Date of Award


Level of Access

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)


Wildlife Ecology and Wildlife Conservation


Joseph Zydlewski

Second Committee Member

Michael Bailey

Third Committee Member

Michael Kinnison


Atlantic salmon Salmo salar populations are diminished throughout their range and high marine mortality is among the drivers of the failure of many stocks to recover. A goal of salmon recovery is to maximize the number of juvenile ‘smolts’ entering the ocean to offset loss therein. Dam removals and changes to hydropower allocation in Maine’s largest river, the Penobscot River, have occurred as part of the Penobscot River Restoration Project (PRRP). These activities, in addition to stocking have the potential to influence the number of smolts reaching the ocean. Telemetry was used to investigate factors influencing initiation of migratory behavior, movement rates, migratory route, and survival through freshwater (FW) before and after changes to the system resulting from the PRRP, and behavior and survival of smolts during estuary migration.

Initiation of migration was influenced by smolt development, stocking location and environmental conditions. Smolts with the greatest gill Na+, K+-ATPase activity (physiological development) initiated migration 24 hours sooner than fish with the lowest gill NKA activity. Fish with the greatest cumulative temperature experience (accumulated thermal units: ATU) initiated migration 5 days earlier than those with lowest ATU. Smolts released furthest upstream initiated migration earlier than those released downstream, and movement rate increased 5-fold from upstream to the estuary. Movement rate increased from 2.8 km·h-1 to 5.4 km·h-1 in reaches where dams were removed, and decreased from 2.1 km·h-1 to 0.1 kmh-1 after powerhouse construction.

Proportional use of the Stillwater Branch was low (0.12, 95% CI = 0.06 – 0.25), and survival through the dams therein was relatively high (0.99) prior to installation of new powerhouses, decreasing slightly thereafter. Survival at Milford Dam, now the lowermost dam in the main-stem, was low (0.91) prior to increased power generation, whereas survival at Great Works and Veazie Dams was high (0.99 and 0.98) prior to removal. Survival was higher through free-flowing reaches (> 0.99·km-1) than reaches containing dams (c. 0.95·km-1). Survival was reduced at high (> 2000 m3s-1) or low (<300 m3s-1) flow, and was optimal between 12 C and 17 C. Survival increased following dam removal, but survival through those dams was high before removal. The greatest increase in survival (8%) followed turbine shutdown at Howland Dam.

Smolts experiencing greatest ATU arrived in the estuary 8 days earlier than those experiencing lowest ATU. Estuary arrival date was 10 days later for fish experiencing high flow than for fish experiencing low flow. Fish released furthest upstream arrived in the estuary 3 days later than those stocked further downstream, but moved 0.5 km·h-1 faster through the estuary. Estuary survival decreased by 40% with increasing number of dams passed (from 2 to 9). Estuary movement rate and survival both peaked in mid-May, and slowed from FW to ocean, likely resulting from tidal influences. Smolts became increasingly surface-oriented during passage from FW to ocean as salt water (SW) became more prevalent. In laboratory experiments, preference for SW by never exceeded 50% during smolt development. Thus, smolts likely select low salinity (i.e. surface) waters during migration through coastal areas.

Smolts with low gill NKA activity spent greater time in FW reaches of the estuary than those with high gill NKA activity. However, there was no difference in travel time through SW reaches of the estuary based on gill NKA activity. Fish with the highest gill NKA activity incurred 25% lower mortality through the estuary than fish with lowest gill NKA activity, and survival was lowest where SW was prevalent. These results underscore the importance of physiological preparedness on performance and the delayed effects of dams on survival of smolts during estuary migration, ultimately affecting marine survival estimates.