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Current location: Home > Biological Effects Model > 1° Driver: Avoid and Tolerate > 2° Driver: Ability to Tolerate Low DO |
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Secondary Driver: Ability to Tolerate Low DO Concentrations Jump down this page to: Steelhead | Chinook Salmon | Delta Smelt | Longfin Smelt | Sacramento Splittail | White Sturgeon | Green Sturgeon | Striped Bass Like all animals, fish require adequate supplies of oxygen to maintain metabolic life processes. However, fish live in an environment where oxygen is much less available than in terrestrial systems. Water has a limited capacity to dissolve atmospheric oxygen. For example, fresh water equilibrated with air can hold only 1/23 the amount of oxygen per volume that atmospheric air can hold at 5°C, and only 1/43 the amount of oxygen at 10°C (Moyle and Cech 2000). Oxygen solubility decreases as water temperature and salinity increase. In addition, because of water’s viscosity and density, it is much more difficult to extract oxygen from water than from air. Fish have developed certain physiological adaptations to these environmental conditions that allow them to extract efficiently what DO is available. In addition, fish possess a suite of mechanisms to compensate for reduced oxygen availability when DO concentrations decline. Fish use specialized structures called gills, which are analogous to lungs in terrestrial animals, to effectively extract oxygen from water. Within the gills, oxygen is transferred from water to the blood by diffusion through tiny finger-like structures called lamellae. Water travels across the lamellae in the opposite direction of blood flow. This “counter-current exchange” provides a consistent difference in oxygen tension between blood and water, allowing maximum diffusion of oxygen into the blood across the gills. For example, Van Dam (1938 in Moyle and Cech 2000) reported rainbow trout were able to extract up to 80% of oxygen available in water passing through the gills. Oxygen uptake increases with greater gill surface area (more lamellae) and thinner lamellar membranes (which reduce the diffusion distance into the blood). Fish vary greatly in their ability to extract oxygen from water across a range of DO concentrations, which subsequently affects their underlying ability to tolerate low DO concentrations. This difference in “extraction efficiency” is often a reflection of the environmental conditions a species has evolved to inhabit. In other words, fish species are more effective in extracting oxygen across a range of DO concentrations that best reflect typical habitats in which they are found. Oxygen extraction efficiency (the percent of total oxygen in water passing through the gills that a fish can extract) depends largely on physiological characteristics such as hemoglobin structure, gill surface area and thickness, and total hematocrit (the packed volume of red blood cells in the blood). Hemoglobin is the respiratory pigment found in red blood cells that allows fish to bind large amounts of oxygen within the blood, dramatically increasing the oxygen content of blood. Different fish species contain specialized hemoglobin types that vary in their affinity for oxygen. Hemoglobins are suited to the particular environments and life stages specific to a species in order to maximize oxygen uptake over a range of conditions (Moyle and Cech 2000). In this way, some species are inherently more able to tolerate low DO concentrations than others. For example, Sacramento blackfish typically are found in still waters with relatively low DO concentrations. As such, blackfish hemoglobin molecules saturate easily in low DO concentrations. Alternatively, rainbow trout are active fish found in cool, highly oxygenated waters. Consequently, rainbow trout hemoglobin begins to saturate at higher DO concentrations (i.e., has a lower affinity for oxygen), but is able to uptake and offload large quantities of oxygen to respiring tissues (Moyle and Cech 2000). These types of hemoglobin work best for exercising fish that occupy a narrow range of DO concentrations (Moyle and Cech 2000). As DO concentrations approach or fall below the incipient limiting threshold, some fish change their behavior to increase their exposure to oxygenated water and decrease their requirements for oxygen. These physiological responses to lowering DO concentrations occur well before the DO concentration reaches acutely toxic levels (Davis 1975). As oxygen tension declines, blood oxygen saturation drops below 100%, requiring fish to increase gill ventilation and circulation to sustain oxygen demands of the tissues. In response to hypoxia, fish can increase their rate and amplitude of breathing (i.e., increase ventilation frequency and volume of water passed through the gills) while simultaneously decreasing their heart rate (Davis 1975). This is often