SALMON help their offspring by making the ultimate sacrifice on their spawning grounds, according to a new study by Scottish ecologists.

According to scientists at Glasgow University, spawning salmon that die after migrating home are actually doing their young a favour.

By making the ultimate sacrifice, they allow their decaying bodies to fertilise the burn and create an environment which favours the growth of the young fish and maintains their genetic diversity.

Spawning salmon migrate hundreds of miles across the ocean back to the burns in which they were born, but then often die just after spawning.

Burns that lack dead adult salmon have fewer insects – less “fish food” – so that the surviving salmon fry are smaller and belong to fewer families. The resulting loss of genetic diversity could make these salmon populations more vulnerable to extinction.

The research, led by scientists from the university’s Institute of Biodiversity, Animal Health and Comparative Medicine, came from a detailed study of salmon burns in northern Scotland.

The researchers tested what happened when they manipulated levels of nutrients that would come from decaying salmon carcasses during the winter spawning season.

Five burns received the normal level of nutrients from carcasses, while five other burns only had low nutrient levels – but all ten watercourses got the same number of salmon eggs from the same group of families.

The team of researchers, which included fishery biologists from the local Fisheries Trust as well as scientists from Marine Scotland and the US Forest Service, then returned five months later to look at the impact of these parental nutrients.

Evolutionary ecologist Dr Sonya Auer said: “Our surveys, backed up by DNA fingerprinting, show that fewer families of young salmon survived in the streams that lacked parent carcasses, and those that were able to survive had higher maximum rates of metabolism, which indicates that the environment favoured a particularly competitive and aggressive kind of fish.

“But we didn’t see that same narrowing of the gene pool in the streams with more nutrients, so a by-product of the parents’ death is that they maintain the genetic diversity of the next generation.”

The university’s Professor Neil Metcalfe added: “The longer-term consequences of these parental nutrients are bound to be complex, but these findings indicate that salmon shape their environment in a way that alters their very own destiny.

“If populations decline so that there are fewer dead adults to fertilise the spawning grounds it could reduce the viability of the remaining fish.

the spawning grounds it could reduce the viability of the remaining fish.”

Salmon hatch in fresh water but live most of their adult lives at sea.

They make an incredible upstream journey to spawn in the same places that they were hatched.

The fish use their sense of smell to navigate their way back up river to their home stream.

The study – Nutrients from salmon parents alter selection pressures on their offspring – is published in the scientific journal Ecology Letters.