Healthy Thames 'key for return of salmon'
Atlantic salmon in detail
- 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)
Salmon in the Thames are more likely to be fish that have "strayed" from nearby rivers rather than a result of a multi-million pound restocking effort.
Researchers collected genetic data from returning fish, which suggested that habitat restoration was more effective than species re-introductions.
Lessons from this study, the team said, could be applied to rivers that the fish had disappeared from.
The findings will be published in the journal Biological Conservation.
"Traditionally, people - such as river owners and anglers - have wanted big fish swimming up their rivers," explained co-author Jamie Stevens from the University of Exeter.
"The easiest option has been to just restock those rivers with exogenous fish (from outside the river system).
"In the southern parts of Britain, the thing would have been to take fish from some of the big Scottish rivers, bring them down and dump them in the rivers rather than improving the riparian [river] habitat," he told BBC News.
Dr Stevens and his team, in a previous paper, carried out a study of Atlantic salmon populations (Salmo salar) in northern Spain that suggested that stable populations developed genetic profiles that become definitive to a particular river system.
Some of the unique characteristics included local adaptation to the river system's water chemistry, temperature and run-time from sea to spawning ground.
"Almost certainly, most fish come with a vast amount of local adaptations which we really are only beginning to scratch the surface," he suggested.
"So as soon as you dump a big fish in a river than is not its native one, it is just not as fit."
In their paper, the team said evidence from Pacific restocking programmes showed that once restocking projects had run their course, the numbers dropped off quite quickly and the ones that did remain had "strayed" from nearby rivers. They were not fish that had be introduced from distant hatcheries.
"In a sense, this is what we believe we are seeing in the Thames study," Dr Stevens observed.
"Those fish that were put in the river were Scottish and Irish hatchery fish, and they were probably not suitable for an English chalk stream environment."
Straying from home
Historically, England's longest river had a "significant" salmon population, the researchers wrote.
"It is mentioned as far back as the Magna Carta (1215), and a substantial fishery existed on the river until the early 19th Century."
But, they added: "The industrial revolution and urbanisation of London led to increased levels of pollution in the river and the last record of a Thames salmon was made in 1833."
Since the late 1970s, there have been a number of attempts to re-introduce the species at a considerable cost. A 2001 paper estimated that £3m had been spent on efforts to establish a population in the Thames.
"Perhaps our paper shows that if your yardstick for showing recovery is the presence of Atlantic salmon then the best way to ensure that is to make sure that you get the environment right and just not dumping in a lot of exogenous fish," Dr Stevens suggested.
He explained that the presence of salmon in a river was important because the fish was an "indicator species".
"If you have large, healthy fish running to sea, and then you have a proportion of those coming back as even larger healthy adults that are ready to spawn and mate, you know that things are working well within that river.
"We then know that we have good gravels, good oxygenation within the water, a good range of [food sources], clean water and a good river flow.
"Everything within the system's pyramid has to be right in order for the apex predator to be present."
When asked whether the findings suggested that conservation funds and efforts would be better invested in habitat restoration than focusing on a single species, he said it was probably true when it came to salmon habitats.
"It is about getting the river system right then the fish will do the rest, really," he said.
"Salmon are good at homing, but the good news is that they are not perfect and a percentage will stray.
"That straying means that the species as a whole has the opportunity to explore new avenues when they become available."