Why do salmon jump?
This is the question our guests most frequently ask about salmon as they watch fish pop out of the water around them. The answer is: no one knows. Some speculate salmon jump to loosen their eggs from the membrane encasing them, but males also jump, so this reasoning doesn’t work. Another explanation is they jump to catch flying insects, but their jumping behavior increases in frequency as they near their spawning stream at the same time their digestive system is shutting down and they stop feeding, so this explanation also is not valid.
I like to tell our guests the salmon they see jumping are teenagers. As a salmon prepares to spawn, its hormones rage and its body changes color and shape. These jumping fish are the equivalent of a human teenager, so they act like teenagers. While this explanation is always good for a laugh, there is no scientific evidence to support it.
My opinion, for what it’s worth (and not much, since there is also no scientific evidence to support this), is salmon have evolved to jump because jumping is beneficial to their survival. Salmon that have inherited the genetic characteristic to jump when they near their home stream are more likely than those who cannot jump to make it upstream and spawn. They then pass along this trait to jump to their offspring. Evolution has selected this jumping trait. Most salmon spawn in small streams and they must navigate shallow water, rapids, and sometimes even waterfalls. If they couldn’t jump, many salmon would never make it to their spawning grounds. Again, this explanation is only my opinion, but I believe it has some merit.
The correct answer to the question, “Why do salmon jump?” is: no one knows.
Do all salmon return to spawn in the same river or stream in which they were born?
No. A small percentage of salmon spawn somewhere other than where they were born. This behavior is called “straying,” and it is adaptive because it allows salmon to colonize streams that do not currently have a salmon population. It also allows salmon to spawn somewhere if the stream where they were born no longer exists. Pink salmon and chum salmon both often spawn close to the mouths of small streams. If these streams are diverted by winter storms, as often happens, the returning salmon will stray to a nearby stream.
When a salmon returns from the ocean, how does it find its spawning stream?
Salmon must navigate a long distance from the open ocean to their spawning stream. Evidence shows they use magnetic cues, the position of the sun, and day length to know when to begin their migration back to their natal stream and how to get there. Once salmon near and enter fresh water, scientists think they use their sense of smell to find not only their home stream or river but the specific tributary or area of the river where they were born. Juvenile salmon imprint on the unique chemical signatures of the waters where they were born and occupied as fry as well as on the waters they migrated through to get to the ocean. When they return to spawn, they follow this chemical smell back to where they were born.
How many salmon eggs hatch, develop, and return as adults to spawn?
Salmon have tough lives. From the egg stage, until they spawn and die, they are a food source for a wide variety of fish, birds, and mammals, including man. Biologists estimate 1 in 1000 eggs will develop, find their way to the ocean, swim back to their natal stream, and spawn. Depending on the species and the age of the fish when she spawns, a female salmon lays between 1000 and 6000 eggs, so the survival rate is not good.
I hope I’ve answered a few of your questions about the amazing Pacific salmon. Next week, I will tackle a big question. What is the difference between Atlantic and Pacific salmon?
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