Effects of Environmental and Anthropogenic Stressors on Chinook Salmon Oncorhynchus

Introduction:

Ecosystem degradation has become more prominent as the effects of anthropogenic climate change are increasingly impacting the structure and function of environments. Climate change and human disturbances have created a cascade of issues in aquatic ecosystems. Disturbances in aquatic ecosystems are widespread due to the connectivity that exists along the ever-changing waters of streams, rivers, and creeks. Increasing temperatures due to climate change are decreasing dissolved oxygen, changing species distribution, and interfering with migration patterns. In addition, humans have been directly impacting waterways for their own needs, including the construction of dams, eutrophication, and the incorporation of fish hatcheries.

Along with this, the development of watersheds, irrigation withdrawals, reduction of riparian cover, flood control, and navigation have added significant thermal inputs into waterways (Goniea et al., 2006). With an abundance of stressors, fish are struggling to combat a variety of pollutants and environmental changes. This paper seeks to analyze some of these stressors in regards to the conservation of salmonid species, Chinook salmon.

Anthropogenic and Environmental Stressors

Increased emission rates and greenhouse gas concentrations have been driving climate change and the warming of Earth. Elevated temperatures are at the heart of a lot of issues plaguing aquatic environments and salmonids. As temperatures increase, the availability of dissolved oxygen in waters is decreasing. Dissolved oxygen is essential to ecosystem function and for the proper metabolic function and survival of fish. Temperature changes and reductions in dissolved oxygen not only impact fish but also more importantly, and heavily impact aquatic invertebrates. This is important to understand due to the fact that invertebrates make up the bottom of the food chain and a change in their density impacts everything dependent on them, including fish. Increasing temperatures have been shown to influence physiological processes, such as metabolism, growth rates, fecundity, and emergence times for aquatic insects (Vannote & Sweeney, 1980). This could result in phenological mismatches and directly impacts fish such as Chinook salmon, due to their reliance on aquatic invertebrates for food availability.

In addition, rising temperatures are heavily changing species distribution and persistence (Goniea et al., 2006). Salmonids are cold-water stenotherms, which makes them especially vulnerable to increasing temperatures due to their thermal tolerances (Ruesch et al., 2012). As temperatures rise, many cold-water species, including Chinook, are retreating towards headwaters in refuge of colder waters. In fact, Chinook have a recorded 25 degrees Celsius upper incipient lethal temperature, which is the temperature at which mortality of the population is 50% after indefinite exposure (Brett, Clarke, & Shelbourn, 1982). Chinook threatened by warmer waters could change historic pathways in order to align their distribution and travel with colder waters. In the John Day River Basin, Chinook Salmon are projected to experience a 69-95% decrease in habitat following a moderate climate change scenario (Ruesch et al., 2012). Therefore, depending on how we move forward with climate change and pollution, we could see dramatic reductions in Chinook populations. As Chinook habitat declines, spawning grounds are being impacted and lost, which is making migration routes more difficult.

Anadromous Chinook are broadly distributed and have a large amount of variation in their migration strategies (Goniea et al., 2006). It is crucial for Chinook, therefore, to have optimal water temperatures over a large scale to accommodate for their journey. In one study on the Columbia River, migration paths were analyzed among warming temperatures in the river. With warmer waters, Chinook in the Columbia were more likely to utilize tributaries for their cold waters, which ultimately slowed or stopped migration (Goniea et al., 2006). In addition, warmer temperatures during migration were associated with heightened bioenergetic depletion and prespawn mortality (Goniea et al., 2006). Ultimately, delays in migrations can alter the timing of spawning and create stress, disease, and death (Richter & Kolmes, 2005). Migration is crucial to the life cycle of Chinook, due to the extent of their migration, Chinook are not only being impacted by rising temperatures but also pollution to streams by human hand.

