One of the UK's rarest fish, the allis shad, is returning to the Tamar estuary in good numbers.

Like the salmon, this relative of the herring spends most of its time at sea and only returns to freshwater to breed.

Barriers to migration including weirs or dams and pollution are thought to be the main reasons for a severe decline in its numbers. Over-fishing is also believed to be a contributory factor. The Tamar estuary and the Solway Firth are currently the only known sites in the UK where allis shad regularly spawn.

Environment Agency officers have noticed an increase in the number of allis shad on the Tamar this year. Not only are they more numerous, the fish are larger – in some cases up to 5 lbs. The population is strongly cyclical with boom and bust years.

‘We’ve caught some fine allis shad in our fish trap at Gunnislake – many of them above average size. We’ve also identified at least three spawning areas on the Tamar. 2011 will certainly be remembered as a year when this species was present in abundance. It is excellent news because it is evidence of the high water quality and favourable river conditions in the Tamar,’ said Paul Elsmere for the Environment Agency.

With its streamlined body and deeply forked tail, the allis shad closely resembles the more common twaite shad. Both species are members of the herring family. Being very bony fish, they are not especially valued for their culinary qualities. The allis shad is referred to by some as the ‘Bony Horseman.’

Outside the breeding season, the fish are mainly found in shallow coastal waters. Around April - June they enter large rivers with strong currents and stony or sandy beds to spawn. Adults spawn at night with a great deal of noisy splashing Young fish remain in the river or estuary of their birth for up to two years before migrating to sea.

A genetic study carried out by the Environment Agency, Bristol University and Marine Biological Association showed that the allis shad in the Tamar have a different genetic make-up to fish using the Solway Firth suggesting they are a distinct population.

The allis shad is protected under the EC Habitats Directive and Wildlife and Countryside Act 1981 which makes it an offence to intentionally kill them or damage or destroy their spawning grounds.

In Europe, the species is targeted by fly fishermen who value its hard-fighting qualities. Singing its praises, one angler on his return from a fishing trip to France, described the allis shad as a ‘turbo-charged kipper.’