A rare type of goose, once thought to be a fish, has been spotted at a nature reserve in Abbey Wood.

The barnacle goose were once believe to have hatched from barnacles in the sea, spawning a 1000-year old myth.

These days, the creatures exist in tiny numbers in the UK, with just a breeding population of 900 feral pairs, and they have amber UK conservation status.

Three barnacle geese have been photographed at Crossness Nature Reserve in Abbey Wood, the site of one of the last remaining areas of grazing marsh in London.

Rare barnacle goose spotted in Abbey Woods Crossness Nature Reserve. Photo - Daniel Nikolov

Rare barnacle goose spotted in Abbey Wood's Crossness Nature Reserve. Photo - Daniel Nikolov

The Thames Water Nature Reserve is next to Crossness Sewage Works - one of the biggest in Britain.

Crossness Nature Reserve, Abbey Wood

Crossness Nature Reserve, Abbey Wood

The myth states that the geese hatch from barnacles in the sea, and meant they were regarded as seafood, so could be eaten on Fridays and during Lent.

As with many wild animals, the lack of scientific knowledge about the barnacle goose spawned myth and legend that survived for nearly a thousand years.

Rare barnacle goose spotted in Abbey Woods Crossness Nature Reserve. Photo - Daniel Nikolov

Rare barnacle goose spotted in Abbey Wood's Crossness Nature Reserve. Photo - Daniel Nikolov

The goose was first documented in 1186 when Giraldus Cambrensis in his Topographica Hibernica gave his explanation for the striking absence of barnacle geese in Ireland during the summer months.

Today we know that Giraldus was referring to the cirrhopod shellfish, the goose barnacle, his belief being that the goose barnacle and the barnacle goose were in fact different life stages of one and the same species.

Since the 1960s barnacle geese found breeding in Arctic Svalbard have been marked with individually numbered leg and neck rings.

Whilst 900 exist in the UK, wild barnacle geese, however, breed in places like Greenland and Svalbard but migrate to Scotland, Ireland and northern England between October and March for the milder weather.

Subsequent recaptures of these birds in the Solway Firth area indicate that almost the entire summer population of over ten thousand Svalbard barnacle geese spend the winter around the estuary.

Karen Sutton, the Abbey Wood nature reserve manager, said: “Protecting the environment and looking after wildlife is one of Thames Water’s key priorities so it’s wonderful to see these majestic birds coming to visit our Crossness Nature Reserve for the first time.

Crossness Nature Reserve, Abbey Wood

Crossness Nature Reserve, Abbey Wood

“It’s one of the few remaining areas of grazing marsh in London and has the biggest reedbeds in Bexley. Along with ponds and ditches, and areas of scrub and rough grassland, it’s a perfect site for wildlife to thrive.

“More than 200 species of birds have now been recorded here. We’re also a major site for water voles, water beetles and scarce plants like hedge parsley, Borror’s Saltmarsh Grass and Frog Rush.”

The barnacle goose is one of Britain’s smallest geese and grazes on vegetation, such as roots, stems, leaves and seeds. Unlike the Canada goose, the barnacle goose does not have any brown plumage.