My family just returned from a quintessential summer road trip to Yellowstone National Park.
While there, I saw something that will stay with me forever. We hiked to a small mountain lake surrounded by fields of wildflowers and views of mountains. On a small stream that comes into the lake, the native cutthroat trout that live in the lake were moving upriver to spawn.
I had never actually seen this spectacle before, but the sight of hundreds of brightly marked fish, each more than a foot long, making their way diligently upstream was a true wonder of nature. We saw places
where almost no water flowed and thought surely they haven’t made it beyond these points. But a few pools ahead we would see more of these determined fish, and, occasionally, we could see them throwing their bodies over the rocky mounds.
Trout are anadromous, which means they naturally want to ascend into rivers from the sea to spawn. Even though most rivers are landlocked here in the U.S., the fish still cannot fight the instinct to move to the upper sections of rivers, close to the place they were born, to lay their eggs.
Before spawning, trout increase their intake of food. They do this to provide the energy needed for the migration to their upstream spawning areas, and also to produce the eggs and sperm needed for the spawn.
The female carefully selects an area to make a redd, or a bed, to lay her eggs.
The female uses the motion of her swimming body as well as the sideways movement of her tail to form a depression in the gravel. This motion results in removing the sand and silt out of the gravel where the eggs will be deposited.
While this is occurring, one or more males will compete for the right to spawn with the female. It is not uncommon to see several males fighting for one female. The female releases her eggs, and the male releases his milt (sperm) in the water to fertilize the eggs as they are released. Depending on her size, the female will release 4,000 to 14,000 eggs.
Trout spawn at different times, depending on the species and the elevation. Will Blanchard, owner of Animas Valley Anglers, says the rainbow and cutthroat trout around here spawn in March, April and May, while the brown and brook trout wait until September.
The eggs face many hazards, including damage by the gravel, too much or not enough current or simply not having the sperm find them. Many of the eggs will be eaten by other fish. Those that survive develop from egg into fry in the rivers, tributaries or sandbars where they were deposited by their parents. As fry, the developing trout must learn to survive without the protection of shelter and must rely on themselves for food.
From the fry stage, which generally lasts about a year, trout develop into parr. As parr, the trout remain in fresh water and accrue parr bars, which are vertical marks that appear on both sides of the trout. As the parr develop into smolt, they leave the streams and rivers where they were born to mature in more rugged water and to prepare for spawning. Then the cycle begins again, with trout moving back toward the place of their birth.
Regardless of the science around spawning, the site of these trout doing what has been programmed into their DNA, regardless of the obstacles, is a sight that reinforces the power of nature. The beauty and cruelty of this migration is an amazing thing to witness.