The Influence of Reduced Forest Cover and Dissolved Oxygen on the Viability of Eggs from Eastern Hellbenders (Cryptobranchus alleganiensis)

Riparian deforestation is a significant threat to freshwater riverine ecosystems and sensitive fauna that depend on clean water. Sensitive aquatic species are vulnerable to the destruction of riparian forest cover which diminishes protection from pollutants, sedimentation, and solar radiation, while also depleting dissolved oxygen. In this thesis, I explore the influence of degraded riparian forest cover and its effect on dissolved oxygen on the embryonic viability of a sensitive freshwater habitat specialist, the Eastern hellbender. Hellbenders are a large-bodied, long-lived amphibian that inhabits fast flowing, cold mountain streams in the eastern United States. Over the last several decades, hellbender populations have experienced declines that are associated with low riparian forest cover, a geriatric population age structure, and high rates of nest failure. Adult male hellbenders normally provide extensive paternal care to embryos and larvae over an 8-month period. However, researchers have recently discovered that in degraded populations, hellbender nests are failing due to whole-clutch filial cannibalism by adult males. The underlying mechanism that triggers males to eat their young remains unknown, but one possibility is that eggs are not developing properly and as a result the attending male ceases to provide care. However, the embryonic viability of clutches developing in habitats with low riparian forest cover is unknown. Given the limited research on hellbender embryonic viability, I first sought to examine whether embryo viability is associated with a forest cover gradient. To accomplish this, I inherited two years of laboratory and field data, and I conducted a final third year of data collection for the study. Over these three years of data collection, I simultaneously evaluated embryo viability in a controlled captive rearing system while classifying nest failure due to whole-clutch cannibalism of sibling embryos in the field. I found significantly lower hellbender embryo viability, faster hatching times, and higher rates of underdeveloped hatchlings in hellbender populations with degraded riparian forest cover. However, I found no relationship between embryonic viability and whole-clutch filial cannibalism. I concluded that elevated specific conductance of water (i.e., dissolved ions) associated with loss of forest cover may affect the embryonic development of hellbenders, but this hypothesis has yet to be tested. To further explore the impact of degraded riparian forest cover on hellbender embryonic viability, I designed a study to evaluate the influence of depleted dissolved oxygen on embryonic development. To accomplish this, I designed and implemented a dissolved oxygen manipulation system to rear sibling embryos in high, medium, and low dissolved oxygen concentrations. I found that hellbender embryos are vulnerable to relatively modest reductions in dissolved oxygen (i.e., 5 mg/L), comparable to those found to affect embryos of some of the most sensitive species examined to date. Nests reared in low dissolved oxygen had a lower percentage of viable hatchlings, lower hatching success, higher rates of underdeveloped hatchlings, and smaller morphometrics compared to those reared at higher dissolved oxygen concentrations. I concluded that hellbenders may be particularly susceptible to reductions in dissolved oxygen because of their high degree of specialization for well oxygenated streams, and I suggest prioritizing additional research on dissolved oxygen to advance hellbender conservation. Taken together, my research established a foundation of knowledge on the sensitivities of hellbender embryos to degraded water quality caused by low riparian forest cover. Further, my work underscored the importance of riparian forest conservation for hellbender populations and for other freshwater biodiversity. Protection of riparian forest will also be critical to build resilience in streams against the existential threat of climate change.