Allis and twaite shad (Alosa alosa and Alosa fallax)
|Size||Allis shad: 30 - 50 cm|
Twaite shad: 25 - 40 cm
Classified as Vulnerable by the British Red Data Book. Protected under Schedule 5 of the Wildlife and Countryside Act, 1981. Listed under Annexes II and V of the EC Habitats Directive and Appendix III of the Bern Convention.
Shads are vertically flattened, silvery-coloured fish of the Herring family. Both UK shad species have large scales, which are somewhat fragile, and cover the entire body except for the head. They are a little-known fish, and are quite often mistaken for other species. A population in Cornwall was not recognised because it was known locally by another name. The species has been known since Tudor times or before, with references being made to 'Shadde' in the River Thames, and laws were passed to prevent people from over fishing.
These two species are found across west and North-west Europe, although there are 14 other species across Europe. The allis Shad inhabits coastal waters and spawning rivers from Iceland to Portugal. The twaite Shad is found from Northern Europe and the Baltic to the Mediterranean. In the UK, twaite spawning populations exist in southern Wales, Hereford and Worcester and are believed to occur in south-west Scotland.
The adult shad is a saltwater fish that move into rivers to breed. A significant part of the marine diet of allis shad is composed of larger plankton, whilst the twaite shad take mostly smaller fish species such as anchovy. The young shads remain in rivers and estuaries, feeding on invertebrates on the bottom, until they are large enough to survive the open sea. Adults of both species enter rivers to spawn.
Twaite shads return to the rivers to spawn between late April and early June and timing is believed to depend on the water temperature. Allis shads begin their spawning cycle a little earlier than twaite shads in the south of their range - as early as March - but in the north they begin the spawning process at the same time. Both species will migrate up rivers as far as possible. Before the widespread construction of weirs and other features that blocked their migration, they were recorded as much as 700km up continental rivers. Now, they are usually unable to proceed to this distance, and rarely more than 100km.
The adults mature at between 3 and 5 years, with a normal lifetime of up to 20 years. The allis shad only spawns once, laying up to 23000 eggs in shallow, gravely water, usually at around 0.5m depth. Twaite shads spawn more than once when they have access to a sufficient food supply, usually laying around 70000 eggs in deeper water. The young hatch after around a week, and feed initially on drifting invertebrates, later foraging on the riverbed. When they are a few weeks old, the young swim down to the river estuaries, which provide protection and a greater abundance of food. The adults feed near the surface on plankton and small fish, usually further out at sea, though older fish do seem to occur in deeper waters.
The two shad species have declined as a result of a variety of factors, including the construction of river barriers such as weirs and barrages, pollution of their spawning rivers, overfishing and habitat deterioration. Weirs and barrages prevent adults from reaching their spawning grounds, and standard salmon fish-passes at weirs or locks encourage turbulent water flow patterns, which shad actively avoid. The shad is more sensitive to pollution than other similar estuary species and have been forced out of many rivers where they formally occurred, including the River Thames.
The shads are vulnerable species, allis shad more so than twaite in the UK. Several plans are in place to try and recover their populations. A plan involving the Countryside Council for Wales (CCW), the Environment Agency (EA) and English Nature has been set up to try and conduct more research into the problems faced by the shad. The fish are now included in English Nature's Species Recovery Programme.The River Wye population of twaite shad is quite strong and could probably form the basis for a sustainable fishery if there was a demand, though the situation elsewhere in the UK is not so good. Allis shad seem particularly elusive.One project will map the genetic signatures of the two species. This will, hopefully, determine the status of the breeding population and place UK fish within a north-west seaboard context. Other research projects have been able to identify populations of the fish that were present in Cornwall, but were known locally as a scad or horse mackerel; closer analysis showed that these fish were, in fact, shads. Law protects the two species, and any caught by anglers must be returned to the river unharmed. Sensitive areas such as known spawning grounds are being protected as Special Areas of Conservation (SACs). A project officer has been appointed to oversee the process, and currently, the shad population seems to be in no immediate danger. However, while the population is still limited in number, these species will enjoy the highest level of protection.
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