What's depleting salmon populations?

­

While a variety of fact­ors impacts salmon population­s, four tend to stand out. Although there are only six species of salmon -- five Pacific and one Atlantic -- each individual stock, or population, is genetically unique and faces challenges specific to its region, making it difficult to label any one of the four Hs more important.

Two of the H factors are somewhat interrelated. The first one, harvests, refers to the commercial, recreational and tribal fishing of salmon -- either out at sea or as they migrate back upstream. Until recently, many governments have failed to put appropriate limits on the number of salmon that can be reasonably caught each year. For example, in Canada, when around 8 million salmon returned from the ocean to spawn in 1917, the government still allowed a catch of more than 7.3 million fish [source: Hume].

Overfishing has led to record low levels of the salmon almost everywhere they're caught. An exception is in Alaska, where well-developed management plans give sustainability a high priority.

­The severe salmon depletions prompted by overfishing have led to a subsequent increase in fish hatcheries, the next big H, to replenish the stocks. Initially created to enhance salmon numbers, these fish nurseries artificially raise young salmon until they're self-supporting and then release them into the ocean. Similar facilities now exist solely for food production and are referred to as fish farms.

Ironically, the hatcheries and farms that popped up to alleviate pressure on wild salmon populations are now endangering them. Research shows that situating fish farms near wild salmon populations can cause declines of more than 50 percent in the wild fish [source: Owen]. Both hatcheries and farms may introduce weaker genes into the wild population's gene pool through interbreeding, thus lowering salmons' chances for survival. In addition, the artificial structures are more conducive to parasites and disease, which then infect wild populations. Competition is also a factor.

Both harvest and hatchery problems can easily be fixed. With proper management and controls, populations can and do improve. Better quality fish cages, for instance, can prevent physical contact between the fish, and stricter controls on harvest quotas can help deter overfishing.

The next two Hs, also man-made, have to do with where salmon live.

The health of salmon populat­ions depends a lot on the health of their living environment. The fact that salmon live in a variety of habitats throughout their lives makes them especially vulnerable to the next H -- habitat degradation. Even small disturbances can make a big difference, since spawning is highly sensitive to things like increased sedimentation. Logging, agricultural practices, trash dumping and oil spills all contribute to poor water quality.

Another factor that can disrupt habitat is the final H, hydropower dams. The Clearwater coho salmon, once abundant in the Snake River Basin of Oregon, Washington and Idaho, became extinct in the early 1960s largely due to the construction of a dam that blocked the fish's passage to and from its birthing grounds [source: Northwest Power & Conservation Council]. Besides potentially blocking access to salmons' habitats, dams may cause changes to water flows and temperatures that can devastate local populations.

Dams aren't necessarily a death knell for salmon, however. Researchers tracking salmon on the dam-laden Columbia River concluded it wasn't the dams impeding their survival but challenges they face at sea including predation, ocean warming and alterations to their prey distribution [source: Owen]. The study showed that with proper modifications to assist the salmon in their migration, dams and salmon can coexist.

The four Hs -- harvests, hatcheries, habitat and hydropower -- aren't problematic in and of themselves. It's only when they're mismanaged that things start to go downhill. While some populations have dropped so low they will take years to restore, if at all, others can be saved from following the path of the Clearwater coho.

­