Cooperative Fish and Wildlife Research Units Program: Maine
Education, Research and Technical Assistance for Managing Our Natural Resources

Maine Project

Does predation limit Atlantic salmon recovery?

January 2021 - December 2025


Participating Agencies

  • NOAA

Among migratory fish, Atlantic salmon have been culturally, economically and ecologically important. These fish have also been impacted by a suite of threats and the Gulf of Maine DPS Atlantic salmon were added to the federal Species of Concern list in 1997 and the Endangered Species list in 2000. In 2009, Atlantic salmon returning to the Penobscot, Kennebec, and Androscoggin Rivers were included in the Gulf of Maine DPS as a federally endangered species.

Atlantic salmon are native to the northern Atlantic Ocean and its drainages. The anadromous form hatches from eggs deposited in cold headwater streams where it develops into a small, territorial drift-feeding fish (parr). As territorial juveniles in freshwater, Atlantic salmon parr may compete for resources with native and introduced species, limiting their access to resources (Callaway and Aschehoug 2000) and decreasing survival (Nakano et al. 1998; Gunckel et al. 2002). Smallmouth bass are notable as they were introduced into New England waters more than a century ago and have expanded into nearly every historical Atlantic salmon watershed (Warner 2005; Valois et al. 2009). Where they overlap, introduced smallmouth bass may compete with Atlantic salmon for physical habitat and food (Fausch 1998; Coghlan and Ringler 2005). Atlantic salmon populations may also suffer from direct predation by smallmouth bass in fresh water.

The stream-dwelling parr undergoes a complex suite of behavioral, morphological and physiological changes resulting in a migratory smolt (McCormick and Saunders 1987). The parr-smolt transition is a critical transitional stage in the life history of this species. Successful transition into the marine environment is thought to occur during a “window of opportunity,” when physiological condition is optimal for survival (McCormick and Saunders 1987). Poor synchrony can result in high mortality in the estuary (McCormick et al. 1999; Stich et al., 2015) or at sea (Virtanen et al., 1991; Staurnes et al., 1993; Stich et al., 2014). As smolts leave rearing habitats and migrate seaward, they encounter geomorphic barriers, changing flow, varying turbidity, and a gauntlet of predators. Atlantic salmon currently travel through waters with a diverse array of nonnative resident fishes, including brown trout, largemouth bass, smallmouth bass, and northern pike. Migration is a time of high risk of predation (Blackwell et al. 1997; Kocik et al. 2009; Hawkes et al. 2013). High mortality of smolts is often observed in the estuaries (Kocik et al., 2009; Holbrook et al., 2011) and near-coastal waters (Lacroix, 2008; Dempson et al., 2011; Thorstad et al., 2012a; 2012b). The proposed work is aimed at better characterizing the role predation may have during their early life history.