NEW SCIENTIST - Daily news
21 July 2017
Baby salmon with ‘old’ DNA more likely to survive epic migration
By Aylin Woodward
There’s something fishy going on. Juvenile Atlantic salmon with shorter telomeres – normally considered a sign of poor health – have a higher chance of surviving the epic migration from their home river to the sea and back again.
Telomeres act as caps on the ends of chromosomes, preserving the DNA after cells divide. But the telomeres shorten with each division and eventually become so short the cells can’t divide any more. In humans, shortened telomeres are associated with cardiovascular diseases and cancer in adults, and are thought to reflect overall cell ageing and health.
No wonder Darryl McLennan at the University of Glasgow, UK, and his colleagues were puzzled by their results. In the spring of 2013, McLennan’s team tagged over 1800 juvenile salmon, or smolts, in the Blackwater river in northern Scotland just before they migrated to sea. The team also took a small fin tissue sample from each fish to measure the telomeres.
In the autumn of 2014 and 2015, when McLennan expected the salmon to return to the river to spawn, his team trapped the tagged fish and took a follow-up fin tissue sample to measure telomere length. Only 21 of the original salmon remained and the survivors were significantly more likely to have shorter telomeres than when they began their migration.
“When we started this project we hypothesised the juvenile salmon with shorter telomeres would have a reduced lifespan and found the complete opposite,” he says.
It’s an unexpected result, but Terry Burke at the University of Sheffield, UK, points out that the analysis ultimately relies on data from very few of the original salmon: only about 1 per cent made it back to spawn. He would like to see the study replicated before we can say with any confidence that young salmon with shorter telomeres outperform their peers carrying longer versions.
But Kjetil Hindar at the Norwegian Institute for Nature Research in Trondheim is not surprised by the dismally low survival rate. He says it’s the same return rate he sees in Norway these days. “Salmon survival at sea is much lower now than it was thirty years ago,” he says. “We had twice as many fish returning in the ’80s.”
A migrating salmon’s life isn’t easy. While it is one of the world’s most studied fish species, we know relatively little about what happens to salmon at sea. Ultimately, predation from coastal birds and larger marine fish, coupled with higher levels of fishing, mean that very few ever make it back to their freshwater birthplace.
Burke says it’s nice to see this kind of telomere work being done with fish – most often these studies examining life history are done with humans and birds. But he points out that there might be other reasons to explain why McLennan’s team found a result that runs contrary to popular wisdom. “We’re not observing these fish dying from illness, but mostly from predation or being caught at sea,” he says. “So there’s a different kind of selection operating here than on humans. The salmon aren’t living long enough to die of old age.”
McLennan’s has his own ideas about why fish with shorter telomeres seem to fare better. Salmon have to undergo physiological changes to prepare themselves for both the taxing migration and the challenge of moving from a freshwater to a marine environment – for example, altering their gills to deal with higher levels of salt. McLennan thinks that fish who invest more energy into preparing themselves for life at sea do so at the cost of maintaining their telomere length. What’s more, unlike humans, fish can repair their telomeres.
Whatever the ultimate outcome of the research, McLennan thinks the salmon are evidence that we need a better understanding of telomeres’ role as proxies for ageing and cellular health. “Telomere dynamics are not universal,” says McLennan. “You need to focus on the species you’re interested in because telomeres tell you different things depending on what species you’re looking at.”
Journal reference: Functional Ecology, DOI: 10.1111/1365-2435.12939