Crayfish invasion in Niwot?

If you tread on Niwot trails regularly, this year you're likely to have frequently sighted some fun small critters – crayfish and Woodhouse's toads.

The word "fish" in "crayfish" speaks volumes as to why they drew the most attention when whatever body of water they crawled out from wasn't even in sight. Media reports of the "invasive Rusty Crayfish" in Colorado hit the headlines just as the numbers of Niwot crayfish sightings hit a high.

Boulder County Senior Wildlife Biologist Mac Kobza explained that this year's record rainfall is why the crustacean has been so noticeable this year. Crayfish migrate from one waterway to another when it's wet, and they especially take a hike when there's a high density of crayfish in one area.

"They fight like mad with each other, "Kobza said. "They're not kind to each other and they try to get away from one another."

In addition to decaying plants and eggs, crayfish eat invertebrates like aquatic insects, snails, and worms. "We've seen a big explosion of invertebrates this year, especially around our waterways," Kobza said.

Anywhere from 20 to 50 of the 5" critters were spotted recently on the trail behind Niwot High School. They are early morning commuters, so maybe it's the low light that makes them unwitting targets for bicyclists and pedestrians, explaining the noticeable amount of crayfish carnage on the hill.

How does a nonindigenous crayfish species come to live here, and are the ones in Niwot the infamous rusty crayfish that are being found elsewhere in Colorado?

Kobza said rusty crayfish escaping from bait buckets are the culprits. "It takes that and it takes some luck for the crayfish to find the right place to live over the winter and to find a mate, and to actually find a place to have their young and for the young to survive."

To discover what type of crayfish are strolling on Niwot trails, one-and-a-half crayfish (thanks to the carnage mentioned earlier) were snatched up and given to Boyd Wright, Colorado Parks and Wildlife Native Aquatic Species Biologist, for assistance.

"Our lead technician working on our new crayfish inventory project, and her supervisor both independently examined it," Wright wrote." We all agree that it is a northern crayfish, but would have liked to see more samples to rule out water nymph crayfish. Crayfish identification is challenging! Both species are widespread in Colorado where they are native to the east slope. The use of terrestrial habitats does not seem to be a significant portion of the life history for either species."

Boyd hypothesized that the extra moisture helped the crayfish colonize newly wetted habitats along Dry Creek, just north of where they were being observed.

Had they been rusty crayfish, they could have displaced native crayfish by eating their food or even physically pushing them out due to their aggressive traits and high breeding rates. They are also a threat to fish populations because they eat their eggs and eat what the fish eat. All of this can lower the diversity of species in general, which is never good for the environment.

Woodhouse's toads were another creature seen in good numbers this year. These toads are also semi-aquatic living along riparian areas. They have scores of young when the conditions are right – where there's plenty of moisture and where there are a lot of insects like mayflies, stoneflies, and caddisflies for them to forage on.

Seeing an abundance of amphibians such as crayfish and toads is a good indicator of environmental health, Kobza said. These creatures are the first to be lost when waterways have toxins in them, when water quality is poor or when the food web is slim. But, seeing a lot of these creatures is a great sign.

Crayfish hibernate when the temperatures drop. Usually around the first frost they start to burrow under the soil or under a boulder in the water keeping them from freezing while they go into a deep sleep.

When asked what to expect next year, Kobza said, given their high numbers this year, we can expect they will breed in abundance next year too. There will be plenty in the water, but we may not see them unless it's another wet year, in which case they are likely to take more walkabouts again.