A “kelt” is a salmon that survives spawning. Again and again.

Hakai Magazine looks at the few mighty salmon who survive the rigors of the spawning run year after year. Where most fish die after (or during) their first trip, what turns some tough salmon into upstream survivors?:

In most parts of the world, the fact that some salmon can be repeat spawners in this way is not very well known. But in the north of Norway, local communities have long understood that kelts are different, says Elina Halttunen, a salmon specialist at Norway’s University of Tromsø.

Locals know how to recognize kelts—they are the unusually large but tatty fish that are often seen in the rivers in early spring.

Having used so much energy to swim upriver, the fish are hungry, and easily caught on a lure.

“That’s how we could do the study—we could catch them by fishing,” explains Halttunen.

The surprising thing that Halttunen and her colleagues discovered about kelts was just how many of them there were in this part of Norway. And with each spawning, survival only becomes more impressive. In general, only 30 percent of Atlantic salmon survive to spawn again. Kelts have already beaten the odds once—with every subsequent journey upriver, they beat the odds again. And yet, female kelts punch above their weight in terms of their egg deposits.

While about 20 percent of the female population in a river might consist of kelts, those fish have been found to contribute nearly 30 percent of all the eggs, on average.

Repeat spawners are clearly capable of strongly influencing the next generation. “They are kind of an insurance policy of the population,” says Halttunen. “They are the buffer in bad times.”

But semelparous salmon, which only spawn once and then die, are still the norm. So what makes a repeat spawner?

Tutku Aykanat, a fish physiologist at the University of Helsinki in Finland, and his colleagues recently published a paper in which they describe a key genetic difference between semelparous and iteroparous salmon. They found that fish whose genomes had a characteristic section around a gene called vgII3 were more than twice as likely to become repeat spawners.

The team discovered this by examining DNA from thousands of archived salmon scales. Aykanat says that the gene isn’t the only thing that determines iteroparity. There must be environmental factors, too, but exactly what they are remains unknown. And there’s another mystery. Repeat spawning has evolved in Atlantic salmon but not, for example, in Pacific salmon.

Whatever makes the kelts tick, they are needed, says Glenn Crossin, a biologist at Dalhousie University in Halifax, Nova Scotia. He has studied Atlantic salmon in eastern Canada where the species is struggling. Years of acid rain has altered the rivers, causing fewer eggs to hatch into healthy fish. In regions with declining populations, kelts, especially the fecund females, could be extremely beneficial. “You really want these iteroparous individuals,” says Crossin.

There are quite a few links to bits of original research in the original article, so if you want to learn more, go and read ’em.

[via Nature Briefing]