accompanied by an increase in stroke volume per heartbeat, more water flow through the gills, and decreased venous oxygen tension. Reducing demand for oxygen through metabolic depression has been suggested as a more effective survival strategy than using anaerobic metabolism (Dalla Via et al. 1994 in Breitburg 2002). In addition to regulating bodily processes, fish will actively avoid areas of low DO concentrations. Although avoidance behavior is reported for many species of fish, the exact threshold of this response varies extensively among species and with environmental conditions. For example, Whitmore et al. (1960 in Davis 1975) found juvenile Chinook salmon avoided 1.5–4.5 mg/L DO during summer high temperatures but displayed no avoidance of 4.5 mg/L DO when temperatures were lower in fall. These changes in behavior are designed to limit the negative effects of exposure to low DO concentrations but may result in decreased growth, increased exposure to predators or pathogens, and other negative effects. Jump to "General Effects" discussion under other Secondary Drivers: Steelhead (Oncorhynchus mykiss) Hypothesis: Steelhead are relatively intolerant of low DO concentrations (4 mg/L and below). 1. How does this driver operate? Compared to many fish species, steelhead and other salmonids are less tolerant of hypoxic conditions because of their high metabolic requirements and scope for activity (Cech et al. 1990). Trout have a large proportion of red muscle that has a resting oxygen consumption rate several times higher than that of white muscle (Gordon 1968 and Lin et al. 1974 in Cech et al. 1990). Prolonged exposure to low DO concentrations may cause a range of adverse effects that can include reduced survival, growth, and spawning success. 2. Are there critical thresholds associated with this driver? Lethal thresholds for salmonid species range from 0.95 to 3.4 mg/L (Alabaster and Lloyd 1982). Generally, mortality increases below 3.0 mg/L and becomes high at 2.0 to 2.5 mg/L, depending on water temperature (Hicks 2000). See also Adverse Effects of Low Dissolved Oxygen: Direct Mortality, for more detailed information. 3. How important is this driver? The ability to tolerate low DO concentrations would be important if numerous steelhead were subject to low DO concentrations (less than 5 mg/L) and high water temperatures during their migration through the DWSC. However, general knowledge regarding the timing of steelhead migration indicates that any adverse effects would be limited to a small fraction of the total population. 4. How well is this driver understood? Lethal oxygen thresholds for O. mykiss have been well studied in the laboratory. Because these thresholds generally define oxygen concentrations associated with direct mortality under laboratory conditions and in the absence of other stressors (e.g., contaminants), they may not accurately reflect the tolerances of fish in their natural environment. In general, little is known about the interactive effects of multiple stressors on the environmental tolerances of fish. Jump to "Steelhead" discussion under other Secondary Drivers: Chinook Salmon (Oncorhynchus tshawytscha) Hypothesis: Chinook salmon are relatively intolerant of DO concentrations below the regulatory minimum. 1. How does this driver operate? Salmonids are less tolerant of hypoxic conditions than many other fish species because of their high metabolic requirements and scope for activity (Cech et al. 1990). Prolonged exposure to low DO concentrations may cause a range of adverse effects that can include reduced survival, growth, and spawning success (General Effects). 2. Are there critical thresholds associated with this driver? Critical thresholds for DO concentrations vary because other factors, such as temperature, affect the responses of fish to hypoxia. Lethal thresholds for salmonid species range from 0.95 to 3.4 mg/L (Alabaster and Lloyd 1982). Juvenile Chinook salmon experienced mortality when exposed to DO concentrations between 2 and 3 mg/L in cold water, but survived in concentrations of 3–7 mg/L (Raleigh et al. in Hicks 2000). The incipient lethal time period (50% mortality) for 1 mg/L at 15°C is from 0.5 to 1.3 hours for rainbow trout, but this criterion could also be applied to Chinook salmon (Hicks 2000). Generally, mortality for Chinook salmon juveniles increases below 3.0 mg/L and becomes high at 2.0 to 2.5 mg/L, depending on water temperature (Hicks 2000). See also Adverse Effects of Low Dissolved Oxygen, Direct Mortality, for more detailed information. 