The Atlantic salmon has shown a steady decline over the last two centuries, seemingly related to increased industrial development throughout their traditional home range. The situation has become drastically worse since the 1970s and catches of wild salmon have fallen by 80 percent. River pollution caused by industrialisation can severely damage local populations as can the increased number of man-made obstacles, such as dams, weirs or the alteration of watercourses, which makes migration impossible.
Salmon has become an extremely popular dish in the western world and commercial farming can affect wild populations in a number of ways; escaped salmon may erode the gene pool through interbreeding, or farms may act as foci for the spread of parasites and diseases to wild stocks.
A further identified threat to the Atlantic salmon is global climate change. As the species’ developmental rate is directly related to water temperature, it is possible that increasing temperatures could result in more rapidly developing juveniles entering the ocean before their planktonic food source has reached sufficiently high levels.
Increasing freshwater temperatures could also create a thermal barrier to migrating salmon, as hotter waters require additional energy to navigate around. Such barriers can also delay or even prevent spawning. In addition, as air temperatures warm, much of the snow that feeds the river systems is expected to melt earlier. This will lead to a reduction in the flow of many rivers in the spring and summer, which will increase water temperatures further and is likely to reduce the overall habitat available to the Atlantic salmon.
In the Atlantic salmon’s marine habitat, climate change may disrupt many food webs. For example, the timing of the planktonic blooms required by the young salmon is governed by climatic factors. Changes in the timing of these blooms could cause a scarcity of food at a critical stage of the salmon’s life cycle.