This week is National Library Week! And there’s no better way to celebrate than taking a closer look at an animal that represents knowledge and wisdom.
Now I could do a ‘typical’ animal representing wisdom like owls, but what’s the fun in that?
Instead, let’s take a closer look at an animal you might not have considered particularly wise: the salmon.
Salmon are frequently associated with wisdom and knowledge in Irish, British, and Scottish folklore. In one story, the Salmon of Knowledge was a salmon who ate nine hazelnuts that had fallen into the Well of Wisdom; the first person to eat this salmon would gain all the world’s knowledge. In another Welsh tale, the Salmon of Llyn Llyw was the oldest and wisest of all the animals.
There are a few different species of salmon: Pacific salmon and Atlantic salmon. The major difference is (you guessed it) where they live – Pacific salmon live in the Pacific Ocean, while Atlantic salmon live in the Atlantic Ocean. There are also seven species of Pacific salmon and only one species of Atlantic salmon.
Since salmon are considered wise in the folklore of areas surrounded by the Atlantic Ocean, we’re going to focus today on Atlantic salmon.
Atlantic salmon (Salmo salar) are fish found on both sides of the northern Atlantic Ocean. They are the largest species in their genus, Salmo, which includes 40 species of trout. (I know I said we wouldn’t talk about Pacific salmon, but I wanted to give you one quick fact: Pacific salmon are part of an entirely different genus, Oncorhynchus).
Atlantic salmon spend the first few years of their lives in freshwater rivers. These rivers must be clear, chilly, and have a gravel bottom. This gravel bottom is where newly hatched salmon (‘alevins’) live until their yolk sac is absorbed.
Once alevins are about 65mm long, they are called ‘parr.’ Parr will continue to grow in the river until they reach 12 – 15cm long. At this point, they are called ‘smolts,’ and their organs begin to change to balance the salt levels in their body.
…and ocean fish…
Atlantic salmon are anadromous, meaning they spend part of their lives in freshwater and another part in saltwater. The chemical change that smolts go through prepares their body to move from freshwater rivers to the saltwater ocean. Most animals cannot survive in such a wide variety of salt levels – Atlantic salmon can because of the changes their body goes through as they move from freshwater to saltwater. For instance, their kidneys differ in how much urine they produce, and their gill cells change to better modulate salt in their blood.
Atlantic salmon go to the ocean to finish maturing. Compared to the freshwater river of their birth, the sea is much richer in nutrients and food sources. This lets Atlantic salmon grow much faster and larger. And they get big: Atlantic salmon can grow to be just under four feet long. This rapid growth is critical, as larger fish are less likely to get eaten by predators, and larger females can produce more eggs.
…and landlocked fish
Some populations of Atlantic salmon are completely landlocked, so unlike their river- and ocean-dwelling cousins, they have no way of migrating to the open sea. Instead, they move between their natal rivers and connected lakes.
How did the Atlantic salmon get trapped in a lake? It’s likely the result of glaciers. As the glaciers retreated at the end of the last ice age, they left in their wake inland lakes that connected to the ocean via rivers and streams. Atlantic salmon eventually made their way into these inland lakes. However, as water levels decreased, the connections between the lakes and the oceans disappeared, trapping the salmon in landlocked lakes forever.
The Great Migration
Okay, back to the ocean-dwelling salmon!
These Atlantic salmon spend one to two years at sea. Then, when they are big and have reached full maturity, they get ready to migrate back to freshwater. They’ll stop eating, and their bodies will change to prepare themselves for returning to freshwater.
This migration is no joke! Some Atlantic salmon end up completing a 2,000-nautical-mile round-trip voyage. And they don’t just migrate to a random river – Atlantic salmon will travel from the sea back to the exact river (and even the exact stream!) where they were born. We don’t know precisely how they do this. Atlantic salmon may be using magnetic fields while they are at sea to orient themselves. Once in the water, they likely use their sense of smell to find their birthplace. Their sense of smell is 1000 times greater than a dog’s!
Once they reach the area where they were born, they are ready to spawn. Females will dig nests called ‘redds’ in the gravel of the riverbed. Once the redd is constructed, the female will deposit her eggs in it, and a male will fertilize the eggs. The female then covers the eggs with gravel, where they will stay until they hatch and the alevins absorb their yolk sac.
An Atlantic salmon typically lays 700-800 eggs per pound of her body weight. That’s a lot of eggs! But laying so many eggs is how Atlantic salmon keep their population steady. After all, Atlantic salmon are an important food source for many other animals. The eggs, alevins, parr, and smolts are food for river creatures like other fish and otters. Until they grow in size, young salmon are popular food for other ocean-dwelling animals. And during the arduous migration from sea to river, many don’t have the energy to return to the spawning point. Out of every 8,000 eggs laid, only two will survive to spawn themselves on average.
What do the adults do after spawning? Not much – the journey to the spawn point and the actual act of spawning takes a lot of energy. In fact, many Atlantic salmon spawn just once before dying. However, some can survive and spawn again in a year or two; a few rare salmon survive to spawn three or more times!
It’s thought that the word ‘salmon’ comes from the Latin word ‘salmo’ or ‘salire,’ which means ‘to leap.’
And leap they do.
Remember, Atlantic salmon must migrate upriver from the sea to reach their spawning grounds. It takes a lot of energy to be swimming against the current! And sometimes, they reach waterfalls. Since Atlantic salmon don’t have hands and can’t climb around waterfalls, they do the next best thing: leap over them. Salmon have been recorded leaping over 15 feet (4.5 meters) tall waterfalls!
But even with their excellent leaping abilities, Atlantic salmon sometimes need help navigating a river. This is especially true when humans have changed the river by adding dams. People will construct fish ladders to help the Atlantic salmon reach their spawning grounds. These ladders help the salmon get over or around barriers that might otherwise impede their progress.
After all, the more Atlantic salmon that can make it to spawn and lay eggs, the healthier their population will be.