Causes of Declining Salmon Populations

Illustration of Pacific Salmon Life Cycle by Peter Lynde (11).

Salmon are migratory fish with unique life cycles.  Spawners, which are mature adult salmon that are ready create offspring, spawn/mate in fresh water tributaries.  The female would release her eggs on the gravel and the male would come fertilize it by releasing its sperm.  These fertilized eggs would hatch into alevins, which are in a stage where they still live near the gravel and rely on the yolk for nutrients.  Once the yolk is used up, alevins emerge from the gravel as fry.  These fry stay in the freshwater habitat and then migrate to marine waters as smolts. The smolts would then grow into large adults in the open sea and migrate back to freshwater tributaries to spawn. And, once the spawners spawn, they come to the end of their life cycle and their decaying bodies up in the spawning grounds replenish the freshwater ecosystem with nutrients from their bodies obtained previously from when they grew in the oceans (10). Derived from Table 7 on p. 19 in Gresh et. al 2000 study, Figure 1 represents overall reduction in the amount of marine-derived nutrients, which includes Nitrogen and Phosphorous, in the freshwater ecosystem by comparing results from a time before decline in salmon to the current time (2). This indicates how crucial of a role salmon has on ecosystems, especially the ones surrounding the spawning grounds.  

Fig. 1 Estimates on marine-derived nutrients
in Pacific Northwest Rivers from previous
times (before dramatic decline of Pacific
Salmon) and current times.

Unfortunately, these migratory fish that are found around the world, are facing great risks of extinction due to anthropogenic activities. The two most detrimental activities are overfishing and urban development.  Overfishing is fishing to a point where fish populations cannot sustain themselves.  Salmon have been fished at alarming rates due to the advancement of technology, high demand, and high commercial value (8).  This has led to declining salmon populations that are unable to sustain themselves.  Urban development also has a huge impact on salmon populations due to habitat degradation and blockage of the migratory pathways of salmon with dams.

Though overfishing is an issue that local communities face, the impact of this issue is on a global scale because overfishing not only affects the entire marine ecosystem but also economic relations among countries (8). The increasing global demand for fish gives local fishers a strong incentive to keep fishing through any method and rate that has a high yield, regardless of negative impact on future stocks and ecosystem (8). The scientific, social, and ethical aspects of fisheries are crucial to know and understand (1) in order to address overfishing properly because these aspects are interrelated and these aspects combined provide a comprehensive explanation of what this issue really means in our world right now.