Salmon's Struggle: Pollution's Impact On Their Survival

Salmon populations are declining due to a variety of factors, including pollution. Climate change, for example, is threatening the cold, clean, and abundant water in rivers that salmon need to survive. Human activities have led to increased carbon dioxide levels, which have in turn increased ocean acidity and damaged the food sources salmon rely on. Additionally, warming temperatures and habitat loss are detrimental to salmon abundance and survival. Water pollution, specifically, has been identified as a major cause of the decline in Atlantic salmon stocks, affecting all life stages of the fish.

Climate change

Salmon are integral to the ecosystem and the culture of the Pacific Northwest. They are considered a "keystone species" by scientists due to the benefits they provide to aquatic and terrestrial ecosystems. Climate change poses a significant threat to the survival of salmon populations, particularly in the Pacific Northwest region of the United States.

One of the primary ways climate change affects salmon is by altering their aquatic habitats. Rising temperatures increase water temperatures, which is detrimental to salmon as they depend on cold, well-oxygenated water. Glacier melt and reduced snowpacks further contribute to lower water levels in streams and rivers, impacting the survival of young salmon during critical life stages. Warmer waters also favour sub-tropical zooplankton, which are poor food sources for juvenile salmon.

Additionally, climate change affects the timing and availability of food sources for salmon. Some salmon migrations coincide with high spring runoff from melting mountain snow, while juvenile salmon return to the sea in sync with seasonal plankton blooms. Climate change disrupts these cycles, making it harder for salmon to find enough food.

The combination of warming waters, reduced food sources, and altered environmental conditions can lead to significant changes in the ocean, potentially resulting in a catastrophic failure of salmon populations.

To address these challenges, adaptation strategies include removing barriers to upstream migration, floodplain reconnection, restoring stream flow regimes, increasing cold-water refuges, and enhancing riparian forest cover. Conservation efforts must focus on maintaining connected habitats and ensuring the availability of cold, well-oxygenated water to support salmon survival.

Industrial pollution

Industrial salmon farming is another source of pollution. Massive floating salmon pens in places like Machias Bay, Maine, release waste directly into the water. A typical industrial farm with several hundred thousand fish produces around one million pounds of waste annually, which is roughly equivalent to the sewage generated by a large city. Unlike sewage, however, this waste is not treated and instead accumulates on the ocean floor, creating a toxic environment for bottom-dwelling creatures. Industrial salmon farms also contribute to nitrogen pollution, which, when combined with warm water, creates favourable conditions for toxic algae outbreaks. These outbreaks poison fish, turtles, and shellfish.

Regulators have been slow to address these issues, and in some cases, have failed to enforce or implement necessary regulations. For example, the US Environmental Protection Agency (EPA) has allowed cleanups to languish, and state regulators have been slow to rein in industrial pollution. Similarly, Maine's fish farm regulations have been criticised for their lack of oversight, with no limits on waste or nitrogen pollution and no requirements for monitoring water quality impacts.

Parasites

Salmon are susceptible to a range of parasites, and pollution can increase the prevalence and intensity of these parasites in salmon populations.

Henneguya salminicola, a myxozoan parasite, is commonly found in the flesh of Oncorhynchus species of salmon. It has been observed in salmon returning to the Queen Charlotte Islands, where the parasite is walled off by the fish into cysts containing milky fluid, which is an accumulation of a large number of parasites. The Henneguya parasite has a complex life cycle, with the salmon being one of two hosts. After spawning, the fish releases the spores, which enter an invertebrate host in the spawning stream. When juvenile salmon migrate to the ocean, the second host releases a stage of the parasite that infects the salmon, which is carried until the next spawning cycle. While the Henneguya parasite does not appear to cause significant harm to the host salmon, it is still detrimental to the economic viability of salmon farming.

In addition to Henneguya, other parasites that affect salmon include sea lice, Argulus species, Gyrodactylus, and Myxobolus cerebralis, which causes whirling disease in salmon. Salmon are also susceptible to endoparasites such as myxozoons, microsporans, and haematozoons, which proliferate within their hosts.

