Study shows crayfish can be harmful

Water district has concerns
PINCERS MOVEMENT—Dragonflies help humans by eating diseaseladen mosquitoes. But the red swamp crayfish, above, eats the dragonfly, which helps puts the mosquito back in business. Acornfile photo

PINCERS MOVEMENT—Dragonflies help humans by eating diseaseladen mosquitoes. But the red swamp crayfish, above, eats the dragonfly, which helps puts the mosquito back in business. Acornfile photo

A new study to be published in the scientific journal “Conservation Biology” indicates that the presence of invasive crayfish could contribute to the spread of mosquito-borne diseases in the Malibu Creek watershed and also reduce its biological diversity.

Research about the invasive species found in the watershed and the potential impact they might have on ecosystem integrity was conducted by the Las Virgenes Municipal Water and the Triunfo Sanitation districts in collaboration with the Mountains Restoration Trust and researchers from UCLA, the U.S. Geological Survey and Pepperdine University.

The study investigated how the presence of the red swamp crayfish (Procambarus clarkii) might impact the ecosystem of local streams. The research was conducted on creeks and streams across the Santa Monica Mountains to determine the health of the waterways with crayfish and those without them.

Early results indicated that the presence of invasive crayfish is likely the single largest factor affecting the vitality of local streams while also negatively impacting the relative abundance of juvenile dragonflies.

Mosquitoes are notorious vectors of diseases such as malaria, Zika and West Nile virus. In local streams, mosquito populations are kept in check by juvenile dragonflies, which rapaciously consume juvenile mosquitoes. But invasive crayfish disrupt that predator-prey relationship by targeting juvenile dragonflies and leaving the young mosquitoes behind. As a result, the streams that have invasive crayfish have a higher abundance of mosquitoes.

“A lot of people don’t know this but before dragonflies are flying around and beautiful, they actually are these voracious predators in streams and ponds,” said Gary Bucciarelli, a conservation biologist with UCLA’s La Kretz Center for California Conservation Science and the paper’s lead author. “They do a great job of preying on other invertebrates in the streams we work on.”

Based on the findings, researchers proceeded with controlled laboratory experiments where they witnessed adult crayfish and juvenile dragonflies hunting mosquito larvae separately and together.

Next, tanks were set up with only adult crayfish, tanks with only juvenile dragonflies and tanks that had both. The results demonstrated that juvenile dragonflies alone wiped out mosquito larvae, while crayfish were much less efficient. The tanks that contained both showed that the crayfish disrupted the predator-prey relationship so severely that there were as many juvenile mosquitoes as there were in the tanks with only crayfish.

“This study is part of a longterm effort to understand the invasive crayfish and eliminate or reduce their numbers as much as possible. The original intent was to protect endangered and threatened species, but today’s findings reveal a human impact that hasn’t previously been examined,” said Robert Fisher, a U.S. Geological Survey research biologist and coauthor of the study.

“Drought appears to be making the problem worse,” Fisher said.

“The red swamp crayfish comes from places in the Southeast United States that are hot or at least warm all year long. As monthly record-setting temperatures have become the norm the last few years, local habitat becomes more suitable to these pests,” Fisher said.

Dave Pedersen, general manager of the Las Virgenes Municipal Water District, said Las Virgenes and Triunfo are committed to ensuring that Malibu Creek’s ecosystem is diverse and healthy.

“This study reveals how invasive species can have a significant impact on the balance of aquatic macro-invertebrate populations while potentially promoting the increase of mosquito populations, which could have public health risks,” Pedersen said.

Acorn staff report