|
Current location: Home > Biological Effects Model > Adverse Effects > Indirect Effects |
                |
Adverse Effect: Indirect Effects Jump down this page to: Steelhead | Chinook Salmon | Delta Smelt | Longfin Smelt | Sacramento Splittail | White Sturgeon | Green Sturgeon | Striped Bass Susceptibility to Predation All fishes have an incipient limiting threshold for DO below which they experience a decline in the ability to perform certain activities and functions. Exposure to low DO concentrations can increase the susceptibility of fish to predation by altering normal behavior, reducing activity levels, and reducing swimming performance. Prolonged or frequent exposure to hypoxia may reduce growth rates enough to reduce the size of fish and thereby increase the period of time that fish are vulnerable to predators. Susceptibility to Parasites/Pathogens Environmental conditions, including natural and anthropogenic stressors, can heavily influence the parasite-host interaction because they regulate the physiological condition of both the host and the parasite (Lenihan et al. 1999). Stressful environmental conditions, such as low DO concentrations, can increase the susceptibility of fish to infectious diseases and parasites. The compounding effects 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). Parasites and pathogens alone are known to cause significant changes in reproduction, survival, and growth of individual fish. Affected fish often become debilitated, reproduce less, and become more susceptible to predation and less able to tolerate environmental extremes (Krebs 2001). Although it may be difficult to separate the combined effects of multiple stressors acting simultaneously, combining low DO concentrations with parasites and pathogens likely amplifies negative effects. Susceptibility to Contaminants Fish species may be negatively affected by chemical pollution in urban or agricultural runoff (Kuivila and Foe 1995; Werner et al. 2000; Kuivila and Moon 2004). The toxicity of particular chemicals to fish often changes depending on water quality parameters, including DO concentrations, pH, salinity, and hardness (Meehan 1991; Palawski et al. 1985; Richards and Rago 1999). Toxic substances and low DO concentrations change the physiology of fish and can affect the function and behavior of fish in the field (Cox and Coutant 1981). In general, organisms living near their environmental tolerance limits (such as low DO concentrations) are more susceptible to additional chemical stress, especially when exacerbated by increased temperatures or low food supplies (Heugens et al. 2002). An increase in susceptibility to toxic substances may be caused by an increase in respiration attributable to low DO concentrations. Fish respiring more bring more water, and therefore more toxic substances, across the gills and into their systems (Lloyd 1961 in Chapman 1986). Jump to "General Effects" discussion under other adverse effects: Steelhead (Oncorhynchus mykiss) Susceptibility to Predation Hypothesis: Low DO concentrations reduce activity levels and swimming performance of juvenile steelhead, increasing their susceptibility to predation. 1. What is the mechanism causing the indirect effects? Altered behavior or reduced swimming performance in response to low DO concentrations can increase the susceptibility of juvenile fish to predation (Reduced Swimming Performance and Altered Behavior). 2. Are there critical thresholds associated with the indirect effects? No studies or observations of the effect of low DO concentrations on the susceptibility of juvenile salmonids to predation were found. 3. How important is this mechanism? The potential for adverse effects associated with a given mechanism increases with the severity and duration of exposure to low DO and the number of fish exposed to such conditions. Because adult and juvenile steelhead migrate through the Delta mostly in the late fall to early spring when DO concentrations frequently exceed the regulatory minimum, high predation rates associated with low DO concentrations are unlikely. 4. How well is this mechanism understood? This mechanism has been described for striped bass (Coutant 1985). Susceptibility to Contaminants/Pathogens Hypothesis: Exposure of steelhead to DO concentrations below the regulatory minimum reduces their resistance to parasites and pathogens. 1. What is the mechanism causing the indirect effects? Fish may be negatively affected by chemical pollution from contaminated runoff or sediments (Kuivila and Foe 1995; Kuivila and Moon 2004). Fish pathogens are continuously present in water but may not affect fish unless water quality is poor and the fish’s immune system is impaired (Wedemeyer and Wood 1974 in Karna 2003). Salmonid hatchery fish are more susceptible to diseases such as furunculosis, aeromonad and pseudomonad hemorrhagic septicemia, and vibriosis when low DO concentrations exist (Wedemeyer 1970 and Wedemeyer and Wood 1974 in Karna 2003). 2. Are there critical thresholds associated with the indirect effects?