There is no question that humans are causing increasing temperatures within aquatic ecosystems. Some of the stressors on aquatic environments are more directly related to human action. As the human population grows, so does agriculture, favoring management towards a more fruitful, efficient, and successful yield. Fertilizers have been implemented to help nutrient-lacking soils grow crops. However, with increased precipitation due to climate change, agricultural nutrient runoff is happening more frequently in these systems. This problem exists because fertilizers are applied too heavily, meaning the runoff picks up all the extra nutrients the plant doesn't need. This runoff, containing excess nutrients, enters rivers, streams, and creeks, causing eutrophication in our waterways. This leads to hypoxia, algal blooms, fish kills, and loss of habitat (Carpenter et al., 1998). Areas referred to as "dead zones" form due to the decreased oxygen in water caused by an overabundance of nitrogen and phosphorus, which kills most organisms. Eutrophication promotes a more homogenous ecosystem and heavily degrades the health of the ecosystem. This impacts Chinook, as eutrophication is causing all of these ecosystem degradations in their habitat, creating a number of additional stressors.

Dams, another human addition to waterways, have created a cascade of issues, especially for Chinook. Given that Chinook are anadromous, they travel extensive distances between the ocean and headwaters. On the Snake River, dams and impoundments have significantly delayed migration and impacted the survival of juvenile Chinook (Raymond, 1979). Dams not only delay migration, but also reduce the likelihood of fish reaching historic spawning sites. The prominent causes of death attributed to dams include passage through turbines, predation, slowed migration, and extended exposure to lethal concentrations of dissolved gases from dam spills (Raymond, 1979). Dams have also altered stream temperatures and caused a surplus of sedimentation issues. In conservation efforts, fish ladders have been implemented to aid fish passage. However, some of these fish ladders fail to adequately provide a proper system for fish to travel upstream, resulting in fish mortality.

In response to reduced fish populations, hatcheries have been constructed to combat these reductions and to alleviate stress on wild populations. If wild populations continue to be supplemented by hatchery-reared fish, genetic interactions between wild and artificial fish will increase (Waples, 1991). There are both indirect and direct impacts as hatchery-reared fish infiltrate wild populations, raising concerns. Hybridization of different gene pools and introgression directly impact Chinook, and predation, disease, and competition all indirectly hurt wild populations (Waples, 1991). As artificial Chinook compensate for the reduced wild populations, genetic variability will decrease, creating a more homogenous gene pool. There is also evidence that adult salmonids are less viable and stray from normal migration patterns (Quinn, 1993). The incorporation of artificial fish will create an entirely new gene pool, which may not function as well as the wild populations.

Conservation

In the end, humans are either directly or indirectly the cause of these issues that Chinook are facing. Chinook habitat will continue to be reduced while stressors will amplify and multiply as temperatures continue to increase. Conservation efforts are needed to help alleviate the grim future that these fish face. The issues, more directly created by humans, are easier to implement such as eutrophication and dams. Solutions exist to combat eutrophication, including the incorporation of riparian vegetation and matching fertilizer application to plants' needs to avoid excess nutrients in the soil (Carpenter et al., 1998). Along with this, the reduction of cattle grazing on riparian vegetation and increased complexity of streams will help keep temperatures from rising (Ruesch et al., 2012). Riparian vegetation is key to conservation, as vegetation not only increases complexity and shading but also can help decrease runoff, erosion, and can take up toxins before they reach the water. Bank stability is very important to streams and rivers and is greatly aided by vegetation that keeps the stream structured while providing habitat for fish. The benefits of vegetation are widespread in river conservation and can help to solve almost all of the issues stated above.

The removal of dams is another huge solution to help decrease stream temperatures and would increase stream complexity. If action were taken with all of these things, wild populations wouldn't need the aid of hatchery fish, eliminating all of the problems associated with artificial fish. Dam removal would help to restore connectivity, decrease fish mortality, allow for fish passage, and historic spawning grounds to be reached again. This is especially crucial if fish habitats are being reduced, as they need unblocked access to cooler headwaters.

Conclusion

The connectivity and complexity of rivers have yet to be fully understood. Even with conservation, the path we are traveling down with emissions and pollution is not in favor of the health of natural environments. Chinook salmon will surely experience a reduction in wild populations, along with other species of fish. The conservation issues discussed in this paper are not specific to Chinook and will impact all life in aquatic ecosystems. Biodiversity will continue to decline, creating a more homogenous natural world. Humans are continually redefining the natural world, and if our values don't align with these ecosystems, they will continue to degrade.

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