The Vermilion Darter (Etheostoma nuchale) is so close to the brink of extinction, it could disappear within the span of a decade. Why is this fish so endangered when other fish species are doing just fine? There are several answers to this question.
To begin with, aspects of the darter’s biology make it vulnerable to extinction. The fish is found only within Turkey Creek’s small watershed. If the population is wiped out, there are no second chances. In addition, the fish lives just three years, and only reproduces during the second and third year. Thus, a poor spawning season can cause a big decline in the population size. Two bad spawning seasons in a row can devastate the species.
Both of the above are natural aspects of the fish’s biology, and yet the fish has survived just fine for hundreds-of-thousands, if not several million, years. It’s the human impact on the fish’s habitat that is driving its population downward.
The greatest threat humans impose on the species is polluted stormwater – the water that flows off of the urban landscape during rainstorms. To understand why stormwater is a problem, one needs to know that the fish needs shallow sections of stream with clear, cool water and a bottom of gravel and larger rocks. It seeks shelter among larger rocks, finds small prey in crevices between rocks, and attaches eggs to sheltered rock surfaces that are clean of silt.
Stormwater is wiping away these habitats. During the past century much of the Turkey Creek watershed has been urbanized as the settlement of Pinson grew into a small city. This has caused two sets of problems for the creek – too much dirt and flash flooding.
Stormwater from an urbanized landscape carries a lot of dirt – clay, sand, and small gravel – into urban streams. This sediment pollution fills in the rock crevices on the stream bottom – the crevices needed by the Vermillion Darter and its prey. Where sediment pollution is really bad, the stream bottom can be completely smothered.
Where does this sediment pollution come from? During construction of buildings and other structures, a lot of sediment is usually exposed to the sky. During a heavy rain storm (and we get a lot of these in Birmingham!), these sediments are washed downhill. If proper sediment erosion controls are not in place, the sediments are washed into our stormwater systems and into the nearest creek.
Flash flooding is a major problem in the Southeast’s urban streams. Back when Alabama was mostly covered in forest, tree leaves would slow down the speed of the rain and a heavy rain would be absorbed by the soil and become groundwater. Groundwater would seep into the nearest creek over the next days, weeks, or months. Urbanization changes that. Streets, parking lots, and buildings are hot, impervious surfaces that don’t absorb water. Stormwater from these structures is shunted to our creeks and rivers during storms. Then, water levels in the creek rise quickly, and the current speeds up. Some sections of creek cannot hold the water and flood regularly. These floods endanger human lives and damage property.
For the Vermilion Darter, a flash flood can sweep away its eggs and small, free-swimming larvae. The strong currents also cause stream banks to collapse, which releases more sediment pollution in the stream. The strong currents also sweep away the logs and large rocks that provide cover for the darter and many other wildlife species.
Another major threat to the Vermilion Darter is barriers to its movement in the watershed. There are more than six dams in the Turkey Creek Watershed. Not only do the lakes above the dams eliminate darter habitat, but each dam is a barrier separating upstream and downstream populations. The isolated populations become vulnerable to inbreeding and other factors that often cause small populations to decline. If an isolated population declines, it will not be rescued by adjacent populations because of the dams. Culverts beneath highways and roads crossing the creek are also barriers. Fish are reluctant to enter these darkened tunnels, and upstream passage by fish is often prevented anyway when culverts create small waterfalls on their downstream end.
Natural vulnerabilities, plus the threats of stormwater and dams, have garnered the Vermilion Darter status as one of the most endangered fishes in the United States. The likelihood of extinction will remain high until significant efforts are made.
-R. Scot Duncan