Assessing Estuarine and Coastal Migration and Survival of Age-1 Hatchery Atlantic Salmon Smolts from the Dennys River, Maine (USA) Using Acoustic Telemetry

The Dennys River Atlantic salmon stock is at the northern extent of the endangered Gulf of Maine Distinct Population Segment’s range.  Although the stock once supported a prominent US salmon rod fishery, the population has since collapsed which is thought to be the result of several factors including dams, pollution, overfishing, and poor marine survival. A broodstock hatchery restoration program (fry and smolt stocking) has been in effect since the early 1990s, but this has produced only low numbers of returning adults; since 2001, annual adult returns have averaged just 7 fish. To better understand the reason for the poor returns, we used acoustic telemetry from 2001-2005 to assess estuarine and coastal migration and survival of age -1 hatchery smolts (n=70-150). The fish we tracked averaged 183.2 mm (±12.9) fork length and 68.8 g (±15.1) wet weight. Most smolts (70%) made multiple reversals within the estuary and bay during outmigration; at least 30% of smolts made 10 or more reversals.  This behavior prolongs estuarine and bay residency times and may be conditioning to the marine environment, but it may also result in migration delays and increased exposure predators.  During each of the five years, we observed significant mortality almost immediately after smolts entered the marine environment.  Our estimate of loss from the estuary to early bay ranged from 36 to 83%.   These early marine losses are higher than documented in many other systems and not surprisingly these rivers have higher return rates.  With few post-smolts making it to the Gulf of Maine and Bay of Fundy, recovery of this stock will be challenging.