Image caption: Texas hornshell (Popenaias popeii)
For over 15 years, we studied Texas hornshell (Popenaias popeii), a rare unionid bivalve found in only a few Texas and New Mexico rivers. After surveying over 250 sites in the entire historical range of P. popeii in Texas, we found that this species has been extirpated from a large portion of its geographic range. The remaining population in the Rio Grande is very fragmented, with only one 190 km stretch above Laredo still supporting high densities (Karatayev, A.Y., L.E. Burlakova, T.D. Miller,and M.F. Perrelli (2018) (1MB)). As a result, the Texas hornshell recently became the only federally-listed critically threatened unionid mussel in Texas.
In the Rio Grande, Texas hornshell live in patches (mussel beds) in narrow gaps beneath large sandstone rocks resting atop bedrock. These habitats provide “flow refuges” for mussels during peak currents. Over four years, we tracked one of the largest mussel habitat patches at La Bota above Laredo, Texas, using an intensive mark-and-recapture study that has been recently published (Karatayev, V.A., L.E. Burlakova, A.Y. Karatayev, L. Yang, and T.D. Miller (2020)). To our surprise, the mark-recapture models estimated high levels of annual downstream emigration of adult Texas hornshell (16–51%) as well as high immigration rate from upstream habitats (32–48%). This means that during high river flows, the La Bota mussels are washed out into an unsuitable polluted area since the bed is located upstream from the Laredo sewage plant, below which no live Texas hornshell were found due to heavy pollution.
Furthermore, in many dammed rivers there are active efforts to restore peak river flow rates. Peak river flows are a critical function in pristine rivers, but they also move many organisms downstream from habitat patch to habitat patch. Therefore, in rivers with high habitat loss, restoring natural flow regimes without restoring habitat patches could accelerate local extirpation if the restored peak flows end up moving animals out of the remaining, good habitats into poor-quality habitats downstream.
Our study suggests several important implications for species protection and restoration. Protection of subpopulations isolated in small river stretches by itself may not prevent local extirpation if elevated peak flows will carry most individuals to downstream degraded habitats. To avoid the adverse effects on threatened aquatic species, we may need to restore both habitats and the flow regime simultaneously. One of the best ways would be to restore and maintain long, contiguous stretches of high-quality habitats.
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