Freshwater is necessary for human society in many ways, especially as a water supply for farming, industry, cities and towns. The most common way people have found to make water readily available is by building dams, which stop free-flowing streams and rivers to create reservoirs of standing water. While dams have been important for human population growth and major technological innovations, dams often negatively affect many of the animals and plants that inhabit rivers.
Fish are important components of properly working stream ecosystems. For example, many fish eat algae and insects that live in streams, helping to control insect populations and problem causing algae. Studying how dams impact fish is important for understanding how dams can change the way these stream and river ecosystems work.
Stream fish need a variety of different habitats to survive, but dams change or eliminate the habitats that some fish species rely on and act as barriers to fish that must swim long distances to reproduce. Despite changes from dams, some fish species seem to do just fine in these highly modified reservoir ecosystems, leading scientists to wonder why some fish thrive in these altered ecosystems while others disappear.
At the forefront of this issue is Ph.D. student Casey Pennock, a fish ecologist with a passion for the conservation of native fish species. His research focuses on how the “river-reservoir interface” affects fish species. A river-reservoir interface is the unique habitat that forms where rivers flow into reservoirs following the installation of a dam. To study this issue, Casey splits his time between classes at Kansas State University and collaborating with state and federal agencies and tribal partners to study the unique fish community of the San Juan River and Lake Powell reservoir in southern Utah. Lake Powell is the second largest man-made reservoir in the USA and was created by the Glen Canyon Dam, which impounds the Colorado River and its historical confluence [where two rivers meet] with the San Juan River (see photo below). The extreme habitat modifications caused by construction of the Glen Canyon Dam have led to the decline of many of the unique native fishes in this region.
Casey is most interested in the endangered Razorback Sucker (Xyrauchen texanus). These fish were once abundant throughout the Colorado River basin, but have suffered massive population declines since the construction of Lake Powell and other large reservoirs. Government agencies have been stocking adult Razorback Sucker trying to prevent the species from going extinct. “Thousands of adult Razorback Sucker are put into the San Juan River every year and larval fish [the life stage just after fish eggs hatch] that are spawned by those adults hatch, but never grow up, resulting in the continued need to stock them,” Casey says. “The reason this is so weird, is that two other common species, Bluehead Sucker (Catostomus discobolus) and Flannelmouth Sucker (Catostomus latipinnis) are doing fine in the San Juan.”
Casey believes that the answer to this question might be found in their stomachs. He compared the diets of larval Razorback Sucker to larval Bluehead and Flannelmouth Sucker to understand how differences in what the fish eat during this critical life stage might influence the success of each species. Because larval fish are so small and can only fit microscopic animals and algae in their mouths, he hypothesized that all three species would be eating relatively similar food types. Interestingly, he found that Bluehead Sucker tend to eat algae and Flannelmouth Sucker eat mostly very small insect larvae. In contrast, larval Razorback Sucker tend not to focus on eating one food type, instead they exhibit a high degree of intraspecific variation in their diets. To understand intraspecific variation, imagine five Bluehead Sucker that all eat salads, five Fannelmouth Sucker that all eat grilled chicken, and five Razorback suckers, where one eats a salad, one a pizza, one spaghetti, one likes hamburgers and the fifth eats burritos. The Bluehead and Flannelmouth Sucker have no intraspecific variation, because everyone is eating the same foods, while the Razorback Suckers have a lot of intraspecific variation, because each fish is eating something different. Although his original hypothesis was not supported, his data has led him to hypothesize that Lake Powell has made less river habitat available to all the sucker species and forced larval Razorbacks Suckers to live in areas further upstream where they must share a limited amount of food with Flannelmouth and Bluehead Suckers (see diagram below). “Historically the distributions of the three species did overlap to some extent, but each species seemed to have partitioned out its own section of the river. By flooding the lower reaches of the San Juan River, the Lake Powell reservoir has totally eliminated a large segment of river habitat that may have been key nursery grounds for larval Razorbacks”. This high degree of habitat overlap with other species in the river may force Razorback Sucker larvae to feed on unhealthy food or even starve.
To understand Casey’s hypothesis, imagine a local diner that serves healthy meals for you and your family, so you all rely on it for every meal. If your diner is replaced by a tire shop, a new building that serves a different purpose now exists but there is not anything for you to eat. This forces your family to crowd into a diner in another town, but the diner only serves unhealthy food. In this case, like the Razorback Sucker, you can’t find healthy food on the menu, so you start trying pizza, tacos, or other foods that might not be as healthy. While your appetite might be satisfied, eating unhealthy food might cause health issues down the road. Knowing that Razorbacks are eating a variety of things, Casey hypothesizes that the loss of downstream river habitat is forcing Razorback and other sucker species to share what habitats they use and could be changing interactions among the species. This might lead to Razorbacks eating unhealthy foods that might not promote growth and survival of juveniles, and might even make Razorbacks more likely to be eaten by other fish.
Reservoirs are found across the globe and there is a need to understand how all fish species use and deal with these artificially constructed environments. Dams create unfamiliar habitats to native species like the Razorback Sucker that need the original habitat to complete their life cycle. From an economic standpoint, many thousands of hours and dollars are spent every year to try and recover native fish species impacted by dams. Data collected by researchers like Casey inform environmental policies that work proactively to prevent species loss and environmental destruction. It is also important to remember that fish play important roles in well-functioning ecosystems, like controlling insect populations and eating nuisance algae. Losing a species like the Razorback Sucker should be a warning sign that we are stepping into unfamiliar ecological territory.
Garrett Hopper is a Ph. D. student interested in the role of aquatic animals in stream ecosystems.