Date of Award
Level of Access
Master of Science (MS)
Wildlife Ecology and Wildlife Conservation
Second Committee Member
Third Committee Member
Substantial declines of anadromous Atlantic Salmon Salmo salar have occurred throughout the range of the species, with many populations at the southern extent of the distribution being extirpated or endangered. While Maine is the last state in the country where adult Atlantic Salmon return to rivers each year to spawn, numbers have decreased dramatically in recent decades, with typically less than 2,000 spawners returning to all Maine's rivers combined. The complex life history of this species, which involves a juvenile freshwater phase followed by a marine phase that can last one to five years before returning to freshwater to spawn has exposed Atlantic Salmon to a series of threats that have contributed to their continued decline. These threats include, among others, dams and changing ocean conditions that can influence marine mortality. This thesis focuses on those two threats by using radio telemetry to assess upstream passage of adult salmon in the lower Penobscot River and scale analysis to interpret ocean growth patterns in relation to ocean conditions.
The Penobscot River Restoration Project (PRRP), completed in 2016, involved an extensive plan of dam removal, increases in hydroelectric capacity, and fish passage modifications to increase habitat access for diadromous species. As part of the PRRP, Great Works (rkm 59) and Veazie (rkm 46) Dams were removed, making Milford Dam (rkm 61) the first impediment to federally endangered Atlantic Salmon and other diadromous species. In 2014 and 2015, a total of 73 adult salmon were radio-tagged to track their upstream movements through the Penobscot River to assess potential delays at 1) the dam remnants, 2) the confluence of the Stillwater Branch and the main stem of the Penobscot River below the impassable Orono Dam, and 3) the Milford Dam fish lift (installed in 2014). Movement rates through the dam remnants and the Stillwater confluence were comparable to open river reaches. Passage efficiency of the fish lift was high in both years (95 and 100%). However, fish experienced long delays at Milford Dam, with approximately 1/3 of fish taking over a week to pass in each year. Telemetry indicates most fish locate the fishway entrance within 5 hours of arrival and were observed at the entrance at all hours of the day. These data indicate that overall transit times through the lower river were comparable before and after changes to the Penobscot River due to the substantial delays seen at Milford Dam fish lift. The results of this study show that while adult Atlantic Salmon locate the new fish lift entrance quickly, passage of these fish was significantly delayed under 2014-2015 operations.
Prior to returning to spawn, Maine's Atlantic Salmon typically spend one or two years at sea feeding. While both one sea winter (1SW) and two sea winter (2SW) spawner numbers for the North American stocks have declined since the 1950s, the decline has been most severe in 2SW spawners. The first months at sea are considered a period of high mortality. However, early ocean mortality alone cannot explain the more pronounced decline of 2SW spawners, suggesting that the second year at sea may be more critical than previously thought. Angler and state agency collected scales from 1946 to 2013 from the five eastern Maine rivers were used to estimate smolt age and ocean age of returning adults. Additionally, seasonal growth rates of maiden 2SW spawners were estimated using intercirculi measurements and linear back-calculation methods. Generalized linear models (Gaussian family, log link function) were used to investigate the influence of average sea surface temperature (SST), accumulated thermal units (ATUs), the Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) indices, smolt age, smolt length (FL), and post-smolt growth on growth rate during the oceanic migration of North American Atlantic Salmon. Results suggest that different factors influence salmon growth throughout their oceanic migration, and previous growth can be a strong predictor of future size. Growth was negatively impacted by the phase of the AMO, which has been linked to salmon abundance trends, in the late winter and early spring following the post-smolt period. This is likely when the 1SW and 2SW stock components separate and these results suggest that this period could play a role in the disproportionate decline in 2SW spawners.
Izzo, Lisa K., "Exploring the Threats of Dams and Ocean Conditions: In-River Movements and Ocean Growth of Atlantic Salmon (Salmo salar) from Maine's Rivers" (2016). Electronic Theses and Dissertations. 2479.