Why do Salmon Swim Upstream? (and other cool stuff)

Fish are weird. We’ve got smart fish like anglerfish, who have developed bioluminescent light to trick prey into basically coming right into their mouths, archerfish who squirt water with seemingly perfect aim to knock bugs into the water for a snack, the sheepshead fish that has teeth eerily resembling a human’s so that they can eat pretty much whatever they want, and then there’s the parrotfish — which covers itself in mucus when it sleeps to help itself heal and protect itself, and it poops sand.

But today, I wanted to talk a little about a fish that is a little less exotic than those guys. Let’s talk about something that may find its way onto your dinner plate: the salmon.

First, let’s talk about their awesome pink color. Salmon aren’t naturally pink, they’re white. As a matter of fact, if you eat farm-raised salmon, they probably put coloring in their food to turn them that way, not because it makes them taste better but because you, as the consumer, expect it (it’s true, they even have something called a SalmoFan to help salmon farmers determine the most profitable colors). Salmon turn pink for the same reason flamingos turn pink: it is a natural reaction to eating krill and shrimp. There’s a compound in krill and shrimp called astaxanthin, and when you eat enough of it, it turns you pink. Alaskan sockeye salmon, because they feed off the huge population of krill in the Bering Sea, are naturally the most pink of all salmon species.

OK, now that we’ve talked about appearance, let’s talk about behavior. Salmon are also kind of weird. You probably already know that they swim upstream, and that seems kind of dumb. For one thing, it’s hard to do. It takes an awful lot of energy. And for another, they’re constantly jumping through the air and then being caught and eaten by bears. Seems like a waste, doesn’t it? So why do they do it?

Oddly, it actually helps them (and some other types of fish, like rainbow trout) survive.

First, these are fairly large fish. In order to keep the species alive, they can’t compete with the ones who are trying to grow up for food. Therefore, the younger fish swim downstream and follow the food. The ones who are big and mature enough to reproduce swim the other way to give the youngsters a chance to survive. Aww!

There’s another reason, too: upstream waters are calmer, which means the eggs will have a better chance of being fertilized and hatching. Downstream, the eggs could be eaten or even swept away before fertilization.Finally, once the eggs hatch and grow out of their yolk-feeding larval stage, their diet consists mostly of insects. And where do insects congregate? Slower water, like that found upstream! Doesn’t seem like such a dumb idea anymore, does it?

So how do the fish know to go upstream and where to stop? Believe it or not, they actually can smell it. They can smell the place where they were born and that’s where they travel back to. Sadly, all that upstream swimming and spawning is exhausting, and it kills the fish within about a week after they’ve done everything they need to. But again, being upstream is a blessing here. Once the mature fish die and start to decay, because their bodies aren’t swept away or eaten by something else, they provide a nutrient-rich environment for their babies to grow. Kind of gross but also very helpful!

The next time you’re eating salmon, think back on all this and marvel about how smart they really are!