Fighting to save native fish: On the brink of extinct
When new species of fish are introduced into a lake, they can wreak havoc on the native fishes. In some cases, the introduced species may eat the native ones, causing the population to crash and sometimes disappear altogether. WSU Biology Professor Tom Dowling is helping to find ways to protect native species and maintain a healthy, freshwater biodiversity.
Dowling has been on the front lines of native-fish preservation for more than three decades. The work began shortly after he earned his Ph.D. from Wayne State University when he accepted a faculty position at Arizona State University and began his still-continuing research on how hybridization (successful reproduction between two species) affects the evolution of fish biodiversity. “When you move out to the western United States, you become a conservation biologist too, because so many of the native species are threatened, endangered, or a candidate for listing. And a lot of that has to do with invasive species,” he said. In fact, he noted, a large majority of the 80 or so fish species in Arizona waters are introduced, and many of the introduced species are invasive, which means they can displace native species and take over habitat.
(In comparison, Michigan has some 150 species of fish, including dozens of introduced species, as well as 24 native species that are listed as endangered, threatened, or of special concern.)
The high level of introduced species in Arizona led Dowling to quickly add a topic to his research program: saving the native fishes. “Much of my work has been focused on conservation genetics and management, and I have this big project now where I’m looking particularly at one native species, called the razorback sucker,” he said. Recognizable by a large hump-like keel behind its head, the razorback (Xyrauchen texanus) grows to more than 2 feet long. “This is a species that used to be very abundant. They were a food resource for the indigenous peoples in the area prior to European settlement, and in the 1930s, there are records of people in Arizona going to canals when the fish were running and using pitchforks to scoop out the fish for the farmers to use as fertilizer.” Today, however, the razorback is listed as critically endangered, which puts it on the brink of extinction.
Much of Dowling’s attention has focused on Arizona’s Lake Mohave (below Hoover Dam), where the razorback population has declined primarily because of three introduced species: small, so-called "panfish" known as bluegill (Lepomis macrochirus) and green sunfish (Lepomis cyanellus), and a larger sport fish called striped bass (Morone saxatilis) that can grow 20-40 pounds and 3-4 feet long. These and other introduced fishes gobble up razorback larvae as fast as the females can produce them, leaving no young to replenish the population. Without intervention, the razorback population was destined to die out.
One solution was to rear razorbacks in a protected location until they got large enough to have a fighting chance and then release them in the lake. The traditional method of rearing involves putting a few razorback adults into a hatchery, allowing them to reproduce, and raising the young there. That approach can produce a large number of razorbacks fairly quickly, Dowling said, but it has a few drawbacks. One is that hatchery young descend from a limited number of adults, so they don’t have the genetic diversity that healthy native populations possess. Another is that hatchery holding areas are typically cement raceways or artificial ponds that have no resemblance to a lake habitat and are filled with only the reared species. As a result, when hatchery-reared fish eventually are stocked in the lake, they not only face a completely unfamiliar environment but have undeveloped predator-avoidance behaviors. The latter is a problem because striped bass are large enough to eat most adult razorbacks.
“That led us to trying a creative approach in Lake Mohave back in the late 1990s,” Dowling said, describing a multi-agency effort to bring back the razorback. “It turns out that razorback sucker larvae are phototactic, so if you can put a light in the water, they’ll come to it,” Dowling said. Using this method, the researchers were able to catch 30,000-40,000 larvae per year in the lake — before the introduced fish had a chance to eat them all — and put these larvae into the hatcheries to be reared until they grew to a large enough size (17 inches or so) to evade a bass, and then returned the razorbacks to the lake. “In doing that, we’ve been able to keep the razorback population at about 5,000, and also maintain most of the genetic diversity that was in the original source population dating to the 1980s,” he said. “The problem, of course, is that the population is not self-sustaining. We need to go out every year, collect the 30-40,000 larvae, rear them and grow them up.”
Dowling is now involved in a new approach that employs backwaters rather than hatcheries. The backwaters are small ponds that are isolated from Lake Mohave and the ravenous bluegill, sunfish and striped bass, while offering the larvae a more natural habitat, he said. The idea is to set up self-sustaining populations in the backwaters, and as the young grow and develop normal survival skills, the researchers can then move the razorbacks between backwater sites, or into Lake Mohave. “My role has been to use next-generation genetic-sequencing methods, such as developing a panel of single-nucleotide polymorphisms, to track the lake and backwaters populations to see how many individuals are reproducing and at what rate, so we can make sure we’re getting a good genetic variation,” he said. In addition, he and other researchers are investigating other aspects of backwater populations, such as the optimal population size and age structure.
While that work is under way, Dowling is working with another research group employing a similar method to attempt to rescue the bonytail chub (Gila elegans) which is a type of native minnow that can live for 30 years and reach 16 inches long. The need is great because the bonytail chub only persists today in its native habitat through hatchery rearing efforts. “So far, researchers have developed some backwaters for them, but unlike razorbacks, they are quite cannibalistic, so it is going to be a challenge in terms of attaining self-sustaining populations,” he said.
Through this work with the razorback sucker and the bonytail chub, Dowling and other researchers are gaining greater understanding of life history, genetic variation, and species preservation, and that insight will have broad application to other native-fish restoration projects in the United States and beyond, he said. “In today’s world with changes in water usage (such as dams) and introduced species, we have a new normal that we’re trying to get the native fish to adjust to, so we can maintain biodiversity and keep our ecosystems healthy.” He added, “It’s going to involve some level of partnership between researchers, environmentalists and users so can we generate co-existence and co-persistence in such a way that we can have everything: the introduced sport fishes and native species."