Adverse Effect: Altered Behavior

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General Effects

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 affect the behavior of fish, resulting in changes in distribution, habitat use, activity, and respiration mode. Fish can avoid mortality and other adverse effects of low DO concentrations through a number of behavioral responses that reduce either their exposure to low DO concentrations or their need for oxygen. Potential behavioral responses to low DO concentrations include avoidance, changes in activity, increased use of air breathing, increased use of aquatic surface respiration, and habitat shifts (Kramer 1987).

The concentration of DO that will trigger avoidance behaviors varies among species and life stages, depending on their tolerances of low DO concentrations. Field and laboratory studies indicate that fish tend to avoid oxygen concentrations that are two to three times higher than those that cause 50% mortality in 24-hour and 96-hour exposures, approximately equal to the concentrations associated with reduced growth in laboratory experiments (Breitburg 2000). Such behavior indicates that fish can avoid hypoxic waters and select more highly oxygenated waters if available. For example, vertical movements to the surface or inshore areas where oxygen levels tend to be higher may be an effective survival strategy during periods of severe hypoxia (Riedel et al. 2002; Breitburg 1992, 1994; Kramer 1987). Some species may also use air breathing or aquatic surface respiration to increase oxygen uptake under such conditions (Weber and Kramer 1983). Where alternative habitats are limited or not accessible, low DO concentrations coupled with high water temperatures can block or delay migration or restrict fish to small refuges where they may experience increased susceptibility to predators, disease, and food limitation (Alabaster 1988; Coutant 1985; Alabaster and Lloyd 1982).

Changes in activity in response to low DO concentrations include increased gill ventilation and swimming activity (associated with avoidance behavior) followed by decreases in activity, depending on the duration of exposure. Reduced activity levels can help reduce oxygen requirements and allow fish to survive exposure to low DO concentrations when avoidance is ineffective. However, such a response can reduce feeding and spawning opportunities, potentially leading to reduced growth or reduced reproductive success, depending on the duration of exposure.

These responses can be important strategies for reducing or avoiding adverse effects caused by direct exposure to low DO concentrations but also can increase the potential for adverse effects from other factors (e.g., increased predation risk). Consequently, the degree to which fish exhibit these responses in nature likely will be influenced by the differences in energy costs and mortality risks associated with alternative responses (Breitburg 2000).

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Species-Specific Effects

Steelhead (Oncorhynchus mykiss)

Hypothesis:

Low concentrations of DO cause changes in behavior of trout that reduce oxygen demand or increase oxygen uptake.

1. What is the mechanism causing this adverse effect?

Trout have been observed to cease activity or exhibit surface breathing when subjected to low DO concentrations (3 mg/L and below).

Tagged fish in a lake environment did not move from low-DO areas probably because of a natural response to decrease aerobic activities. Active avoidance (i.e., swimming away from low-DO areas) was not observed (Barrow and Peters 2001). Reductions in activity levels and feeding can lead to growth effects (See Reduction in Growth section).
Juvenile trout in isolated pools with temperatures of 22.4°C and DO of 3.5 mg/L swam up near the surface to maximize oxygen uptake (Erman and Leidy 1975 in McCullough 1999).
Trout have been reported to surface constantly at 3.0 mg/L and rarely at 5 mg/L (Dean and Richardson 1999).

2. Are there critical thresholds associated with this adverse effect?

The studies cited above indicate that DO concentrations below 5 mg/L can alter the behavior of juvenile trout.

3. How important is this mechanism?

The potential for adverse effects resulting from altered behavior in steelhead increases with the severity and duration of exposure to low DO concentrations and the number of fish that encounter such conditions. Because adults and juveniles migrate through the Delta mostly in the late fall to early spring when DO concentrations frequently exceed the regulatory minimum, the potential for a significant population impact is low.

4. How well is this mechanism understood?

Several studies have demonstrated a general ability of trout to detect low DO concentrations and modify their behavior to increase survival. The potential outcome of such responses is largely unknown.

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Chinook Salmon (Oncorhynchus tshawytscha)

Hypothesis:

Low concentrations of DO will cause changes in behavior, such as avoidance of areas with low DO concentrations and straying.

1. What is the mechanism causing this adverse effect?

Adult and juvenile Chinook salmon have been observed in both the laboratory and natural conditions to avoid areas of low DO concentrations.

2. Are there critical thresholds associated with this adverse effect?

Several studies have been done with wild migrating adult Chinook salmon and with juvenile Chinook salmon in the laboratory.

