Survival of the fishiest: Roaring Fork Conservancy releases trout into Roaring Fork River

Six “Halloween baby” trout were released into the Roaring Fork River on Wednesday.

The trout, born on Halloween in 2023, were raised at the Roaring Fork Conservancy for almost a year before they were released into the Roaring Fork River in Basalt.

It was survival of the fittest for these six fish, as they were the only ones to survive out of the 200 other eggs they came from.

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Roaring Fork Conservancy Executive Director Rick Lofaro said that this is normal. Two hundred eggs will usually yield six to 10 fish.

“That gives you an idea of nature,” he said. “Trout, and salmon especially, are laying thousands of eggs with a very small percentage of surviving.”

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Raising the trout from eggs to fingerling size, which is about three to four inches long, is made possible through a partnership program with Colorado Trout Unlimited, known as Trout in the Classroom.

For the Trout in the Classroom program, trout are raised by educators and students. The program is sitting at a 30% survival rate for trout right now. For wildlife professionals, the success rate increases to 80%.

These professionals have more access to hatchery systems that aid in the higher survival rate of the trout, like a flow-through system. The classrooms typically have a closed-system hatchery.

“It’s a little more difficult to raise these finicky, cold-water-loving fishies,” Colorado Trout Unlimited Youth Education Coordinator Natalie Flowers said.

Louisville Middle School near Boulder raised 175 out of the 200 trout eggs, which Flowers called a “phenomenal success rate.”

When Colorado Trout Unlimited and Roaring Fork Conservancy staff first got out to the Roaring Fork River at around 11:30 a.m., the water measured 50 degrees. Staff were looking for a five-degree difference between the river and what the water the trout were formally living in measured at. The fish were living at around 52 degrees, and although this is within the five-degree difference, staff still “tempered” the fish, or took some of the water from the river and put it in the tub the fish were being kept in.

Another reason to temper the fish is for differing pH balances. The difference in pH comes from several factors, including minerals found in the river. If the fish are placed into a new ecosystem, like the Roaring Fork River, and are shocked, whether from temperature or pH balance, it could lessen their chance of survival.

The reason humans do not receive this severe of a shock when doing polar plunges or swimming in alpine lakes or enjoying a sauna is because humans are warm-blooded, or endothermic. Possessing this trait allows a human or other wam-blooded animals to regulate temperature and warm or cool them.

Cold-blooded, or ectothermic, animals, like fish, do not have this same trait. These animals are affected by their surroundings, so they cannot regulate body temperature.

Trout can survive at a range of temperatures, but they are most comfortable at around 52 degrees. Anywhere between 48 to 62 degrees is a range that trout can survive in. Temperatures over 65 degrees is not suitable for trout, but rainbow trout can stand a little bit of a higher temperature.

“In many aspects, trout are as fragile as a newborn baby,” Lofaro said. “But in other aspects, they’re tough as a bull.”

Staff also caught aquatic macro-invertebrates, or water insects, with nets in the river and by flipping over river rocks. After the trout have spent almost the first year of their lives eating hatchery food, or pellets from Colorado Parks and Wildlife, staff wanted to know what the trout would be taking a liking to once released.

These water insects hide under rocks because trout and other predators, like birds, like to eat them.

“I suggest also trying them yourself,” Lofaro said. “They’re not quite as good as sushi, but they’re pretty good.”

The most common aquatic macro-invertebrates found in Roaring Fork Valley rivers are mayflies, stone flies, and caddisflies.

“We have great water quality here in the Roaring Fork Valley, so we always will see a nice, high variety of insects and high diversity, as well as high numbers,” Lofaro said.

Regan Mertz can be reached at 970-429-9153 or rmertz@aspentimes.com.