Twaite Shad

Alosa fallax 

A twaite shad

Characteristics

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 alosaHybrids are fertile so the genetic mixing persists. 

Diet

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.  

 

Habitat

Shad are found across the coastal regions of the North Sea and Mediterranean, but they have struggled in most rivers in these areas. In the last 180 years weirs were built on rivers to aid navigation for boats. Through our monitoring and tagging the twaite shad on the River Severn we have found that there are populations still in existence in the Severn estuary, and every spring adult fish attempt their migration upstream to their ideal spawning grounds which are north of Bewdley and as far as the Welsh border. They are unable to progress upstream north of Worcester, apart from in flood conditions. Shad are a strong swimming fish in terms of body lengths per second, but their inability to jump over obstacles has meant weirs are a significant problem to them. Most adult shad on the River Severn will return to the sea after spawning. They spend an average of just 17 days per year in our rivers, so they really are a sea fish that has to enter freshwater to reproduce. 

Did you know?

The first fisheries laws in Britain were introduced by the Crown to protect shad on the Severn, where both species were eaten by Royalty, particularly around Lent and by the local population.  

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