Whitmore et al. (1960 in Karna 2003) observed that juvenile Chinook salmon showed avoidance of DO concentrations of 1.5–4.5 mg/L at high water temperatures, but showed little avoidance at 4.5 mg/L in the fall when water temperatures were lower.
In a laboratory experiment performed by Whitmore et al. (1960 in Karna 2003), Chinook salmon selected areas of 9 mg/L or higher over DO concentrations of 1.5 mg/L. Moderate selection against DO concentrations of 3.5 mg/L was seen, and selection against 4.5 and 6.0 mg/L was detected in some cases.
Hallock et al. (1970) determined that adult San Joaquin River Chinook salmon would not move past Stockton until DO concentrations rose to 4.5 mg/L, and that the run did not migrate steadily until the DO concentrations were above 5.0 mg/L. Flows and export rates in the Delta also may affect the number of adults that migrate up the San Joaquin River (Mesick 2001; Resources Agency of California 1964). Mesick (2001) found evidence of increased straying of adult Chinook salmon into the Sacramento River and Mokelumne River when Delta exports exceeded about 100% of the flow in the San Joaquin River and San Joaquin River flows were less than 2,000 cfs. This caused Sacramento River water to be pulled into the Delta and eliminated the San Joaquin River water as an attractant for migrating adults.
In the Willamette River, water temperature of 22.4°C and a minimum DO of 3.3 mg/L caused adult spring-run Chinook salmon to stop migration (Alabaster 1988 in McCullough 1999).
During an 11-year study, upstream migration of Chinook and coho salmon through a fish pass in an estuary was inhibited when DO concentrations were lower than 5.0 mg/L, but when fish were abundant in a given year, the fish migrated at concentrations of 3.0 mg/L. However, water temperature and flow were not taken into consideration (Alabaster and Lloyd 1982).
No information is available about avoidance of low-DO areas by downstream migrating smolts (Alabaster and Lloyd 1982).

3. How important is this mechanism?

This mechanism may be important because of the potential for overlap in the timing of upstream migration of adult Chinook salmon and the occurrence of low DO concentrations in the DWSC. Avoidance or prolonged delays in migration could also adversely affect spawning success by increasing energy expenditures and reducing the amount of energy available for gonad development and spawning. Juveniles are less likely to encounter low DO concentrations in the DWSC because of the timing of their migration (winter and spring).

4. How well is this mechanism understood?

Several studies have demonstrated an avoidance response of Chinook salmon adults and juveniles to low DO concentrations. The relative importance of low DO as a causal factor for migration delays or increased straying is uncertain because of the potential role of other factors such as high water temperatures and export-inflow ratios in the lower San Joaquin River and Delta.

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Delta Smelt (Hypomesus transpacificus)

Hypothesis:

Delta smelt exposed to DO concentrations below the regulatory minimum exhibit changes in behavior that lead to adverse effects.

1. What is the mechanism causing this adverse effect?

These changes in behavior are designed to limit the negative effect of exposure to low DO concentrations, but they may result in other negative effects, such as decreased growth and increased exposure to predators or pathogens. Given their general intolerance for handling and their response to extremes of other environmental variables (Swanson et al. 1996, 1998, 2000), delta smelt would probably lose equilibrium after exposure to DO concentrations below their limit of tolerance. Delta smelt generally occur in the upper portion of the water column; under low DO concentrations, however, they may concentrate at the water surface to increase their exposure to oxygenated water. By swimming close to the surface, they increase their exposure to predation (particularly by birds).

2. Are there critical thresholds associated with this adverse effect?

Neither the incipient limiting threshold nor another threshold that produces behavioral changes has been determined for delta smelt.

3. How important is this mechanism?

The importance of this mechanism is unknown because the degree to which delta smelt behavior changes as a result of exposure to low DO concentrations is unknown. The effect of these altered behaviors is also unknown; therefore, the degree to which low DO concentrations produce altered behaviors with negative consequences for delta smelt cannot be evaluated at present.

4. How well is this mechanism understood?

This mechanism is not well understood because no studies of the effect of low DO concentrations on delta smelt behavior have been published.

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Longfin Smelt (Spirinchus thaleichthys)

Hypothesis:

Longfin smelt exposed to DO concentrations below the regulatory minimum exhibit changes in behavior that lead to adverse effects.

1. What is the mechanism causing this adverse effect?

2. Are there critical thresholds associated with this adverse effect?

Like all other fishes, longfin smelt have an incipient limiting threshold for DO below which they experience a decline in the ability to perform certain activities and functions. Neither the incipient limiting threshold nor the threshold that produces behavioral changes in longfin smelt has been determined.

3. How important is this mechanism?

The degree to which longfin smelt behavior changes as a result of exposure to low DO is unknown. The effect of altered behaviors is also unknown. Thus, the degree to which low DO produces altered behaviors with negative consequences for longfin smelt cannot be evaluated at present.

4. How well is this mechanism understood?

No studies of the effect of low DO on longfin smelt behavior have been published.

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Sacramento Splittail (Pogonichthys macrolepidotus)

Hypothesis:

Sacramento splittail exposed to DO concentrations near the regulatory minimum exhibit changes in behavior that lead to adverse effects.

1. What is the mechanism causing this adverse effect?

These changes in behavior are designed to limit the negative effects of exposure to low DO concentrations, but they may result in decreased growth, increased exposure to predators or pathogens, and other negative effects. Sacramento splittail exhibit increased agitation when exposed to low DO concentrations (Young and Cech 1996).