3. How important is this driver? The ability to tolerate low DO concentrations is important because of the potential for relatively large numbers of adult Chinook salmon to encounter low DO concentrations in the DWSC during their upstream migration through the Delta. The mortality rate attributable to low DO concentrations is positively correlated with the duration of exposure (days) and water temperature. The longer the exposure (>1 day) to low DO concentrations (below 3 mg/L) and high water temperatures (>19°C), the more likely mortality could occur. From the study conducted by Hallock et al. (1970), adult Chinook salmon moved around the Delta and would migrate into the Mokelumne and Sacramento Rivers, avoiding the DWSC. It is unlikely that mortality would occur because of low DO concentrations in the DWSC if fish can move out of the area. See also Activity While in the Deep Water Ship Channel and Exposure to High Water Temperature for more detailed information. 4. How well is this driver understood? The critical threshold for juvenile Chinook salmon mortality has been well studied in the laboratory. Generally, levels below 3 mg/L are considered lethal for a variety of water temperatures (Alabaster and Lloyd 1982; Raleigh et al. in Hicks 2000; Hicks 2000). These thresholds serve as general guidelines for evaluating the ability of adults and juveniles to tolerate low DO concentrations in their natural environment. Jump to "Chinook Salmon" discussion under other Secondary Drivers: Delta Smelt (Hypomesus transpacificus) Hypothesis: Delta smelt cannot tolerate prolonged (e.g., 24 hours) exposure to DO concentrations below the regulatory minimum. 1. How does this driver operate? Several observations suggest that delta smelt may not be able to tolerate the low DO concentrations found in the DWSC:
In addition, Ishitobi et al. (2000) indicate that populations of H. nipponensis, a delta smelt congener, had declined in a Japanese estuary in response to a long-term decline in local DO concentrations. Those results suggest that delta smelt may be sensitive to low DO concentrations. However, inferring precise limits of delta smelt tolerance for low DO concentrations from the Ishitobi et al. study is not possible because (a) the data analyses employed by those authors were not very precise (e.g., DO concentrations were presented as monthly and annual averages rather than as minima), (b) their analysis was correlated, and (c) H. nipponensis and delta smelt may differ in their ability to tolerate low DO concentrations. The professional judgment of one research physiologist who has worked extensively with this species is that DO concentrations less than 50–60% of saturation (e.g., at 20°C, less than 4.55–5.46 mg/L) may present a problem for this species (Swanson pers. comm.). 2. Are there critical thresholds associated with this driver? DO tolerance levels are defined by critical thresholds (General Effects). These thresholds have not been studied in delta smelt. 3. How important is this driver? It is not known whether delta smelt can tolerate the DO concentrations found in the DWSC during the periods when smelt are present. 4. How well is this driver understood? This driver is not well understood because no studies have been published on delta smelt tolerance for low DO concentrations, much less their behavioral or physiological responses to low DO concentrations. Thus, it is not known what DO concentrations represent important thresholds, what exposure levels produce lethal or sublethal effects, or what behavioral or ecological drivers (e.g., swimming, feeding, fecundity) are most affected by exposure to low DO concentrations. In addition, no information exists on how DO thresholds or dose-response curves change throughout the delta smelt lifecycle. The generally frail nature of delta smelt and the limited evolutionary exposure of Osmeridae (the smelt family) to low DO concentrations suggest that delta smelt are not able to tolerate low DO concentrations. Jump to "Delta Smelt" discussion under other Secondary Drivers: Longfin Smelt (Spirinchus thaleichthys) Hypothesis: Longfin smelt will not tolerate prolonged (e.g., 24-hour) exposure to DO concentrations below the regulatory minimums. 1. How does this driver operate? Longfin smelt are not likely to tolerate the low DO concentrations found in the DWSC. Longfin smelt would not be expected to tolerate low DO concentrations that the species did not experience during its evolutionary history; this fish typically occurs in relatively cool, fast-flowing and well-oxygenated waters (U.S. Fish and Wildlife Service 1996; Moyle 2002; Rosenfield and Baxter in prep.). Longfin smelt are closely related to nearshore marine species (Stanley et al. 1995) that rarely would encounter low DO concentrations. Based on their ecological and life-history patterns and their evolutionary history, longfin smelt are expected to be even less likely to tolerate low DO concentrations than delta smelt (see delta smelt section of this conceptual model) (J. Rosenfield pers. comm.). 2. Are there critical thresholds associated with this driver? DO tolerance levels are defined by critical thresholds (General Effects). These thresholds have not been studied in longfin smelt. 