Atlantic salmon's natural breeding habitat is rivers. With dams in the way they aren’t able to reach their spawning grounds. This is a major factor towards making it harder for them to repopulate.

Salmon are built to naturally be able to jump up small waterfalls, but when it comes to dams, it is too much for them. 

Even with fishways, the number of surviving salmon is drastically below what it should be. Although fishways allow fish like salmon to get through, they still change the river's ecology.

According to the National Oceanic and Atmosphere Administration,

“More than 90% of Maine's rivers and streams are affected by dams, which directly kill or injure a significant number of Atlantic salmon on upstream and downstream migrations.” 

Some of the reasons why dams are built are for energy, to control water flow, and to create artificial lakes. Dams tamper with the natural flow of rivers and their ecosystem, making it impossible for fish to get by. Over time dams may erode and pollute the waters beneath them. 

The warming climate pushes species to migrate further north than natural, leading them into unfamiliar and deeper waters. As well as this it leads them outside of the environment that they are naturally accommodated to.

 Climate change not only affects salmon but also their prey species. The warming waters push both species further out of their natural habitats. In some cases certain species move further north faster than others, throwing off the balance even more. Scientists call this a phonological mismatch. 

Atlantic salmon are very sensitive to bad water quality. Human interference and things like runoff put salmon at a much higher risk for disease and death. Man-made changes in rivers strongly affect salmon populations. The dams provide an obstacle for spawning and also change the river and the ecosystem, itself. 

Some things that can affect a river's quality are 

• dams

• bridges and roadways over rivers

• towns alongside rivers 

• runoff 

Many of these things not only pollute the water but also take away important aspects of a good-quality river like good water flow, healthy vegetation, and a proper predator-to-prey balance. Any of these factors being out of order can impact an ecosystem's quality negatively.

In Maine, salmon are protected by the government due to the extremely low number of adults that return to breed. Sadly, in other places they aren't as protected, hence the lack of returning adults. Atlantic salmon's range goes as far as Canada, Greenland, the Labrador Sea, and beyond. In some of these places, regulations aren't as strict, creating a conflict of interest.

Along with the overfishing of Atlantic salmon themselves, their food sources are also being overfished. Additionally, many salmon prey species are suffering from the same things that affect salmon.