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Adult returns to many Atlantic salmon wild and hatchery stocks of the North Atlantic have declined or collapsed since 1985. Enhancement, commercial fishery closures, and angling restrictions have failed to halt the decline. Human impacts such as dams, pollution or marine overexploitation were responsible for some stock declines in the past, but adult returns to river and hatchery stocks with no obvious local impacts have also declined or collapsed since 1985. Multiple studies have postulated that the recent widespread occurrence of low adult returns may be caused by climate change, salmon farming, food availability at sea, or marine predators but these possibilities are unsupported by stocks that persist near historic levels, loss of stocks remote from farm sites, a diverse marine prey field, and scarcity of large offshore predators. The decline and collapse of stocks has common characteristics: 1) cyclic annual adult returns cease, 2) annual adult returns flatline, 3) adult mean size declines, and 4) stock collapses occurred earliest among watersheds distant from the North Atlantic Sub-polar Gyre (NASpG). Cyclic annual adult returns were common to all stocks in the past that were not impacted by anthropogenic changes to their natal streams. A flatline of adult abundance and reduction in adult mean size are common characteristics of many overexploited fish stocks and suggest illegal, unreported, and unregulated (IUU) fisheries exploitation at sea. Distance from the NASpG causing higher mortality of migrating post-smolts would increase the potential for collapse of these stocks from IUU exploitation. By-catch of post-smolts and adults in paired-trawl fisheries off Europe and intercept adult fisheries off Greenland, in the Gulf of St. Lawrence, and off Europe have been sources of marine mortality but seem unlikely to be the primary cause of the decline. Distribution in time and space of former, legal high-sea fisheries indicated fishers were well acquainted with the ocean migratory pattern of salmon and combined with lack of surveillance since 1985 outside Exclusive Economic Zones or in remote northern regions may mean high at-sea mortality occurs because of IUU fisheries. The problem of IUU ocean fisheries is acute, has collapsed numerous stocks of desired species worldwide, and is probably linked to the decline and impending collapse of the North Atlantic salmon population.

The Decline and Impending Collapse of the Atlantic Salmon ( Salmo salar ) Population in the North Atlantic Ocean: A Review of Possible Causes