Endangered animals frequently survive in natural habitats that have been grossly altered or have largely disappeared due to human or natural causes. In the case of the endangered Rio Grande silvery minnow, water development projects and practices on the Rio Grande and the Pecos River have contributed to the elimination of this fish from most of its original range. Today, they are only found along the Middle Rio Grande, from Cochiti Dam to Elephant Butte Reservoir. Reintroduction efforts have been made in other reaches, but river conditions are different and success is not ensured.
This is why the work of Corps Biologist Michael Porter and Hydrologist/H&H Section Chief Tamara Massong is so important. During the last decade, their studies have focused on the interaction between hydrology, channel morphology, minnow nursery habitat requirements and minnow life history in order to recreate optimal conditions for spawning and recruitment (survival to adulthood). These concepts form the foundation for many Corps and Middle Rio Grande Endangered Species Collaborative Program activities aimed at helping to preserve the species, including habitat restoration, population viability modeling and adaptive management.
If their efforts to adaptively manage the environment for minnows in the Middle Rio Grande are successful, they could provide crucial information to successfully reintroduce this species throughout its former range.
The key to successful recruitment is the size and duration of the peak river flow during spring runoff in early June. Therefore, Massong and Porter pushed for the experimental manipulation of peak spring river flows as a means to improve spawning and, indirectly, recruitment of a new generation.
“In 2002 and 2003,” Porter recalls, “We collaborated with the [U.S. Fish and Wildlife] Service to experimentally ramp flows up to around 1,000 cubic feet per second (cfs) at the Central Avenue gauge. This caused an increase in the number of fish spawning but didn’t translate into greater recruitment, leaving the scientists baffled.
In the following year, natural flows kept the river above 3,000 cfs for five days, which flooded shoreline areas and sandbars. This produced little evidence of spawning (as measured by fish egg counts downstream), yet there was a huge increase in recruitment.
“Since the number of eggs going downstream was abysmal, some folks thought the minnow was gone,” Porter said. Instead, he said, the numbers caught in nets in the fall increased 38-fold.
What Porter concluded was that improved recruitment required overbanking floods so that fertilized eggs could hatch, and juvenile fish could mature in the shallow slackwater environments outside of the main channel.
The idea of a “recruitment flow,” a manipulation of the spring runoff to create a peak flow of at least 3,000 cfs for at least five days at the Central Avenue gauge, was developed within the Collaborative Program. This was achieved naturally in 2004 and 2005, due to large snowpacks in the watershed, but not in 2006. So, in 2007, the Service supported the Corps plan to manipulate the spring flows to create a recruitment flow. As a result, recruitment rose. Beginning in 2008, the Corps worked closely with Pueblo de Cochiti, the Service, the Bureau of Reclamation, New Mexico Interstate Stream Commission, the Rio Grande Compact Commission, Santa Ana Pueblo and others to develop a multi-year plan to temporarily detain a portion of the spring runoff behind Cochiti Dam and then release it during the peak of spring flow in order to ensure minnow recruitment in the Middle Rio Grande.
This project required a year-long approval process in order to obtain a deviation from the Corps’ water control plan. The objectives of the plan are simple: in drier years, the goal is to achieve the minimum recruitment flow; in wetter years, higher peak flows would also benefit native floodplain vegetation that provides habitat for an endangered bird species, the Southwestern Willow Flycatcher.
The operation for recruitment flows in 2009 led to a significant increase in minnow recruitment. In 2010, higher spring flows permitted the Corps to not only achieve recruitment flows, but also some overbanking flows.
“We had very good spawning in response to the 2010 deviation,” Porter recollects, “but fish numbers declined dramatically in late summer, indicating some other environmental factors are limiting populations, such as declines in food supply or available habitat.”
Due to drought, spring flows this year in the Middle Rio Grande are too low to be manipulated as in previous years. Instead, Porter and colleagues from the Collaborative Program will monitor the natural peak flow and silvery minnow response to that flow.
According to Porter, “Naturally, the Rio Grande produces recruitment flows in the Middle Rio Grande in about 50 percent of years. Through manipulation of the hydrograph, we should be able to raise this to about 75 percent of years.”
The Corps hopes to work with the Pueblo de Cochiti and other stakeholders at Cochiti Lake to seek ways to continue this adaptive flow management in the future. But, that still leaves one in four years when flows are insufficient in this stretch of the river. 2011 is looking to be one of these years. Porter is hopeful for recruitment in some areas but doesn’t think recruitment will occur in all reaches.
“Overall, it’s not good for 2011,” Porter said. “But the fish is well-adapted to a desert river and it is likely some recruitment will occur this year, setting the stage for a rebound in future years.”