Long-term monitoring of two endangered freshwater mussels (Bivalvia: Unionidae) reveals how demographic vital rates are influenced by species life history traits

To meet monitoring and recovery planning needs, demographic vital rates of two endangered freshwater mussels (Bivalvia: Unionidae)—the Cumberlandian Combshell (Epioblasma brevidens, Lea 1831) and Oyster Mussel (Epioblasma capsaeformis, Lea 1834), species endemic to the Tennessee and Cumberland river basins, U.S.A—were estimated and compared using census methodologies. Annual variation in population density and size, recruitment rate, mortality rate, sex ratios, and female fecundity of both species were observed from 2004–2014 at three fixed sites, spanning a 33.8 kilometer (KM) reach of the Clinch River, Hancock County, Tennessee. Mean population size of E. brevidens estimated from 11 censuses was 2,598 individuals at Swan Island (KM 277.1), 8,744 at Frost Ford (KM 291.8), and 879 at Wallen Bend (KM 309.6); collectively, these demes grew at an annual rate of 7% over the study period. Mean population size of E. capsaeformis was 7,846 individuals at Swan Island, 265,442 at Frost Ford, and 11,704 at Wallen Bend; collectively, these demes grew at an annual rate of 6%. Population size, variability in population growth, recruitment, and mortality of the shorter-lived E. capsaeformis (maximum age = 16 yrs, rarely >10 yrs) were higher than those of the longer-lived E. brevidens (maximum age = 25 yrs). Stream discharge was associated with realized per-capita population growth rate for both species when juvenile (Ages 1–3) data was included. Linear regression analysis showed that the growth rate of E. brevidens was negatively associated with median annual discharge (p = 0.0274) and that growth rate of E. capsaeformis was negatively associated with the number of days having extreme high discharge preceding a census (p = 0.0381). Fecundity of female E. brevidens averaged 34,947 (SE = 2,492) glochidia and ranged from 18,987 to 56,151, whereas fecundity of female E. capsaeformis averaged 9,558 (SE = 603) glochidia and ranged from 3,456 to 22,182. Estimated vital rates indicated that the two species are characterized by different life-history strategies, with E. brevidens exhibiting a periodic strategy (between K- and r-selected) and E. capsaeformis an opportunistic strategy (r-selected). These life history strategies are likely influenced by each species’ longevity and habitat preference, in addition to the life histories and population dynamics of their primary fish hosts.