Crawfish Frogs Reintroduced at Angel Mounds

Historic site collaborates with Indiana DNR on conservation efforts to restore state-endangered species to their native habitat.

A colony of state-endangered crawfish frogs has successfully been reintroduced at Angel Mounds State Historic Site in Evansville, marking the first time in nearly 40 years the species has been seen at the site.

Starting in 2022, the multi-year conservation effort was a collaboration between the Indiana Department of Natural Resources Division of Fish & Wildlife and Angel Mounds.

"We are a multi-faceted site. It's about the culture, history, and natural and environmental components that we can tap into, like this," said Mike Linderman, site manager for Angel Mounds and southwest regional director for the Indiana State Museum and Historic Sites. "We're helping an endangered species return to its natural habitat where it once thrived, that's pretty exciting."

With their spot pattern and loud croak, crawfish frogs are prairie-dwelling amphibians that spend much of their lives underground. The adult frogs, which can grow the size of a fist, rely on quality grassland habitat with high densities of crayfish burrows, which lends to their name. After sheltering in those burrows during the extreme weather months, the frogs emerge in the spring and relocate to nearby ponds to breed, leading to a chorus of croaking that can be heard more than a mile away.

A large population of crawfish frogs once thrived at Angel Mounds, potentially as far back as when Native Americans inhabited the site, until the mid-to late-1980s when the amphibians vanished suddenly and mysteriously, possibly triggered by an extended drought. Before the recovery project, the number of crawfish frogs in Evansville had dropped to zero, and the main threat to the species has been habitat loss due to land development.

"Indiana only has two robust crawfish frog populations remaining in the state," said DNR Herpetologist Nate Engbrecht. "The recovery effort at Angel Mounds is somewhat unique for being a historic restoration project - a biological one - that aims to recover a historic component of the site that has been lost."

A habitat suitability study conducted by DNR determined Angel Mounds has the ideal environmental conditions to support crawfish frogs. In particular, the analysis identified several small ponds in the northwest corner of the site that fill with water after each winter but dry out during the summer. The ponds were formed centuries ago from "borrow pits," or large holes left after Native Americans built the mud wall at Angel Mounds.

In March, eight crawfish frog egg masses were collected from a large population in Greene County and relocated to the pond at Angel Mounds, where they were placed inside cages to protect the eggs as they developed. Engbrecht said the translocation process played out exactly as hoped, and within weeks, thousands of tadpoles filled the ponds. Engbrecht observed that the tadpoles began growing and developing legs, leading up to metamorphosis. Finally, following decades of absence, dozens of young spotted frogs began hopping by early July.

Crawfish frogs take two to three years to reach sexual maturity, meaning that the frogs that were moved to Angel Mounds this spring will not return to the pond from where they hatched to mate until 2026. In the meantime, the DNR plans to move additional egg masses to supplement the local population during the spring of 2025.

The Angel Mounds recovery project has also been supported by the Indianapolis Zoo, which provided grant funding for the habitat study, as well as Evansville resident and retired herpetologist Mike Lodato, who documented the Angel Mounds crawfish frog population crash in the 1980s.

More information about DNR's work with crawfish frogs, including the ongoing project at Angel Mounds, is available through its website.