3. How important is this driver? It is not known whether longfin smelt can tolerate DO concentrations found in the DWSC during periods when this fish is present there. 4. How well is this driver understood? No studies have been published on the ability of longfin smelt to tolerate low DO or their ability to acclimate to low DO concentrations. Thus, it is not known what DO concentrations represent important thresholds, what exposure levels produce lethal or sublethal effects, and what behavioral or ecological factors (e.g., swimming, feeding, fecundity) are most affected by exposure to low DO. In addition, no information exists on how DO thresholds or dose-response curves change throughout the longfin smelt life cycle. Jump to "Longfin Smelt" discussion under other Secondary Drivers: Sacramento Splittail (Pogonichthys macrolepidotus) Hypothesis: Sacramento splittail can tolerate prolonged (e.g., 24-hour) exposure to DO concentrations as low as 1.5 mg/L. 1. How does this driver operate? Sacramento splittail are likely able to tolerate relatively low DO concentrations in their natural habitats. Young and Cech (1996) found that Sacramento splittail were tolerant of low-DO and high-temperature conditions (see also Secondary Driver: Exposure to High Water Temperatures). Sacramento splittail life history suggests that they should be relatively tolerant of low DO concentrations encountered in their natural habitats. This species occurs principally in shallow sloughs and channels with slow currents (Moyle 2002; Moyle et al. 2004) and is a benthic feeding specialist (Moyle 2002; Moyle et al 2004). These two attributes suggest that Sacramento splittail are frequently exposed to relatively low DO concentrations over their life span. Indeed, Young and Cech (1996) speculated that Sacramento splittail may use low DO concentrations to their advantage by foraging in areas with DO concentrations below those that their predators can tolerate. 2. Are there critical thresholds associated with this driver? DO tolerance levels are defined by critical thresholds (General Effects). Under laboratory conditions, Sacramento splittail of various ages that were acclimated to either 12°C or 17°C were able to tolerate DO concentrations of 1.1–1.3 mg/L; older fish acclimated to 12°C were able to tolerate DO concentrations of 0.6 mg/L. These acclimation temperatures are much lower than the maximum temperatures tolerated in the laboratory (28.9–32°C) or temperatures found in their natural habitats. It is, therefore, not clear what concentrations of DO Sacramento splittail can tolerate under natural conditions; however, they probably can tolerate extended (i.e., greater than 24-hour) exposure to DO concentrations well below the regulatory minimum at temperature conditions found in the DWSC. These concentrations are reasonable estimates of the incipient lethal threshold for fish of the size tested under temperature and other conditions present in the laboratory. 3. How important is this driver? Sacramento splittail are likely to tolerate low DO concentrations better than many other native fish species in the Delta. Direct impacts on this species are possible when DO concentrations reach extremely low levels, which are uncommon in the DWSC. Thus, low DO concentrations in the DWSC are expected to affect Sacramento splittail infrequently. 4. How well is this driver understood? One aspect of this driver is fairly well understood because Young and Cech (1996) described the temperature and DO tolerances of Sacramento splittail of different ages (life stages) acclimated to different temperatures. However, no studies on the ability of Sacramento splittail to acclimate to low DO concentrations have been published. If Sacramento splittail have the ability to acclimate, this ability might reduce the DO concentration at which adverse affects occur. Jump to "Sacramento Splittail" discussion under other Secondary Drivers: White Sturgeon (Acipenser transmontanus) Hypothesis: White sturgeon do not tolerate prolonged (e.g., 24-hour) exposure to DO concentrations below the regulatory minimums. 1. How does this driver operate? Juvenile white sturgeon metabolic activity decreases under hypoxic conditions (51% of saturation or 4.7–5.7 mg/L) over a range of temperatures from 15°C to 25°C (Cech et al. 1984). In part, this decrease in oxygen consumption rate results from a decrease in swimming activity under hypoxic conditions (Cech et al. 1984; Cech and Crocker 2002). These fish also eat less and decrease their metabolic rates under low DO concentrations (Cech and Crocker 2002). This ability to decrease aerobic respiration appears to allow white sturgeon to tolerate short-term exposure to hypoxic conditions (Burggren and Randall 1978; Cech and Doroshov 2004). Burggren and Randall (1978) found that white sturgeon survived 25–35 minute exposures to very hypoxic (5–10% normoxic) conditions without increasing ventilation frequency or oxygen consumption after exposure. This metabolic response is unique