3. How important is this mechanism? Low DO concentrations in the DWSC are not likely to be an important stressor on migrating adult and juvenile steelhead because of the timing of their migration relative to the occurrence of low-DO events. 4. How well is this mechanism understood? The indirect effect of low DO concentrations on the susceptibility of fish to certain contaminants and pathogens has been investigated in salmonids under hatchery and laboratory conditions. Jump to "Steelhead" discussion under other adverse effects: Chinook Salmon (Oncorhynchus tshawytscha) Susceptibility to Predation Hypothesis: Low DO concentrations reduce activity levels and swimming performance of Chinook salmon juveniles, increasing their susceptibility to predation. 1. What is the mechanism causing the indirect effects? Reductions in swimming performance (Reduced Swimming Performance) can reduce the ability of juvenile fish to escape predators. 2. Are there critical thresholds associated with the indirect effects? No studies of the effect of low DO on susceptibility to predation in juvenile Chinook salmon have been published. 3. How important is this mechanism? The potential effect of increased predation associated with low DO concentrations is greatest for juvenile Chinook salmon. The magnitude of such an effect is unknown but is likely small based on the limited occurrence of low DO concentrations during the primary juvenile migration period through the DWSC 4. How well is this mechanism understood? No studies of the effect of DO on susceptibility to predation have been published. Susceptibility to Contaminants/Pathogens Hypothesis: Low DO concentrations (less than 9 mg/L) cause fish to become stressed and less resistant to contaminants and pathogens. 1. What is the mechanism causing the indirect effects? Fish may be negatively affected by chemical pollution from contaminated runoff or sediments (Kuivila and Foe 1995; Kuivila and Moon 2004). Fish pathogens are continuously present in water but may not affect fish unless water quality is poor and the fish’s immune system is impaired (Wedemeyer and Wood 1974 in Karna 2003 ). Salmonid hatchery fish are more susceptible to diseases such as furunculosis, aeromonad and pseudomonad hemorrhagic septicemia, and vibriosis when low DO concentrations exist (Wedemeyer 1970 and Wedemeyer and Wood 1974 in Karna 2003). 2. Are there critical thresholds associated with the indirect effects? No critical thresholds have been established for the effect of low DO concentrations on the susceptibility of salmonids to contaminants and pathogens (General Effects). 3. How important is this mechanism? The importance of this mechanism is unclear because of limited understanding of the effects of contaminants and pathogens on Chinook salmon and the effect of multiple stressors on fish in their natural environment. 4. How well is this mechanism understood? The indirect effects of low DO concentrations on the susceptibility of fish to contaminants and pathogens are not well understood. Jump to "Chinook Salmon" discussion under other adverse effects: Delta Smelt (Hypomesus transpacificus) Hypothesis: Indirect effects include such mechanisms as susceptibility to predation and susceptibility to pathogens. Delta smelt exposed to DO concentrations below the regulatory minimum suffer indirect negative effects. 1. What is the mechanism causing the indirect effects? Stressors such as reduced availability of food-web species and exposure to predators, pathogens, and toxic substances can make delta smelt more susceptible to negative effects when exposed to low DO concentrations. However the reverse is also true—exposure of delta smelt to low DO concentrations can make them more susceptible to these stressors. Delta smelt exposed to low DO concentrations may be more susceptible than normal to predation or infection. Low DO concentrations can lead to decreased swimming performance or altered behaviors (e.g., surface breathing) that may make them more susceptible to predation by other fishes or birds (General Effects and Swimming Performance). Also, under low DO concentrations, fish may reduce their metabolic demands by decreasing activities of the immune system; this decrease may increase their susceptibility to infection by pathogens or parasites (General Effects). 2. Are there critical thresholds associated with the indirect effects? The incipient limiting threshold for delta smelt has not been determined. 3. How important is this mechanism? The extent and importance of the indirect effects of low DO concentrations on delta smelt are not known. 4. How well is this mechanism understood? This mechanism is not well understood because no studies relating low DO concentrations to indirect effects such as susceptibility to predation, parasites, or pathogens have been published. Jump to "Delta Smelt" discussion under other adverse effects: Longfin Smelt (Spirinchus thaleichthys) Hypothesis: Longfin smelt exposed to DO concentrations below the regulatory minimum suffer indirect adverse effects. 1. What is the mechanism causing the indirect effects? Reduced availability of food and increased exposure to pathogens, predators, and toxic substances can make longfin smelt more susceptible to adverse effects when they are exposed to low DO concentrations. However, exposure of longfin smelt to low DO concentrations also can make them more susceptible to other stressors. Longfin smelt exposed to low DO concentrations may be more susceptible than normal to predation or infection. Low DO concentrations can lead to decreased swimming performance or altered behaviors (e.g., surface breathing) that may make these fish more susceptible to predation by other fishes or avian predators (General Effects). Also, under low DO concentrations, fish may reduce their metabolic demands by decreasing activities of the immune system, which may increase their susceptibility to infection by pathogens or parasites (General Effects). No research on the indirect effects of low-DO exposure on longfin smelt has been published at this time. 2. Are there critical thresholds associated with the indirect effects? The incipient limiting threshold for longfin smelt has not been determined. 