Date of Award

Fall 12-8-2020

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)


Wildlife Ecology and Wildlife Conservation


Jospeh Zydlewksi

Second Committee Member

Erik Blomberg

Third Committee Member

Alessio Mortelliti


After beginning life in the Sargasso Sea, American eel enter river systems as juveniles and swim upstream in pursuit of freshwater habitat. Many encounter dams during this migration which act as barriers to upstream movement and limit eel establishment in headwater systems. Some dams have been retrofitted with fishways to improve watershed connectivity, but the individual selection imposed by these structures remains uncharacterized. We considered whether individual differences in behavior (i.e., personality) may be used to predict the propensity of juveniles to use a passage structure, suggesting that eel personality may predict access to habitat upstream of dams. Migrating, juvenile eels (n=63) were captured from a tidal tributary, and we measured the expression of bold and exploratory behaviors in classic animal personality assays (open field and emergence). Then we assessed the propensity for individuals to volitionally climb through a passage structure and assessed passage outcomes. Finally, we compared consistent behavioral tendencies and climbing propensity.

We show evidence for personality in young eels by demonstrating among-individual variation in bold and exploratory behaviors that were consistent across repeated trials in open field and emergence assays. Mean swimming speed in the open field was a predictor of climbing propensity; faster fish were less likely to climb through a passage structure. For successful climbers, climbing time was negatively associated with fish length, offering evidence for potential size-based selection on climbing ability during upstream passage at dams. Our results suggest strong potential for selective pressure on both climbing motivation and ability during fish passage Preventing a subset of individuals from accessing upstream habitat may have unintended consequences for both aquatic ecosystems and American eel populations.

Eels that successfully recruit to habitat upstream of dams may spend decades in freshwater systems before making a single, terminal migration to spawn in the Sargasso Sea. Therefore, individuals that ascended dams as juvenile, must navigate these same dams while moving downstream as mature adults, where passage is commonly associated with mortality and delay. We conducted a four-year acoustic telemetry study that characterized passage risks through two hydropower dams (West Enfield and Milford) in the Penobscot River, Maine, USA. We released tagged fish (n=355) at two sites, estimated survival and delay under variable river conditions, and compared performance among dammed and free-flowing river sections. Survival rates (standardized per river km) were lower at West Enfield (Φrkm = 0.984 ± 0.006 SE) and Milford (Φrkm = 0.966 ± 0.007 SE), compared to undammed River sections (Φrkm = 0.998 ± 0.0003 SE). This accounted for 8.7%, 14.2%, and 8.7% cumulative mortality through sections classified as West Enfield (4.4km), Milford (5.5km), or River (58.1km) respectively. Fish that already passed an upstream dam incurred higher downstream mortality compared to individuals without passage experience. Additionally, fish endured long delays at dams, and >10% of fish were delayed >24h. Low flows exacerbated the risk of mortality and delay. These results offer evidence for direct, latent, and sub-lethal consequences of dam passage for migrating eels.