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Most animals, often including humans, can have a kid and then have more kids, if the whole parenthood thing doesn’t put them off of the idea of doing it more than once. This is called iteroparity.

But not all animals are like this. Some creatures use a strategy called semelparity. They reproduce one time, then die immediately afterwards.

Examples of semelparous animals include gypsy moths, mayflies, and marsupial mice. But the most famous species are salmon. The journey these fish make upstream is one of the most hardcore things in the animal kingdom.

And it almost always ends with death. Except, some species manage to survive against the odds. A lucky few live to spawn again.

And learning more about how they do it could teach us a lot about reproduction as a whole, and possibly help save endangered fish. Now, when it comes to perilous expeditions,. Pacific salmon and Atlantic salmon are very different.

Pacific salmon are an example of what most salmon are like. They’re famous for being delicious, and for willingly embarking on a voyage of death shortly after reaching sexual maturity. They’re born in rivers, reared on floodplains, then gradually make their way out to sea.

But once they get there, they aren’t content to just hang out by the beach and enjoy coastal living. They eventually decide it’s time to head back up the river, settle down, and raise a family. Only for salmon, "settling down" means settling down eternally; as in, sleeping with the fishes.

The reason Pacific salmon don’t survive spawning is partly because their bodies just don’t last long after they reach maturity. But it’s also partly because the journey literally kills them. Some of these determined fish travel more than 3000 kilometers upstream to their spawning grounds.

And even when they manage to avoid bears, humans, and a myriad of other dangers, they may die from exhaustion. The ones that make it aren’t usually in good shape when they arrive, either. Along the way, they use up so much energy that they have to resort to some pretty extreme strategies, including absorbing their own bones so they won’t run out of calcium.

They also stop eating. That way, they can put eggs or sperm where their now-disintegrated stomachs used to be. So, by the time they arrive at their spawning grounds, their fat stores are used up, and their muscles are wasting away.

It seems awful to us, but it’s what salmon do. Unless, that is, they want to mess with our understanding of the natural order. Like some Atlantic salmon.

Unlike Pacific salmon, Atlantic salmon don’t always die after they spawn:. Some live to reproduce again, and not just one more time, but as many as seven times. They’re iteroparous like humans.

And when they’ve survived the first spawning, they’re called kelts. Most are female, and they’re super tough, because after all the deprivation and exhausting travel and predator-dodging they're all like, "That was fun! Let's do this again next year!" And they’re not just the occasional oddity, either:.

In Norway, up to 20% of female salmon might be multi-year spawners. So, for these fish, survival isn’t an accident. And while there’s a lot we don’t know about kelts, they do seem to be really important for salmon populations.

Repeat spawners have an extra year of growth, so they’re bigger. And bigger fish are better at producing offspring. Like, in Norway, 20% of females are respawners, but they produce 27% of the eggs.

Part of this seems to be that respawners build better redds, which are the carefully-constructed piles of gravel that they lay their eggs in. Larger females can build deeper nests from heavier gravel, which will better protect their eggs from predators and from the dangers of a fast-moving current. But also, the kelts lay more eggs, and their eggs are larger than those produced by first-time spawners.

It’s not clear why, but it does help boost populations of Atlantic salmon, which is especially important now that they’re threatened by pollution and other changes to their environment. Like, during years when not so many first-time spawners make it upstream, up to 60% of the eggs present on the spawning grounds might belong to kelts. At this point, though, there are still more questions than answers about these fish.

Like, we’re not exactly sure what makes them different from one-time spawners, or how they survive spawning season so well. It may have something to do with their metabolisms, or maybe their genetics? Like, we know kelts have a gene variant that makes them mature earlier.

And for some reason, salmon with this variant can be more than twice as likely to survive and respawn. Beyond that, we’re also not sure why this happens in Atlantic salmon but not in Pacific salmon, but figuring out why could teach us a lot. We know that the two species diverged several million years ago, so they’ve had a lot of time to develop differences in their reproductive strategies.

And now, studying the differences between the environment and genetics of these groups could help us understand why they did. There’s a lot to learn here! And of course, knowing more about these die-hard super-spawners could help us conserve critically endangered salmon populations, too, groups that may depend on these older, wiser fish when the going gets tough.

So, while studying kelts can be fun just for the wonder of it, they might also help us answer some big questions. Now, salmon migrations are pretty extreme, but some animals have migrations that are even more impressive. Like certain geese, which fly over mountains.

You can learn about them after this. And as always, thanks for watching this episode of SciShow. [♪ OUTRO].