Moapa Dace Numbers Tick Up Once Again

By VERNON ROBISON

The Progress

There are still plenty of fish in the stream. In fact, more than have been seen in nearly three decades, according to biologists.

The annual summer count of the endangered Moapa dace, which took place on Aug. 10-11, tallied a total of 2,444 of the finger-sized fish currently living in the Warm Springs headwaters of the Muddy River.

“It was the highest count since 1994,” said Southern Nevada Water Authority (SNWA) biologist David Syzdek. “It was a 4 percent increase from the August 2020 count and an 85 percent increase over August 2019.”

The count brought together scientists from Southern Nevada Water Authority, US Fish and Wildlife and Nevada Department of Wildlife. The group also received support from Coyote Springs personnel who helped to gather and record the data during the two-day count.

The biologists donned wetsuits, masks and snorkels and plunged into the stream. Over the two days they methodically swam, crawled or waded through more than six miles of streams counting every fish they could see. A data-taker accompanied each snorkeler, clipboard in hand, recording the data and keeping careful notes about stream conditions.

On Wednesday, Aug. 11, NDOW biologist Kevin Guadalupe was found crawling his way up the Lower Pederson Stream at the Warm Springs Natural Area, a former ranch operation now owned by SNWA.
Guadalupe would surface every few seconds and call out the number of dace he had just observed. Right behind him, Michael Yetter, also from NDOW, was wading through the stream recording Guadalupe’s observations.

Though it appeared to be entirely natural, this segment of stream was actually an artificial channel designed and built by biologists in 2008 to re-create the ideal dace habitat. The new stream segment had replaced a failed irrigation ditch that had not been able to support the dace.

That morning, Guadalupe and Yetter counted 502 dace in that segment of the stream alone.
Syzdek explained that the Moapa dace are typically found in pockets of “slack water” that are immediately adjacent to faster-moving stream flows. When a food object; such as a small invertebrate or piece of algae; floats by, the dace darts into the fast water to grab the tasty morsel. Then it moves back to the slack water to await the next meal drifting by.

“That makes them fairly easy to count,” Syzdek said. “The snorkeler crawls up the stream and when a dace, or school of dace, are seen, they are counted. When the snorkeler crawls further up the stream the fish will swim around the snorkeler. Due to the current, and the narrow width of the streams, the dace can’t really pass the snorkeler and won’t be double-counted.”

The dace are counted twice each year. Once in August and again in February. “August numbers are generally higher than February due to recently hatched larval fish,” Syzdek explained.

This month, the scientists counted 1,836 adult dace, 484 juveniles and 124 larvae.
“That indicates that we are likely to have good numbers for our next count in February 2022,” Syzdek said.

Perhaps the best news for the scientists is that the dace seem to be expanding their habitat. They are being found in more reaches of the stream and in greater numbers. And most importantly, the fish are beginning to use the main stem of the river as a more permanent habitat.

For many years, the dace had not been seen in the main stem. Early on, it was a perilous place because it was frequented by tilapia, an invasive predator fish. Then between 1998 and 2014, a fish barrier was put in place to keep the tilapia out of the tributaries where the dace had retreated.

Eventually, scientists were able to eradicate the non-native tilapia from the main stem and the fish barriers were removed. That allowed the dace to return to the main stem of the river. But it has taken a long time for the fish to find their way back.

“While we knew that dace would occasionally move between tributaries using the river, they were not staying in this habitat and we were unsure why,” Syzdek said. “However, this summer count was different.”

Many of the deeper areas of the main stem actually had large, adult dace observed feeding in the current. Nearly sixty dace were counted in the three reaches of the main stem. That is a marked increase from the 24 fish counted in those reaches in the 2020 summer count. In 2019, the number found there was less than 5.

“This is exciting because we think that life in the tributaries is hard for these fish,” Syzdek said. “It is close to their thermal limit and is energetically expensive. Fish in the cooler water of the main stem should grow bigger, live longer and produce more eggs.”

The Warm Springs Natural Area (WSNA) will be open again for the season on Sept. 7. Visiting hours will be Tuesday through Sunday from 7 am to 3 pm.

Guided one-hour tours of the WSNA will occur on Sept. 25, Oct. 2 and Nov. 6 at 10 am, 12 pm, and 2 pm each day. Visitors can sign up for the tours at the kiosk at WSNA.

The WSNA will also be hosting a planting event for the public to help restore wildlife habitat on Saturday, Oct. 9. Lunch will be provided. Registration can be done at www.eventbrite.com/e/warm-springs-natural-area-green-up-tickets-165045528619.