Swimming Depth, Behavior, and Survival of Atlantic Salmon Postsmolts in Penobscot Bay, Maine

To gain information on postsmolt dynamics of emigrating Atlantic salmon Salmo salar through Penobscot Estuary and Penobscot Bay, Maine, we conducted a telemetry experiment in 2005. We implanted 26 salmon smolts with ultrasonic depth tags, and monitored movement activity and fish passage with linear detection arrays through 44.2 km of the estuary and 45.5 km of the bay. During daylight in the bay, greater than 95% of the detections occurred in water depths of 5 m or less, but depths to 37 m were recorded. At night, 99% of the detections were in the top 5 m of the water column and maximum depth was 9 m. Overall survival was 39% and was highest for smaller fish and those released earlier in the smolt run, when river discharge was greater. Rapid emigration (i.e., approximately 1 km/h) and preferential surface orientation improved survival. These results verify that postsmolts are primarily surface oriented in the waters of Penobscot Bay and that they may experience high rates of nearshore mortality despite their short residence time. Detailed emigration and behavioral data such as these allow scientists and managers to delineate areas of high mortality to develop strategies that improve survival, and provide marine spatial planners information to minimize impacts of coastal zone development.