In April of this year, a project initiated in 2021 concluded, during which researchers sought methods to combat invasive crayfish species. Additionally, they aimed to increase public awareness of the threats posed by these non-native species. In Estonia, the invasive crayfish include the signal crayfish (Pacifastacus leniusculus), spiny-cheek crayfish (Faxonius limosus), marbled crayfish (Procambarus virginalis) and narrow-clawed crayfish (Pontastacus leptodactylus).

Katrin Kaldre, a junior professor of aquaculture biology at the University of Life Sciences, stated that the invasive species from America are primarily dangerous due to the plague they spread. "They are immune themselves, but the mortality rate among European crayfish species after infection is nearly 100 percent. The problem is becoming increasingly serious each year. We practically discover a new outbreak site every year," Kaldre told ERR.

Signal crayfish were first discovered in Estonia in 2008 in the Mustjõgi River in Harju County. According to Kaldre, they are now known to inhabit twenty different bodies of water. The popular marbled crayfish, kept as a pet due to its striking appearance, was first found in the wild in Estonia in 2017 in the discharge canal of the Baltic Power Plant. That same year, the first spiny-cheek crayfish was discovered at the mouth of the Pärnu River.

Last fall, the narrow-clawed crayfish was detected for the first time in Estonia, which, according to Katrin Kaldre, is of European origin and therefore does not spread the plague. However, this species is not native to Estonia and manages better than the native crayfish in poorer environmental conditions. Native crayfish, as is well known, inhabit only clean freshwater bodies.

"If these invasive species continue to spread in the habitats of the native crayfish, then after some time, the native crayfish will simply disappear from these water bodies. This has already happened in several bodies of water," acknowledged the aquaculture biologist. For example, the signal crayfish has almost completely displaced the native crayfish in Saaremaa's Riksu Stream.

Poison, electricity and eels

Katrin Kaldre and other researchers initially planned to test chemical control, essentially poisoning the water body. "Our project partner, the Norwegian Veterinary Institute, has experience in using chemicals to control signal crayfish. However, the Environmental Board did not grant permission for such an action. Potentially, it could have only been tested in a couple of water bodies anyway. Most of the signal crayfish locations are in flowing water bodies, where the water would simply carry the poison away," she noted.

Crayfish fishing is usually done with traps, but according to Kaldre, crayfish smaller than six centimeters do not enter the traps: "The problem with trap fishing is that smaller specimens remain in the water body, grow and reproduce." Therefore, the researchers also experimented with electrofishing. While effective for catching fish, it proved inefficient for crayfish. "Crayfish live under rocks at the bottom of water bodies, and the electric pulse simply does not reach them. This also proved to be an ineffective method," the scientist acknowledged.

Ultimately, the researchers primarily engaged in intensive trapping, during which they caught thousands of invasive crayfish. "In the Ropka Reservoir and Reo Quarry, we also experimented with stocking eels. Eels eat smaller crayfish, and the larger ones can be caught with traps. They could be a supportive additional control measure, but how well eels can control the crayfish population will only be assessable in years to come," said Kaldre.

Generally, scientists consider eels to be an effective biological control method. These fish feed from the bottom of water bodies and can reach crayfish in their burrows due to their streamlined body shape. Additionally, eels are primarily nocturnal, when crayfish are more active. "In water bodies stocked with eels, we will continue monitoring in the coming years to evaluate their effectiveness," the scientist summarized.

eDNA as a promising field of research

During the project, researchers at the University of Life Sciences also conducted environmental DNA (eDNA) studies. Every living organism leaves behind genetic material in its environment, and in aquatic life, this can be detected from water samples. "We developed a methodology for detecting invasive crayfish species and crayfish plague from water samples," explained Katrin Kaldre.

"Trapping is very time-consuming and labor-intensive, but knowing whether foreign species are present from a laboratory analysis of a water sample makes their detection considerably easier. One of our goals was to learn how to apply this methodology in Estonia. This research is still in its infancy, and we were able to get started on it in this project," she added.

In addition to other activities, participants in the project also conducted a public awareness campaign on invasive species. "We taught in schools and at festivals how to distinguish between different crayfish species. It's important to raise awareness that not all crayfish are the same; you cannot simply move them from one body of water to another. Such actions have consequences," emphasized the scientist.

Researchers from the University of Life Sciences encourage people walking near water bodies to photograph any crayfish they observe and report them. For example, environmental information can be relayed through the national information phone line 1247.

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