Genetic study finds Ice Age salmon refuge

Salmon (Image: PA) Salmon populations found in British and Irish waters among the most genetically diverse in Europe

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An area of coastal waters around North-West France has been identified as a site for a previously unknown ice-free refuge for salmon during the Ice Age.

Researchers said the isolated marine haven would help explain the "genetic mosaic" of British and Irish salmon.

They added that fish from this refuge bred with fish from the Iberian peninsula as they migrated into UK waters as the ice receded.

The findings have been published in the journal Heredity.

"There has been a lot of work done on terrestrial organisms and their refugia at the time of the last glacial maximum," explained co-author Jamie Stevens, an evolutionary biologist from the University of Exeter, UK.

In their paper, the team of European researchers said that it was possible, as a result of genetic differences, to "trace the movement of (marine) species from refugial areas into previously glaciated regions".

Atlantic salmon in detail

Salmon leaping (Image: PA)
  • Scientific name: Salmo salar
  • Found throughout the North Atlantic region
  • After long migrations, the fish return to their natal river to spawn
  • Abundance of Atlantic Salmon has declined markedly since the 1970s
  • Increased mortality at sea appears to be a major factor in this decline
  • Other threats include river pollution, overfishing and dams

(Source: IUCN Red List)

They added: "For most European species, ancestry from the Pleistocene period can be traced back to one or more of the three main refugia in the Iberian, Italian or Balkan peninsulas.

Unrecognised refuge

However, they explained, that there was "no evidence for their extension into the Italian or Balkan regions at that time".

"One of the key findings of this paper is that we can now explain the genetic mosaic of salmon in Britain and Ireland as being made up from fish that migrated in from the Iberian peninsula and a previously unrecognised refuge for salmon in North-West France," Dr Stevens observed.

"What this evidence shows is that there was almost certainly a refuge in this big scour in the ocean at the western end of the English Channel, which is referred to as Hurd Deep.

"It may have offered conditions - both climatically and in terms of access - to rivers that were open from ice, providing a refuge for salmon while other areas were largely inhospitable."

He added that the salmon populating the two regions were already genetically distinct before the arrival of the last Ice Age pushed them into the refugia.

"And when they came out and recolonised the British and Irish waters, they were very distinct," Dr Stevens explained.

"Because of that, we can now recognise the mosaic of contemporary populations in Britain and Ireland as being a product of fish that have migrated out of those two refugia."

Understanding the historical sources of the species' genetic diversity had a range of contemporary applications, he said.

"Broadly speaking, there is a general feeling that safeguarding diversity within a species mean that species has a level of genetic preparedness within it, then if some form of change happens within the environment, there is enough genetic variation within an organism to be able to drive an increase in genes that are most [apt] within the new environment."

But, he added: "If an organism has gone through a process where it has experienced much reduced genetic diversity then when that change happens, the chances of it being able to respond just would not be available."

A greater genetic diversity also provided salmon populations with the ability to display local adaptations, such as being able to inhabit chalk streams.

Dr Stevens adds that genetic research has shown the populations found in British and Irish waters to be among the most distinct, genetically, in Europe "so there is a real value in knowing where they came from and trying to preserve them as well."

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