Date of Award


Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)




Joseph D. Zydlewski

Second Committee Member

Gayle B. Zydlewski

Third Committee Member

James D. McCleave


Atlantic salmon Salmo salar management is complicated by their complex anadromous life cycle and extensive marine migrations through international waters. A primary enhancement strategy is stocking the smolt life-stage, but marine mortality of smolts is high. Hatchery reared Atlantic salmon smolts produced from captive-reared Dennys River and sea-run Penobscot River broodstock are released into their source rivers in Maine. The adult return rate of Dennys smolts is comparatively low. This may be a function of ecological factors unique to the Dennys River and bay, or reflect poor quality of hatchery smolts produced by Dennys River stocks. The Dennys River environment compares favorably to the Penobscot River, and telemetry studies indicate in-river mortality for migrating smolts is much lower than in the Penobscot. There are ecological factors unique to Dennys Bay (Atlantic salmon aquaculture, complex geomorphology and hydrology) but the impacts of those on smolt survival are unknown. Smolt mortality is much higher in Dennys Bay than in Penobscot Bay based on telemetry studies. Broodstock genetic composition and rearing environment differ between watersheds and may affect smolt quality. Despite decades of stock transfers between watersheds, Penobscot and Dennys smolts maintain some genetic separation, and differences in stock specific traits (e.g. fecundity, morphology) have been identified in common garden studies of other Maine stocks. Genetic differences may reflect local adaptations which convey survival advantages or result from other factors. Domestication is implicated in reduced performance of salmon stocks and the captive-rearing environment of Dennys broodstock may increase domestication effects relative to anadromous Penobscot broodstock, reducing smolt quality. Diminished smolt quality may be most evident at critical life-stages such as the parr-smolt transition (PST) and seawater entry. To assess smolt quality and performance indices for these stocks during the PST, behavioral trials were conducted using smolts placed in naturalized annular tanks with circular flow. "Migratory urge" (downstream movement) was remotely monitored via PIT tags and gill Na+, K+ -ATPase activity was measured periodically as an index of smolt development. Migratory urge of both stocks was low in early April, increased 20-fold through late May, and subsided by the end of June. Migratory urge of Penobscot smolts increased with greater magnitude (up to 43% greater) than Dennys smolts early in the migration, but cumulative (seasonal) distribution of downstream movement was independent of stock. Gill Na+, K+ -ATPase activity of Dennys smolts was higher than Penobscot smolts in April, but otherwise similar. Gill Na+, K+ -ATPase activity of both groups increased in advance of migratory urge, declining while migratory urge was increasing. There was no clear relationship between migratory urge and enzyme activity at the individual or stock level. Maximum diel movement was observed from 2 h after sunset through 1 h after sunrise, but varied seasonally. Dennys smolts were slightly more nocturnal than Penobscot smolts. Penobscot and Dennys smolts were generally similar in the characteristics measured, and it is unclear if the observed modest differences could be responsible for the large differences observed in adult return rates.