It’s Tuesday night March 10th at Angel Mounds State Historic Site near Evansville.
State Herpetologist Nathan Engbrecht and Assistant Jason Mirtl have trekked through the dark across the sodden grass to listen for the mating calls of crawfish frogs.
Small glistening amphibians are out in the grass, enjoying the rain, but not the intrusion of headlamps in the dark.
“So all those ones we've been seeing were leopard frogs,” Endbrecht said. “(It) seems just the warmth and humidity, a lot of these leopard frogs seem to be loving it.”
They’ve just seen lots of crawfish and frogs, but no crawfish frogs.
The DNR is in the midst of a program to bring the crawfish frog back to this site. Earlier in the day, they deposited clumps of crawfish frog eggs from a donor location to help grow the population here.
They’re out, hoping to hear survivors from the last two years of egg deposits, calling out along with the leopard frogs and chorus frogs.
Engbrecht describes the crawfish frog’s call as a “loud snore.”
“It's almost comical,” he said. “It actually has tremendous carrying power on a good night with good weather, you can hear them from over a mile away.”
But after about an hour of waiting, they heard none. It could be that they’ll call on another night.
But one concern is that the historic flooding from 2025 could have washed frogs away, and made tadpoles vulnerable to predators.
“If that did, it could have certainly set back last year's egg mass translocation, and perhaps even the previous year, if it flooded frogs out of the burrows and washed them away. We just don't know for sure.”
They might know more later in the season. Something new the DNR is trying, is a digital recorder set up near the frog’s mating pool. They'll use software to automatically scan the audio for the crawfish frog call.
“And so you can save a ton of time doing it that way, create this call recognizer, this digital call recognizer, and scan like a four or eight hour recording in like 30 minutes, and then find out if you've got the calls on there or not,” he said. “We could potentially confirm if they're even laying eggs or not. And that, again, that would be another huge benchmark for us in our recovery project out here.”
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Angel Mounds is the first site the DNR has attempted to reestablish, with eggs deposited initially in 2024, and 2025. They’ve just begun the second location in Pike County at Sugar Ridge Fish and Wildlife Area.
Wetlands were constructed last summer; the pair laid 10 crawfish frog egg masses into the ponds the same day. “We're also trying to establish those frogs there using pretty much the same technique that we've been using down here as well.”
They could be calling in 2028 or 2029.
On this night Engbrecht and Mirtl did locate several crawfish. This is useful to their new study on what crawfish species live near crawfish frogs.
The frogs make a home in the crawfish’s burrow. They ended up grabbing a few to have identified later.
Most importantly, they’re hoping some of the frogs did survive the flood and might show up in recordings.
“It takes these frogs two to three years before they reach the age where they would start calling, and so it may be that some of them just haven't matured yet,” Engbrecht said. “So all hope is not lost if we don't hear them tonight.”