopped feeding once they moved into freshwater to spawn, but a recent survey on the River Ulla in Spain seems to suggest that active feeding may take a while to cease on entering freshwater. It was discovered that shad feed on a variety of small freshwater animals, including small fish. It was also discovered that they will readily feed on the surface for terrestrial invertebrates like mayflies, spiders and other insects that fall onto the surface of the water. This might explain why historically they were easy to catch on fly fishing tackle, although today in the UK they are fully protected and angling is not allowed.