among the Acipenser species studied to date. Despite this physiological and behavioral response to low DO, white sturgeon experience higher mortality and significantly reduced growth (21–62% depending on temperature) after exposure to intermediate hypoxic conditions (e.g., 51% of saturation) of moderate duration (10 days; Cech et al. 1984). Indeed, all Acipenseridae seem to be sensitive to DO concentrations that other benthic fish tolerate well (Secor and Gunderson 1998; Cech and Doroshov 2004; National Marine Fisheries Service 2004). For example, Atlantic sturgeon (A. oxyrinchus) display increased mortality under hypoxic conditions (3 mg/L) (Secor and Gunderson 1998), and shortnose sturgeon (A. brevirostrum) of all ages show complete mortality at DO less than 2mg/L. (Jenkins et al. 1993 in Cech and Doroshov 2004) 2. Are there critical thresholds associated with this driver? DO tolerance levels are defined by critical thresholds (see General Effects). These thresholds have not been precisely defined for white sturgeon. Cech et al. (1984) demonstrated significant physiological, behavioral, and survival impacts at DO concentrations as high as 51% of saturation (4.7–5.7 mg/L, at the temperatures they tested). The threshold for impacts may be at higher DO concentrations than those that have been tested. 3. How important is this driver? Their general intolerance of and response to low DO (e.g., dramatically reduced swimming activity) suggest that low DO concentrations in the DWSC will present a major impediment to those white sturgeon that attempt to migrate through the DWSC during periods when low DO concentrations prevail there. 4. How well is this driver understood? White sturgeon intolerance of, and response to, low DO concentrations are well documented (Burggren and Randall 1978; Cech et al. 1984; Cech and Crocker 2002; Cech and Doroshov 2004). In addition, intolerance of low DO concentrations has been documented for several species in the sturgeon family, Acipenseridae (e.g., Secor and Gunderson 1998; Cech and Doroshov 2004; National Marine Fisheries Service 2004). These findings increase our understanding and certainty regarding white sturgeon response to low DO concentrations. Jump to "White Sturgeon" discussion under other Secondary Drivers: Green Sturgeon (Acipenser medirostris) Hypothesis: 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 Secondary Drivers: Striped Bass (Morone saxatilis) Hypothesis: Striped bass have a limited ability to tolerate prolonged (e.g., 24-hour) exposure to DO concentrations below the regulatory minimum, with young bass more likely to be affected by low DO concentrations. 1. How does this driver operate? Prolonged exposure to low DO concentrations may cause a range of adverse effects that can include reduced spawning success, reduced growth, and increased rates of mortality for all life stages (General Effects). 2. How important is this driver? Critical thresholds for DO concentrations are difficult to identify because other factors, such as temperature, can affect susceptibility to hypoxia. Adult striped bass can withstand DO concentrations as low as 3–5 mg/L for short periods, but juvenile striped bass are less able to tolerate such conditions (Moyle 2002). Several studies generally support these thresholds:
Across a range of temperatures (13–25°C), Krouse (1968 in U.S. Environmental Protection Agency 2003) found DO concentrations of:
Young striped bass are less able to tolerate low DO concentrations. Some findings related to specific thresholds of juvenile striped bass are:
Following an extensive literature review, the EPA established instantaneous minimum DO criteria of 5 mg/L to protect early life stages of migrating anadromous fishes in Chesapeake Bay, including striped bass (U.S. Environmental Protection Agency 2003). Given these findings, conditions in the DWSC could potentially adversely affect striped bass, depending on the severity, duration, timing, and extent of low DO concentrations in relation to the timing and abundance of various life stages in the DWSC. 3. Are there critical thresholds associated with this driver? Because we do not know the contribution of spawning above the DWSC to overall striped bass production, it is unknown whether and to what extent this driver may affect the population status of this species in the Delta. However, this driver is potentially important because of the sensitivity of early life stages. A more definitive assessment of potential population-level effects will require a better understanding of the timing, abundance, and distribution of eggs, larvae, and juveniles relative to the timing, duration, and extent of stressful DO concentrations in the DWSC. 4. How well is this driver understood? This driver is fairly well understood because a large amount of literature is available on the effects of hypoxia on striped bass. Jump to "Striped Bass" discussion under other Secondary Drivers: |