Studying the aquatic animals that keep our water clean

Population monitoring, exploring understudied locations, and tolerance testing are all part of the work the Brazos River Authority is undergoing to better understand two Central Texas mussels whose livelihood is in danger and are important for a healthy river.

The work aims to determine the optimal habitat conditions for two particular mussel species – the Texas Fawnsfoot and the Balcones Spike – and understand the ecological drivers of their current distribution, and identify actionable land use and water quality management strategies to promote their recovery.

“A lot of people don’t know how many different species of mussels there are,” said Justin Grimm, Brazos River Authority environmental programs coordinator. “They hear ‘zebra mussels are bad,’ so they assume all mussels are bad. So, part of our efforts includes working to show the public that there are different kinds of mussels, good mussels versus bad mussels, and how the good mussels are important to a healthy river.”

Grimm manages the Candidate Conservation Agreement with Assurances, or CCAA, a 2021 agreement with the U.S. Fish and Wildlife Service, USFWS, that commits the BRA to implement a voluntary conservation strategy for the Balcones Spike and Texas Fawnsfoot mussels.

It’s the first system-wide conservation partnership designed to protect the diversity of aquatic species in the Brazos River Basin.

In 2016, the USFWS began evaluating six freshwater mussels known to occur in the Guadalupe, Colorado, and Brazos rivers to determine if the species’ status warrants federal protection under the Endangered Species Act. Two of the six freshwater mussels live in the Brazos River Basin: the Truncilla macrodon, known as the Texas Fawnsfoot, and the Fusconaia iheringi, known as the Balcones Spike.

So, how do you track all those species day in and day out to ensure they don’t present complications to the necessary water supply while still protecting them? The answer is to create an evaluation methodology and then monitor the species that could have a likelihood of impact, said Tiffany Malzahn, BRA chief environmental officer. Once those species are identified, staff can recommend appropriate responses, such as further monitoring, a special study, or additions to the CCAA.

Since the agreement was approved, the teams have been making active progress, Malzahn said.

Discovering novel sampling methods has been instrumental to the process, she said.

“One of the biggest lessons learned is there’s been a real tendency to just do what’s always been done,” Malzahn said. “We’ve explored a variety of different sampling methodologies and have actually identified a methodology that is U.S. Fish and Wildlife Service-approved that has not historically been done in Texas, but has been done in other places in the U.S. That has greatly improved our ability to identify and detect juvenile mussels.”

Finding these mussels to study them in their environment requires a lot of field work, Grimm said. Teams have looked at 80 different sites, using weighted 10-meter lines and putting them in different natural areas to indicate areas for review. Each study area was for other areas of the habitat, including where the water moves, the still part, and the riffle area, which is the shallow area in a stream where the water flows quickly and is broken up by rocks.

The Texas Fawnsfoot is smaller, though. More often than not, those in the field use a colander to sift through the streambed to make sure they don’t miss anything, he said. These mussels tend to burrow, so just feeling around with your hands is not the most efficient way to locate them, Grimm said.

Malzahn said they’ve already visited many previously unsurveyed locations within the basin to see if the two different types of mussels live there. So far, none of these locations have generated new findings, which is in itself informative.

As part of the CCAA work, work is also in progress to generate a drought plan for the mussels.

“We have trigger points outlined for water levels,” Malzahn said. “In that plan, it discusses if things like water levels, temperature impacts, and dissolved oxygen reach a certain point, we’ll coordinate with the Texas Parks & Wildlife Department and the U.S. Fish and Wildlife Service, and if everyone agrees, we’ll try to move the mussels further downstream to a better location in the same stream.”

Another step has included sending a few of the mussels to Auburn University, where teams are working to determine what high temperature is lethal to the species or causes them distress, she said.

Teams in the field are studying substrate and channel morphology, essentially determining how the shape and make of the channel provide for these mussels’ habitat and how it’s changing over time.

Also, while in the field, monitoring is being done for invasive species, such as zebra mussels. Zebra mussels will attach to any hard surface they settle on, including native mussels. Excessive growth of zebra mussels on and around these native mussel beds can suffocate them over time, making them a danger to the native mussels’ survival.

The CCAA includes activities related to research and monitoring to further knowledge of the two species, the development of conservation zones and future hydrology modeling to prioritize areas for implementation of specific conservation measures designed to reduce current and future threats to the species.

There are more than 50 mussel species in Texas. When the populations are at risk, there will be problems for other fish and wildlife species, and people, too.

Freshwater mussels serve as indicators of a thriving aquatic ecosystem, Grimm said. As filter feeders, they play a crucial role in maintaining water clarity and quality by removing pollutants, he said.

The sudden disappearance of mussels in an area not recently subject to prolonged drought could indicate water pollution problems.

Historically, freshwater mussels were commercially exploited for food, pearls, and button making. The Texas Parks and Wildlife Department now regulates commercial harvesting and mandates licenses and annual reporting.

The Balcones Spike is currently listed as Federally Endangered, and the Texas Fawnsfoot is currently listed as a threatened species.

When a species is threatened or endangered, different levels of protection may be implemented at the state and federal levels. The state of Texas provides two levels of protection: threatened and endangered species listing. On the federal level, species can also be designated as threatened or endangered. A plant or animal can be protected as endangered at both the state and federal levels at the same time. The level of protection provided by state and federal regulations is different, with federal laws providing stronger protections than the state level.

The Texas Fawnsfoot has been found in multiple locations within the Brazos River Basin, including the Clear Fork of the Brazos River, the Brazos River between Possum Kingdom Lake and Lake Granbury, the Brazos River below Waco, the Navasota River, and the Little River.

Little is known regarding the life history requirements of the Texas Fawnsfoot. Like all freshwater mussels, the Texas Fawnsfoot is relatively inactive and capable of moving only small distances. Their primary mode of moving or colonizing new areas is accomplished by the movement of the fish hosts or by high-flow events that scour adult mussels from their current location and move them downstream.

Within the Brazos River Basin, the Balcones Spike is currently known to reside in the Little River, San Gabriel River, and Brushy Creek.

The Balcones Spike is usually seen in riffles and runs within streams. Like other native freshwater unionids, Balcones Spike parasitize or latch on to a host fish during the larval stage, receiving nutrition and transport from the host until dropping off as fully developed juveniles, according to the Journal of the North American Benthological Society.

Grimm said the BRA is collaborating with organizations like the Texas Parks & Wildlife Department to host landowner meetings.

“That’s been really helpful,” he said. “We tell people about mussels and find landowners that are willing to implement conservation efforts, such as keeping riparian areas clean and keeping sediment out of the water. These are things considered best land management already. Everything rolls downhill to the water.”

Learn more about the CCAA here.