Sep. 28—The splash is almost silent as Rio Grande silvery minnows dart to the surface of green water, devouring flakes of fish food before they disappear again.
“It’s a nondescript fish, but it’s a special fish,” said Patrick Horley, Aquatic Conservation Facility curator.
The refugium, a lazy river in the Albuquerque BioPark somewhere behind the botanical gardens, is stocked with 31,000 of the endangered minnows. Nearby are rows of silver tanks, each capable of holding 5,000 fish.
In coming weeks, 30,000 to 60,000 of the minnows will be tagged with a line of latex paint injected below the skin. In a month or so, those small silver fish will return to the river where many were spawned, but where they have yet to swim.
The fish struggle to survive in the Rio Grande. The species only lives in 7% of its native range. In late summer months the water can reach over 100 degrees and the river runs dry for long stretches, leaving them stressed in small puddles.
Once, Rio Grande silvery minnows were the most prolific fish in the river. But dams built in the early to mid-1900s and channelizing the river in years that followed made it difficult for the fish to thrive. Dams separated the fish populations, limiting genetic diversity, and the inconsistent flow of the river disrupts their spawning.
“The biggest impact is the dams. It lets water out — it trickles it out. It doesn’t come flooding out like it would have before the dams. Then also, the fish can’t swim past it. So it creates this genetic bottleneck. … Then once you get down into Elephant Butte, there’s so many other non-native fish that if they get there, they’re just gone,” Horley said.
Now, the silvery minnows live in just three parts of the river: the Albuquerque, Isleta and San Acacia reaches.
“Humans caused a problem. Now we’re trying to fix the problem,” Horley said. The BioPark, the state and federal government and 13 other partners work together to support the silvery minnow’s survival.
Starting in spring, BioPark staff along with U.S. Fish and Wildlife Service employees stand in the river’s flows with boxes that filter the eggs into a water-filled tray. They use a spoon to scoop the eggs and a clicker to count how many they collect. It’s easy to mistake an air bubble for one of the tiny clear eggs.
“That happens to everybody on their first day,” Horley said.
Some years, the fish egg haul is slim. Three years ago, only 16 eggs were found, forcing them to also capture larval fish later in the year. The larval fish are much harder to tell apart, and it’s easy for a shiner or carp to slip into the collections.
But this year, conservationists timed collections well. They started wading into the river earlier than usual in April, because the river started drying earlier than usual. River managers released water in a specific area of the Rio Grande in early May, mimicking the rush of water that would come down the river after winter snowmelt, to encourage the fish to spawn. They captured 2,500 eggs.
Two days later, a large rainstorm filled the river. They gathered another 46,000. Of those, the BioPark kept 19,000 and sent 26,000 to a federal hatchery.
While that sounds like many fish, roughly half the eggs are viable. For the viable eggs, there’s a 75% chance the fish will grow up to swim, Horley said.
“In nature, their likelihood of survival is tiny, tiny, tiny, tiny, tiny,” he said.
The BioPark raises the fish from eggs to adulthood. The young minnows feast on baby brine shrimp.
“We can see their little pink bellies; We know that they’re eating well,” Horley said.
As they grow older, the silvery minnows eat custom fish flakes and algae. New Mexico gets 800 pounds of the shimmery golden-green fish flakes annually to feed the endangered fish, small poundage compared to commercial fish farms that can go through closer to 26,000 kilos of fish food a day.
Most of the adult minnows are released into the river, but some are held back so that they can spawn fish eggs for future years. That brooding stock is key, since some years few eggs are captured.
The biggest challenge for conservation work to keep the species alive is funding, Horley said.
The Aquatic Conservation Facility’s annual operations are estimated at half a million dollars. The facility holds refuge populations of several native fish and invertebrates, such as the Socorro isopod and the Zuni bluehead sucker, not just Rio Grande silvery minnows.
The Albuquerque Bernalillo County Water Utility Authority contributes $165,000 toward the program, an amount meant to cover the cost of 33,000 fish. The New Mexico Department of Game and Fish contributes $50,000 per year and U.S. Fish and Wildlife contributes $58,722. The BioPark’s operating budget covers the rest, according to Associate Director Matthew Peterson.
Horley has been applying for federal grants that seem like they could help with funding needs, but as the federal government has cut back on spending, some of the grant programs have been cut, never opened for applications or did not close by their original deadlines. He’s not optimistic about securing one.
Keeping the species alive is an uphill battle, Horley said.
“I don’t expect to win. I don’t know what winning even looks like.”