Pollution can have a significant impact on the prevalence and intensity of parasitism in salmon populations. Chronic exposure to pollutants can cause biochemical, physiological, and behavioural changes in the host, impairing their immune response and making them more susceptible to parasites. Pollutants may also favour the survival and reproduction of intermediate hosts. For example, studies have shown that exposure to pollutants increases the number of ectoparasites such as trichodinid ciliates and monogeneans on the gills of fish. Additionally, endoparasitic protozoons such as myxozoons, microsporans, and haematozoons have been found to increase substantially in prevalence and intensity when interacting with pollutants.

The impact of parasites on salmon populations is a complex issue that requires further research to fully understand the cause-and-effect relationships between pollution, parasitism, and the health of salmon populations.

Habitat degradation

Human activities have led to habitat degradation, which is a significant threat to salmon populations. Intensive agriculture, gravel extraction, commercial forestry, and substrate removal for drainage schemes are some of the practices that alter the structure of rivers, increase sedimentation, and reduce the quality of salmon habitats. Water extraction and hydro-regulation also impact salmon by altering the hydrology of rivers, affecting their flow, temperature, and quality. This, in turn, has negative consequences for the productivity of salmon populations.

The construction of dams, weirs, barriers, and other "food prevention" measures severely impact river habitats and the accessibility of those habitats to salmon. This is particularly evident in the Pacific Northwest, where numerous dams have been built in river systems, including over 400 in the Columbia River Basin. These structures impede the natural migration of salmon and disrupt their life cycle, even with the presence of fishways intended to help them bypass these obstacles.

Modern farming methods and various sources of pollution have also contributed to habitat degradation. The loss of invertebrate diversity and population density in rivers reduces the availability of food for salmon. Additionally, agricultural practices such as removing riparian plants and destabilizing stream banks through irrigation and livestock grazing result in a loss of suitable low-gradient stream habitats for salmon.

The impact of habitat degradation is further exacerbated by climate change, which affects both the marine and freshwater phases of the salmon life cycle. Rising temperatures influence the southern range of salmon populations and their ability to survive. Climate change also affects the quality and availability of freshwater habitats, as glaciers and mountain snowpacks, which provide cold water to streams, are vanishing due to increasing temperatures.

Overall, human activities that lead to habitat degradation have severe consequences for salmon populations, disrupting their life cycle, reducing their food sources, and degrading their spawning grounds. Addressing these issues is crucial for the conservation and recovery of salmon populations.

Migration barriers

Salmon face many challenges during their arduous migration from river to sea and back. One of the most significant obstacles is the presence of man-made barriers, such as weirs, locks, hydroelectric projects, culverts, and tidal barrages, which block their path and obstruct their progress upstream. These barriers can have a detrimental impact on the salmon population by preventing them from reaching their spawning grounds.

In their upstream journey, salmon need to swim through oceans and rivers to reach their native habitats for reproduction. However, engineered structures like dams and culverts often block their path, making it difficult or even impossible for them to complete their journey. This has severe consequences for the salmon population, as their inability to reach their habitat directly affects their ability to reproduce and rebuild their numbers.

Dams, for instance, can generate hydroelectric power and provide irrigation, but they also pose a significant challenge to salmon migration. Hydropower dams, in particular, can prevent fish from swimming upstream to their breeding grounds. Similarly, culverts, which are tunnels that carry streams under roads, can become barriers when they are too small, too steep, or perched too high for salmon to pass through.

The impact of these barriers is evident in the decline of salmon populations. For example, the Atlantic salmon, once found in every river north of the Hudson River, now has less than 0.1% of its historic population remaining in U.S. waters due to the presence of dams and other threats. These reduced fish populations have far-reaching effects on the entire ecosystem, as salmon are an important prey for other animals and play a crucial role in commercial and recreational fisheries.

Recognizing the significance of this issue, organizations like the NOAA Fisheries are taking action to improve fish migration. They work with various partners to protect and restore river habitats, evaluate specific conservation actions, and, in some cases, completely remove or bypass barriers to open up rivers and streams for salmon migration.

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