3. How important is this mechanism? The extent and importance of the indirect effects of low DO on longfin smelt are not known. 4. How well is this mechanism understood? No studies relating low DO concentrations to indirect effects such as susceptibility to predation, increased parasite loads, or pathogenic infections have been published. Jump to "Longfin Smelt" discussion under other adverse effects: Sacramento Splittail (Pogonichthys macrolepidotus) Hypothesis: Sacramento splittail exposed to DO concentrations below the regulatory minimum suffer indirect negative effects. 1. What is the mechanism causing the indirect effects? Reduced availability of food and increased exposure to pathogens, predators, and toxic substances can make Sacramento splittail more susceptible to negative effects when exposed to low DO concentrations. However, exposure of Sacramento splittail to low DO concentrations can also make them more susceptible to these other stressors. Sacramento splittail exposed to low DO concentrations may be more susceptible than normal to predation or infection. Low DO concentrations can lead to decreased swimming performance or altered behaviors (e.g., surface breathing) that may make these fish more susceptible to predation by other fishes or avian predators (General Effects). Also, under low DO concentrations, fish may reduce their metabolic demands by decreasing activities of the immune system; this decrease may increase their susceptibility to infection by pathogens or parasites (General Effects). No research on the indirect effects of low-DO exposure on Sacramento splittail has been published. 2. Are there critical thresholds associated with the indirect effects? The incipient limiting threshold for Sacramento splittail has not been determined; however, given that their incipient lethal threshold is very low, it is likely that their incipient limiting threshold is well below the DO regulatory minimum. 3. How important is this mechanism? The extent and importance of the indirect effects of low DO concentrations on Sacramento splittail are not known. 4. How well is this mechanism understood? No studies relating low DO concentrations to indirect effects, such as susceptibility to predation, increased parasite loads, or pathogenic infections, have been published. Jump to "Sacramento Splittail" discussion under other adverse effects: White Sturgeon (Acipenser transmontanus) Hypothesis: White sturgeon exposed to DO concentrations below the regulatory minimum suffer indirect negative effects. 1. What is the mechanism causing the indirect effects? White sturgeon exposed to low DO concentrations may be more susceptible than normal to predation or infection. Low DO concentrations can lead to decreased swimming performance or altered behaviors (e.g., surface breathing) that may make fish more susceptible to predation by other fishes or avian predators (General Effects). Also, under low DO concentrations, fish may reduce their metabolic demands by decreasing the activity of their immune systems; this may increase susceptibility to infection by pathogens or parasites (General Effects). No research on the indirect effects of low-DO exposure on white sturgeon has been published at this time. 2. Are there critical thresholds associated with the indirect effects? The incipient limiting threshold for white sturgeon has not been determined, but it is apparently above 58% saturation (4.7–5.7 mg/L), the level at which Cech et al. (1984) found significant declines in swimming activity, growth, and survival among white sturgeon. 3. How important is this mechanism? The extent and importance of the indirect effects of low DO on white sturgeon are not known. 4. How well is this mechanism understood? No studies relating low DO concentrations to indirect effects such as susceptibility to predation, increased parasite loads, or pathogenic infections have been published. Jump to "White Sturgeon" discussion under other adverse effects: Green Sturgeon (Acipenser medirostris) Although little species-specific information is available for green sturgeon, it is likely that information for white sturgeon is generally applicable to green sturgeon. Jump to "Green Sturgeon" discussion under other adverse effects: Striped Bass (Morone saxatilis) Hypothesis: Striped bass exposed to DO concentrations below the regulatory minimum suffer indirect negative effects, such as predation and increased rates of disease. 1. What is the mechanism causing the indirect effects? Striped bass exposed to low DO concentrations may be more susceptible than normal to predation or infection. For example, when aquatic environments become stratified along gradients of DO concentrations, larval striped bass, along with their predators, may be forced to concentrate in areas of acceptable DO concentrations, where striped bass become increasingly vulnerable to predation (Breitburg 1994). Evidence exists that avoidance of high water temperatures and reduced DO concentrations by adults can result in crowding and increased rates of predation, fishing mortality, and disease (Coutant 1990). Moore and Townsend (1998) found that tadpoles suffered increased predation largely as a result of changes in behavior associated with low DO concentrations and temperature changes. 2. Are there critical thresholds associated with the indirect effects? A threshold specific to striped bass cannot be defined. Indirect effects are often the result of cumulative effects, and it may not be possible to clearly distinguish the individual effects (and their associated thresholds) that make up these cumulative effects. 3. How important is this mechanism? The effect of increased predation associated with low DO concentrations may have the greatest effect on juvenile life stages. The magnitude of such an effect is unknown. 4. How well is this mechanism understood? The indirect effects associated with low DO concentrations are not well understood. Jump to "Striped Bass" discussion under other adverse effects: |