Size Selection of Adult Atlantic Salmon at Fish Passage Facilities on the Penobscot River, Maine

Successful upstream passage of Atlantic salmon through hydropower dams is critical to the long-term stability of sea-run populations. Large, multi-sea winter fish may be less able than smaller salmon to successfully surmount these dams. Passage success of adult Atlantic salmon was investigated at six fishways at hydropower dams in the lower Penobscot River. In-migrating adults were tagged with Passive Integrated Transponders (PIT) at the most seaward dam on the Penobscot River from 2002-2004 and 2010-2012 (n = 4,610). Salmon passage at passage facilities was monitored using PIT antenna arrays. For each dam, generalized linear modeling was used to analyze relationships between passage success and factors thought to influence passage. Environmental variables such as water temperature and river height as well as arrival date in the river influenced passage success, with fish arriving during midsummer, warm, low flow conditions exhibiting the lowest passage success. At passage facilities in the lower river, success was negatively related to fork length, meaning that those facilities may have been exerting size-selective pressure. Coupled with the additive effects of having to navigate through multiple fishways within the Penobscot system, size selection at multiple passage facilities may effectively cull the largest fish from each year’s spawning cohort.