Simulating Migration Mortality of Atlantic Salmon Smolts in the Merrimack River

Abstract

Successful restoration of Atlantic salmon Salmo salar to New England rivers involves the identification and management of mortality sources at different life history stages. The purpose of this study was to examine the effects of mortality during migration on Atlantic salmon smolts exiting the Merrimack River. Our objective was to review data pertaining to smolt production, migration, passage at hydroelectric facilities, and predation in the Merrimack River and construct a simulation model of smolt migration. We constructed a migration model incorporating river-flow-based decision rules affecting migration rate, delay at dams, dam passage mortality, and migration mortality. Mean model estimates of in-river survival ranged from 0.7% to 23.5%. Estimated transit times generally increased in migration scenarios in which smolts began migration later in the season; beginning migration later in the season also resulted in lower in-river survival. The model was evaluated by comparing records of returns of two-seawinter adults to the Merrimack River to a likely range of marine survival rates. For 9 of 14 smolt years, model estimates for the number of smolts exiting the river were comparable with the range of smolt output necessary to achieve the corresponding adult returns. Model estimates of in-river survival that fell below the the lower threshold for 5 of the 14 smolt years could be explained in part by relatively high marine survival experienced by these cohorts. We argue that this model can have important applications in population assessment, river management, and salmon restoration.