2. Are there critical thresholds associated with this adverse effect?

Sacramento splittail display increased agitation when exposed to low DO concentrations, indicating that they may detect and swim away from unfavorable DO concentrations (Young and Cech 1996). There is no information about what this threshold is for Sacramento splittail.

3. How important is this mechanism?

Sacramento splittail exhibit increased agitation when exposed to low DO concentrations. The degree to which this behavior exposes Sacramento splittail to increased predation is unknown, although such rapid, undirected swimming may draw the attention of predatory fishes. The negative consequences (if any) of their behavioral response to low DO concentrations cannot be evaluated at present. The effect of these altered behaviors is also unknown; therefore, the degree to which low DO concentrations produce altered behaviors with negative consequences for Sacramento splittail cannot be evaluated at present.

4. How well is this mechanism understood?

No studies of the effect of low DO concentrations on Sacramento splittail behavior have been published.

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White Sturgeon (Acipenser transmontanus)

Hypothesis:

White sturgeon exposed to DO concentrations below the regulatory minimum will exhibit changes in behavior that lead to adverse effects.

1. What is the mechanism causing this adverse effect?

As DO concentrations approach or fall below the incipient limiting threshold, some fish change their behavior to (a) increase their exposure to oxygenated water or (b) decrease their requirements for oxygen. These changes in behavior are designed to limit the negative impact of exposure to low DO, but they may result in decreased growth, increased exposure to predators or pathogens, and other negative effects. Juvenile white sturgeon display markedly reduced activity when exposed to low DO concentrations (Cech et al. 1984; Cech and Doroshov 2004). In addition, white sturgeon may use surface water to obtain oxygen under low DO concentrations (Cech and Doroshov 2004); this represents a significant change in behavior because in estuarine environments sturgeon are usually exclusively benthic organisms. Similar surfacing behavior is strongly suspected to occur among Atlantic sturgeon (A. oxyrinchus) under low DO concentrations (Secor and Gunderson 1998).

2. Are there critical thresholds associated with this adverse effect?

Neither the incipient limiting threshold nor the threshold that produces behavioral changes has been determined for white sturgeon, but they are apparently above 58% saturation (4.7–5.7 mg/L), the level at which Cech et al. (1984) found significant declines in swimming activity.

3. How important is this mechanism?

This mechanism is likely to be important in white sturgeon because their response to low DO concentrations is to reduce their activity and overall metabolic levels. In adults, this could result in failure to reach spawning localities elsewhere in the San Joaquin River or decreased egg development caused by a reduction in metabolic activity. Among larval and juvenile fish, decreased activity and feeding clearly produce lower growth rates, a response that likely puts exposed white sturgeon at a disadvantage when they enter the marine phase of their life. Also, larval and juvenile white sturgeon that resort to surface breathing would be at greater risk for predation from avian and fish predators—sturgeon are generally bottom-dwelling organisms and are thus expected to show reduced predator-avoidance efficiency in surface waters.

4. How well is this mechanism understood?

The relationship between low DO concentrations and altered behavior is well established for white sturgeon (Cech et al. 1984; Cech and Crocker 2002). Also, Secor and Gunderson (1998) noted behavioral changes among Atlantic sturgeon (A. oxyrinchus) exposed to low DO concentrations.

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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.

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Striped Bass (Morone saxatilis)

Hypothesis:

Striped bass exposed to DO concentrations below the regulatory minimum exhibit changes in behavior that lead to adverse effects.

1. What is the mechanism causing this adverse effect?

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 they are vulnerable to increased 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 a similar case where 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 this adverse effect?

Striped bass probably begin to actively seek alternative habitats when DO concentrations begin to approach 3–4 mg/L. 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 concentrations (Moyle 2002). Coutant and Benson (1990) suggested that suitable habitat criteria for striped bass were temperatures below 25°C and DO concentrations above 2–3 mg/L. Striped bass become stressed when DO concentrations approach 3 mg/L, and areas with DO concentrations below 2 mg/L are uninhabitable (Coutant 1985). Chittenden (1971a in Coutant 1985) found that striped bass distribution was bound largely by the 3 mg/L-DO isopleth in Delaware Bay. Across a range of temperatures (13–25°C), Krouse (1968 in U.S. Environmental Protection Agency 2003) found that DO concentrations of 1 mg/L resulted in 100% fatality, 3 mg/L resulted intermediate survival, and 5 mg/L resulted in minimal mortality.

3. How important is this mechanism?

The degree to which striped bass change their behavior in response to low DO concentrations is unknown. The effect of these altered behaviors is also unknown; therefore, the degree to which low DO concentrations produce altered behaviors with negative consequences for striped bass is also unknown.

4. How well is this mechanism understood?

Understanding of the specific behavioral responses of striped bass to low DO concentrations (5 mg/L or less) is limited. However, some inferences may be drawn from the available literature describing preferred habitats occupied by the species and the physiological limitations of the species (Susceptibility to Adverse Effects of Low Dissolved-Oxygen Concentrations).

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