Challenges for Diadromous Fishes in a Dynamic Global Environment

<em>Abstract</em>.-Understanding estuarine and early marine ecology of Atlantic salmon <em>Salmo salar </em>smolts within the relatively large and spatially dynamic environments of eastern Maine is challenging. Using ultrasonic transmitters and a large network of fixed receivers, we monitored natural smolt migration in the Narraguagus River, Narraguagus Bay, and coastal environment of the western Gulf of Maine. Our 30-km-long study area began in the lower river and extended 8 km downstream to head-of-tide, 7 km through the estuary, then fanned out seaward 15 km into the western gulf along the interface with Maine Coastal Current. From 1997 to 2004, we increased sampling network density in the estuary and expanded marine arrays further into the Gulf of Maine. We designed receiver networks to monitor all smolt exit routes and were able to (1) estimate smolt survival to the Gulf of Maine, (2) map primary migration paths, and (3) document emigration timing. Survival ranged from 36% to 47% to our outer receiver array. Median migration rates were 0.7 km/h in the estuary to middle bay and 1.0 km/h in the outer bay. Smolts generally traveled with the tides and upon entering saltwater most commonly used the western 6 km of a 23-km-wide embayment. These are among the first quantitative data to estimate survival during early marine migration of wild Atlantic salmon smolts. A Cormack- Jolly-Seber model estimated site efficiency and smolt survival simultaneously, providing a useful methodology and information benchmarks for other studies to better understand emigration dynamics and to help identify mortality factors at sea.