Primary Driver: Susceptibility of Fish to
This primary driver is affected by the five secondary drivers shown above.
The effects of low DO concentrations on fish depend on many interacting factors. Once fish are exposed to potentially harmful low DO concentrations, a number of drivers can influence the severity of these effects. These drivers include effects of reduced DO concentrations on a speciesí food web and their exposure to parasites, pathogens, toxic substances, and high water temperatures. Additionally, the activity of fish while in the DWSC can have an important role in their demand for oxygen and thus how severely they will be affected.
The effects of low DO concentrations on trophic transfer may be highly complex and variable among systems and between different predator/prey species assemblages. It is important to consider impacts on species based on species-specific biology and trophic position because each species may be affected differently, depending on the effects of low DO concentrations on a particular prey item. Low DO concentrations in the DWSC have the potential to alter both the abundance and the composition of food resources in this area, subsequently affecting the fish that depend on them. Low DO concentrations may have a different impact on prey availability, depending on the prey species and the predators that feed on them. Apart from causing outright mortality of prey species, low DO concentrations may make prey species unavailable to predators.
Parasites and pathogens alone are known to create significant changes in the reproduction, survival, and growth of individual fish. Although it may be difficult to separate the combined effects of multiple stressors acting simultaneously, combining low DO concentrations with parasites and pathogens likely exacerbates negative effects. Stress conditions such as low DO concentrations increase the susceptibility of fish to infectious diseases and parasites. Parasites and diseases may have both lethal and sublethal effects, but the sublethal effects are usually of greater importance than the lethal effects (direct mortality) to animals in an ecological context. Such ecological consequences may include changes in reproduction, survival, and growth that can reduce individual fitness and perhaps even lead to population-level effects. One of the side effects associated with disease and parasitism may be an increased metabolic rate of fish as they attempt to fight infection, which subsequently will increase DO demands. The compounding impacts of multiple stressors elicit significant physiological and behavioral responses (Sigismondi and Weber 1988; Mesa and Schreck 1989; Jarvi 1990 in Mesa 1994) that may result in increased rates of mortality (Mesa 1994). In this respect, it becomes reasonable to expect fish to be increasingly susceptible to parasites and pathogens while under other physiological stresses.
Large volumes of saline subsurface agricultural drainwater enters the San Joaquin River each year that includes ions of sodium and sulfate in addition to chromium, mercury, selenium, and other trace elements in concentrations near or exceeding maximum limits of the EPA for protecting aquatic life (Saiki et al. 1992). Toxic substances and reduced oxygen levels change the physiology of fish and can affect the function and behavior of striped bass in the field (Cox and Coutant 1981). In general, organisms living near their environmental tolerance limits (such as low DO) are more susceptible to additional chemical stress, especially when exacerbated by increased temperatures or low food supplies (Heugens et al. 2002).
As water temperature increases, the solubility of oxygen in that water decreases, so that warmer water becomes saturated with oxygen at lower concentrations (Moyle and Cech 2000). Higher water temperatures simultaneously elevate metabolic rates, so fish require more oxygen at warmer temperatures, when DO is less available. However, the relationship between temperature and DO concentrations and its effects on fish is not always linear and predictable. Cech et al. (1984) described temperature as a controlling factor, while DO concentration was a limiting factor, suggesting that fish may survive and grow in warm waters, provided that DO concentrations are sufficient. Harmful effects of low DO concentrations will be amplified when combined with temperature-related stress.
Species and life stage tolerances reflect adaptations to different environments and differences in metabolic requirements, activity levels, size, and morphology. Fish species found in the DWSC are often migrating to and from spawning and feeding grounds. Increased activity levels associated with migration result in greater oxygen demands to sustain higher metabolic rates. Low DO concentrations may impair fish activity levels, which can, in turn, affect many aspects of behavior, including migration, feeding, and predator avoidance.