Environment Agency and English Nature scientists have completed in-depth research into a fish under threat.
Allis and twaite shad can be found in seas as diverse as the Baltic and Mediterranean but their numbers are dropping.
In the UK, twaite spawn in just four rivers – Severn, Wye, Usk and Tywi (Midlands and Wales) – and allis in one, the Tamar (South West). Allis have disappeared from the Severn and twaite from the Thames and there is evidence that both were once present in the Trent.
Now the Agency and English Nature have completed reports that look into distribution, biology and ecology of the two species of shad (Alosa alosa and Alosa fallax) in south-west England and review literature available on the fish that give biologists and fisheries managers an overview of conservation options.
Paul Smith, Fisheries, Recreation and Biodiversity Technical Specialist from the Environment Agency said: “There has been a considerable decline of this species across Europe to the point where it is now listed under both the Bern Convention and the EC Habitats Directive and in the UK under the Wildlife and Countryside Act.
“The main conservation options for the protection and enhancement of Alosa populations are reviewed in one of the reports.
“The other presents the most detailed information about shad distribution in south-west England to date.”
Shad have been reported from as far north as Iceland while others have been reported near the Nile.
They have a similar lifecycle to salmon, reproducing in freshwater but spending most of their life at sea. Adults enter UK rivers to spawn around May before returning immediately to the sea. The juveniles do not follow the adults straight away by stay in freshwater until the autumn before heading to sea.
The main reason for the decline is the construction of dams that prevent the fish from reaching their spawning grounds. Their presence has led to either extinction or hybridisation.
Miran Aprahamian, Principal Fisheries Scientist at the Environment Agency said: “The barriers stop migrants moving upstream and various species of shad end up using communal spawning areas which result in hybridisation. Shad congregating below the barriers are easily fished and this can lead to their exploitation.
“Poor water quality, river engineering works and gravel extraction also impact on shad numbers.
“These new reports will be invaluable to anyone involved in the study, management or protection of these fish.”