Atlantic Salmon Spawning Migrations in the Penobscot River, Maine: Fishways, Flows and High Temperatures


S L. Shepard




adult, Atlantic salmon, attraction, Bangor Hydro-Electric, entrance, fish passage, high velocity, hydroelectric, hydroelectric dams, impoundments, Maine, migration, mortality, Penobscot River, releases, restoration, Salmo salar, salmon, spawning, upstream, upstream passage


University of Maine


Restoration of Atlantic salmon, Salmo salar, to the Penobscot River in Maine has met with limited success to date. Research in freshwater has emphasized migration problems created by dams. Fish passage research was initiated jointly by the Maine Atlantic Sea Run Salmon Commission and the Bangor Hydro-Electric Company in 1987. This research was later broadened to examine environmental issues of flow and temperature. From 188 through 1992, radio transmitters were placed in the stomachs of 108 returning adult Atlantic salmon which were then released at several locations in the lower Penobscot River. Radio-tagged Atlantic salmon were monitored with receivers and data loggers at six hydroelectric dams and by mobile tracking from airplanes, trucks, and boats. Radio-tagged Atlantic salmon made no net upstream progress for weeks at a time during the summer. When directed upstream movements were initiated, salmon passed through multiple fishways and often made net progress of 20 to 40 km in five to ten days. Atlantic salmon passed through fishways below 23ºC, but higher temperatures were associated with fewer radio-tagged Atlantic salmon passages and reduced captures at the Veazie Dam fishway trap. Above a threshold of approximately 23ºC, increasing temperatures caused Atlantic salmon to cease movement and seek refuge in stream mouths where they usually remained until high temperatures abated. Daily average temperatures of 26ºC to 27ºC were associated with salmon mortalities. When radio-tagged Atlantic salmon migrated upstream to the Piscataquis and Mattawamkeag rivers, migrations often ceased again, apparently due to low flows in these unregulated tributaries, Main stem and East Branch flows were artificially high due to releases of stored water and there was no indication that migrations were impeded by the lowest flows observed in those areas. In contrast, high main stem flows may have inhibited upstream passage at lower Penobscot River dam as a result of attraction to spillway flows which compete with turbine flows where fishway entrances were typically located. Low velocity areas such as impoundments were not suitable holding habitat and radio-tagged Atlantic salmon moved rapidly through these areas. Radio-tagged Atlantic salmon were generally found in areas of moderate to high velocity and moderate to shallow depth.





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