Moapa Dace Continues Its Baby Boom

Original Post by Vernon Robison, Moapa Valley Progress

USFWS biologist Lee Simons watches as Darrick Weissenflugh snorkels through a Warm Springs stream counting dace. PHOTO BY VERNON ROBISON/Moapa Valley Progress.

It has been another good year for the Moapa dace. The population of the endangered fish has more than doubled over the past two years, according to a survey conducted last week by the U.S. Fish and Wildlife Service (USFWS).

On August 6-7, biologists conducted their bi-annual snorkel survey of the Warm Springs area. Donning face masks, snorkels and wetsuits, the scientists slogged through the shallow waters of 17 stream reaches at the headwaters of the Muddy River counting the finger-sized dace one by one.

What they found was encouraging. The results turned up a total of 1,727 dace. That is a 46 percent increase over the 1,181 fish observed a year ago, and a 41 percent increase just since February, when the most recent survey was conducted and found 1226 fish in the stream.

“It’s good to see these numbers,” said USFWS Biologist Lee Simons. “It tells us that we are heading in the right direction. We have found what has gone awry and fixed it.”

Simons attributes the comeback of the dace population to the careful restoration of habitat that has been ongoing in recent years. Key to that has been returning the stream flow to a more optimal foraging environment for the tiny fish.

The Moapa dace, which is found only in the artesian spring-fed headwaters of the Muddy River, is a warm water fish that is adapted to somewhat rapid stream flows, Simons said.

“They dart in and out of the current feeding off of the flow,” Simons said. “That stream flow is like a conveyor belt carrying food. The dace dive into the fast water and pick it up.”

Simons claims that modifications to that natural stream flow; made, in the past, by agricultural and recreational infrastructure at Warm Springs; were what originally set the dace on the path to being an endangered species.

“We have said that if we can produce the optimal habitat, the dace will reproduce again and come back,” Simons said.

The numbers now seem to be bearing that out. The dace have not been seen in these numbers since the mid 1990s. During that time, the fish count plummetted from around 3,800 fish down to 1,000 with about five years.

Scientist attributed this sudden drop to the arrival of an invasive species to the area at that time: the tilapia. The much larger tilapia fish preyed upon the dace causing the disappearance of the native fish in some of the lower reaches of the stream.

So in recent years, a chemical eradication program has been employed to eliminate the tilapia from the system. But it is a problematic task.

“It is like a cancer where you have to kill every single cancer cell or it might come back,” Simons said. “We have to get every tilapia out of the system or the species will suddenly bounce back.”

By 2010, scientist thought that they had eradicated the tilapia. But in August 2011, a small infestation of the species popped up in the south Fork. The Nevada Department of Wildlife was called in quickly for another chemical eradication treatment which seemed to stop the relapse before it spread, Simons said.

Still, scientists are watching vigilantly for any signs of a tilapia come-back. No tilapia were reported in last week’s snorkel survey.

But the dace have yet to return to those lower reaches of the stream. The entire dace population is currently concentration in the upper reaches of the stream under the management of USFWS and the Southern Nevada Water Authority (SNWA).

The most dramatic increase in fish population has occurred in newly restored spring flows on the Warm Springs Natural Area, owned by the SNWA. In stream reaches #2 and #3, which are located near what was traditionally the “home ranch” area of the old Warm Springs ranch, the dace population numbers have exploded. In reach #2, there were 310 dace counted this month, compared to 79 from last August (139 last February). In reach #3, 248 were counted, up from only 31 in August 2012 (127 last February).

The numbers in Pedersen Springs reach #5, which flows through the Moapa Valley Wildlife Refuge, has decreased to 85 fish, down from 94 last August (128 in February).

Just downstream and across the street, on SNWA property, reach 5.5 has also seen a decrease from the 376 observed in August 2013 to 318 observed last week. But that is still up from the 244 fish that were observed there in February.

Though this month’s survey shows a very positive increase for the dace population, there is still a long way to go before the fish is considered to be out of danger. The USFWS recovery plan for the dace sets delisting goals at 6,000 fish in five springs systems for five consecutive years, restoration of 75 percent of the historic habitat and effective control of non-native, invasive fish.

Click here to view the Dace Graph from 1994 to 2013.

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