Abstract
Little empirical evidence exists of the biology of Atlantic salmon (Salmo salar L.) as they enter the marine habitat or of the factors which influence their subsequent survival and migration. This is primarily due to the difficulties in locating and capturing individuals during this transit period. Mortality in the early marine stages of migration is thought to be high, reported mortality rates greater than 5% km-1 are not uncommon in estuaries and average approximately 1% km-1 in the early marine phase of migration (Thorstad et al., 2012). Reported mortality of smolts is highly variable and application of mortality rates derived from one site and applied to another should be done with caution. Currently there is no estimate of mortality of migrating smolts in estuaries or the marine environment within Scotland. Smolt distribution at sea has previously been inferred from the recapture of fish in surface trawls within the Atlantic Ocean (NASCO, 2011). It remains to be seen if the capture of a few hundred individuals accurately represents the movements of, potentially, millions of individuals leaving European rivers each spring. As such, there is a keen interest in determining the offshore movements of fish as they enter the open ocean. Indeed, the first priority research need listed by Marine Scotland Science (MSS) in its national monitoring strategy for diadromous fish is “What routes and depths do salmon smolts use as they leave Scotland?” (The Scottish Government, 2017)