Todd D. Levine1, Brian K. Lang2, and David J. Berg3.
1Department of Zoology, Miami University, Oxford, OH 45056. 2New Mexico Department of Game and Fish, Santa Fe, NM, 87507. 3Department of Zoology, Miami University, Hamilton, OH 45011.

Long term data for freshwater mussel populations are rare, leaving substantial uncertainty about many demographic processes in these taxa. A ten-year dataset has been developed while monitoring a critically endangered species, the Texas hornshell (Popenaias popeii), in southeastern New Mexico. Using mark-and-recapture, we estimated survival and found it to be high (60-95%, annual apparent survival). Using program MARK, we evaluated competing models of survival and found a negative correlation of survival with water flow; 79% of mussels occupy habitat with velocities less than 0.05 m/sec that are protected by travertine shelves, root mats, and boulders. Using length as a proxy for age, we evaluated the age structure of the population. The length-frequency relationship was similar across years, with a modal length near 100 mm. Use of length-frequency distributions is predicated on a reliable relationship between age and length; therefore we created age-length curves using both annual length data from the M/R study and annual ring counts to explore this relationship. Using a VonBertalanffy growth curve, we estimated an asymptotic length near 100 mm, which corresponded to estimates derived from counted external annuli. Lengths substantially above estimated asymptotes indicate the potential for extreme longevity and indeterminate growth. Overall, these estimates and recent observations of small individuals (indicating recent recruitment) suggest that this population is currently stable. However, due to the restriction in P. popeiiís range, it remains vulnerable because there is little chance for recolonization in the event of an environmental catastrophe such as an exceptionally high discharge event.