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.