Date of Award

Summer 8-20-2021

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)


Wildlife Ecology and Wildlife Conservation


Joseph Zydlewski

Second Committee Member

Nishad Jayasundara

Third Committee Member

Stephen Coghlan, Jr.


For any migratory organism, habitat connectivity is critical for population stability. Structures that impede movement between necessary habitats can be damaging to population persistence. In riverine systems, dams act as migratory barriers, altering ecosystems and delaying, injuring, or otherwise impairing migratory fish movement into essential habitat. Critically endangered Atlantic salmon (Salmo salar) populations in Maine have been on the decline since the 1800s. Because most Atlantic salmon rivers are now highly dammed systems, hydropower dams have been cited as causal to the decline in returning adult populations. Previous studies have demonstrated that Atlantic salmon experience substantial delays below dams while moving upstream, but current state of knowledge with respect to metabolic costs and fitness outcomes for delayed Atlantic salmon provides no clear quantification of risk associated with this delay. I sought to understand consequences of delay in the context of an increased thermal experience below dams. With my collaborators, I have documented that water temperatures below surface-release dams remain several degrees warmer throughout peak summer months than water temperatures in upstream sections of river. Thus, adult Atlantic salmon experiencing delays below dams will be subject to warmer thermal experiences than if they had moved rapidly to upstream sections of river. As ectotherms, ambient water temperatures directly impact physiological processes, but salmonids generally have a narrow optimum temperature range. As waters warm to outside that range, metabolic processes become more energetically costly. That excess energy use might manifest itself in reduced individual reproductive success, or in the case of Atlantic salmon, a decrease in population iteroparity rates. In Chapter 1, we quantified the energetic cost of dam-mediated delays of adult migrating Atlantic salmon using HOBO temperature loggers, temperature-logging radio tags, and a Distell Fish Fatmeter as a noninvasive surrogate for full-body energy estimation. On the Penobscot River and Kennebec Rivers of central Maine, we tagged fish, released them a short distance below the dams, and tracked their movements back upstream, taking a Fatmeter measurement first at tagging and then after ascending the fish way at the respective dams. We found that Atlantic salmon experienced delays of two to three weeks on average while attempting to pass dams on their respective rivers. I that time, these fish lost between 11.1 and 19.4% of their starting endogenous fat reserves. Using the temperature-logging radio tags, we documented the individual thermal experience of each study salmon and determined that the thermal experience did indeed predict fat loss. Our results suggest that dams cause delays, not only extending migration times but also exposing adult Atlantic salmon to warmer water temperatures, compounding energetic impacts of dams and depleting available energy reserves. In Chapter 2, we utilized a bioenergetic model to further understand energy allocation of migrating adult Atlantic salmon in the context of delays at dams and corresponding water temperatures. Atlantic salmon can survive after spawning to be iteroparous, engaging in multiple reproductive life cycles over the course of a lifetime. Iteroparity is thought to be important to the conservation of this federally endangered species through greater population recruitment but currently, repeat spawners in Maine populations have been nearly eliminated. We modeled five different Atlantic salmon spawning runs in five different hypothetical rivers with either zero, one, two, three, or four dams presenting as migration barriers causing delays and subjecting salmon to warmer water temperatures. The results of our model showed that as the number of dams on a river increased, the number of post-spawn surviving Atlantic salmon decreased. On the unimpeded river, 6.3% of the run survived after spawning but on a river with four dams, only 2.8% of the returning salmon run survived. We also found that the number of salmon able to complete the single reproductive event decreased and the number of salmon that died before spawning increased as dams were added to the modeled system. Our results from Chapter 2 suggest that rapid movement through warmer downstream sections of rivers to upstream, cooler waters is key to increasing iteroparity rates in spawning populations and overall reproductive success.