Altantic salmon in detail Image caption The study looks to distangle the factors that weaken the strong homing instinct of salmon 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)

The distinct genetic characteristics of salmon populations in Spain are being lost as a result of climate change and human interference, a study has warned.

A team of UK and Spanish researchers say disrupting the species' migratory behaviour and strong homing instinct could have long-term consequences.

They added that they were working on ways to "disentangle" the individual threats affecting salmon populations.

The findings appear in the journal Global Change Biology.

The team's study focused on the changes recorded over a 20-year period to Atlantic salmon populations in Asturias, Spain - considered to be the "vulnerable" southern limit of the species' natural range.

"Salmon develop quite distinct population structures because of their ability to home to their natal rivers," explained co-author Jamie Stevens, from the University of Exeter's School of Biosciences.

"If you have such a defined system, they will quite quickly develop genetic profiles that become definitive to a particular river system."

He said the unique characteristics meant that the fish adapted to the conditions found within a particular river.

"There is a whole bunch of things: river chemistry, ability of the fish to withstand things like temperature, behavioural factors like run time to the sea and return time to spawning grounds," Dr Stevens told BBC News.

"The reason why we do not want those structures broken down is because we know that those local populations have a range of adaptations that can give the fish an advantage within that river."

'Straying fish'

The team analysed 924 tissue samples taken from adult fish returning to five rivers in Asturias between 1988 and 2007. They found that the results were consistent with high levels of mixing between local populations of salmon.

Dr Stevens said the study identified two distinct periods. Until 1992, there were a lot of "foreign" fish being introduced to the river systems. After 1992, this practice was halted but there was still a high number of "straying fish".

"Historically, people who like fishing like to have big fish coming up their rivers, so there has been human movement of fish, such as from highly productive rivers in Scotland to other areas," he said.

"Introduced fish would, typically, be chosen for larger sizes and faster growth rates but they may have had very poor survival rates at sea or poor at returning to the river or spawning.

"That stopped in 1992, but we still had problems. As our paper shows, there is still not a big recovery towards the genetic differences that is a signal of healthy populations.

"So as the impact from the movement of fish by humans is subsiding, we have got other things coming into play that are also causing disruptions."

Previous studies had suggested that increased water temperature was linked to an increase in fish straying between rivers and a breakdown of population structures.

"Increased water temperature appears to disrupt the fidelity of salmon returning to their natal rivers," Dr Stevens added.

In their paper, the researchers from the universities of Exeter, UK, and Oviedo, Spain attempted to untangle how the different factors were undermining the salmon population structures of the five rivers.

They suggested that while the impact of the introduction of foreign fish was decreasing over time, the influence of changes to water temperature was becoming increasingly important.

However, they said that more data was required to get a clearer picture.

"Without many additional studies, it is not possible to determine the exact moment or life stage when the population structuring was eroded," they wrote.

"Long datasets of well-monitored populations could serve for this purpose."

They added: "The ability to disentangle the effects of climatic changes and anthropogenic factors (fisheries management practices) is essential for effective long-term conservation of this iconic species."

Dr Stevens also explained that salmon was often used as an indicator for the state of rivers: "Monitoring a fish that is a top predator gives you a really good feel for the overall health of river systems that you might want to manage."