The impact of marine fish predation on Atlantic salmon smolts ( Salmo salar) in the Tana estuary, North Norway, in the presence of an alternative prey, lesser sandeel ( Ammodytes marinus)

Abstract

The Tana river, in northern Norway has the largest wild Atlantic salmon stock in the world. Each summer more than 1 million smolts, representing nearly 20% of the total number of Atlantic salmon smolts produced annually in Norwegian rivers, descend through the large estuary of this river. Several marine fishes feed in the estuary during the summer months, and because the descending smolts are suitable prey, predation by these species was expected to be significant for the survival of the smolts. In 2000 (mid-June to early August), nearly 500 cod ( Gadus morhua L.), saithe ( Pollachius virens L.), whiting ( Merlangius merlangus), haddock ( Melanogrammus aeglefinus) and sea trout ( Salmo trutta) were caught by hand jigging and troll fishing in the estuary and inner part of the Tana fjord before, during and after the smolt run. Based on analyses of the fish stomach contents, lesser sandeel ( Ammodytes marinus) was found to be the dominant prey, while no remains of Atlantic salmon, including otoliths, were found. We conclude that neither marine fishes nor anadromous trout are significant predators on salmon smolts in the estuary and inner fjord, probably due to a large abundance of lesser sandeel. Sandeel may thus be a key factor for the status of the Tana river as one of the world best Atlantic salmon rivers, by reducing the high smolt and post-smolt mortality as observed in several other estuaries and fjords.