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Adverse Effect: Impaired Development Jump down this page to: Steelhead | Chinook Salmon | Delta Smelt | Longfin Smelt | Sacramento Splittail | White Sturgeon | Green Sturgeon | Striped Bass All fishes have an incipient limiting threshold for DO below which they experience a decline in the ability to perform certain activities and functions. Fish embryos and larvae are often more sensitive to low DO concentrations than older life stages (juvenile and adults) of the same species because of their limited ability to detect and avoid low DO concentrations and, in some cases, their higher oxygen requirements. Several studies have shown that the oxygen requirements of fish eggs (embryos) and larvae increase (i.e., their tolerance to low DO concentrations decreases) as development proceeds (Chapman 1986). The observed effects of hypoxia on embryos and larvae include respiratory dependence, reduced growth, retarded development, deformities, and mortality. The sensitivity of embryos and larvae appears to be greatest around the time of hatching and other key transitional stages, including development of the circulatory system in embryos and the transition to external feeding in larvae. Jump to "General Effects" discussion under other adverse effects: Steelhead (Oncorhynchus mykiss) Hypothesis: Low DO concentrations exacerbate the adverse effects of high water temperature on parr-smolt transformation in juvenile steelhead. 1. What is the mechanism causing this adverse effect? The transformation of freshwater juveniles (parr) to smolts involves behavioral, morphological, and physiological changes that adapt anadromous fish for living in salt water. Elevated water temperatures can impair or reverse smolt development, resulting in reduced survival in salt water or the reversion of juveniles back to parr (Hoar 1988 in Myrick and Cech 2001). It is assumed that low DO concentrations can contribute to impaired smolt development by limiting metabolic inputs necessary to sustain the physiological processes associated with smolt development. 2. Are there critical thresholds associated with this adverse effect? Juvenile steelhead have been reported to exhibit impaired smolt development at water temperatures above 15ºC (Adams at al. 1973 and 1975 in Myrick and Cech 2001). The role of DO concentrations has not been investigated. 3. How important is this mechanism? Downstream migration of steelhead smolts typically occurs from January through May when spring flows are high and DO concentrations in the DWSC commonly exceed the regulatory minimum. Consequently, low DO concentrations in the DWSC do not pose a significant threat to smolts. Water temperature is expected to be a more important stressor during the late spring (April through June) when temperatures frequently reach levels associated with impaired smolt development (greater than 15°C). 4. How well is this mechanism understood? The effect of low DO concentrations (independent of water temperature) on smolt development is unknown. Most studies have focused on the effect of water temperature on smolt development and survival in salt water. Jump to "Steelhead" discussion under other adverse effects: Chinook Salmon (Oncorhynchus tshawytscha) Hypothesis: Low DO concentrations exacerbate the adverse effects of high water temperature on parr-smolt transformation in juvenile Chinook salmon. 1. What is the mechanism causing this adverse effect? As juvenile Chinook salmon move toward the ocean, they begin to transform from parr (freshwater juveniles) into smolts. Smolt development involves behavioral, morphological, and physiological changes that prepare juveniles for the transition from fresh water to salt water. Elevated water temperatures can impair or reverse the parr-smolt transformation, depending on the timing of exposure (Hoar 1988 in Myrick and Cech 2001). It is assumed that associated reductions in DO concentrations can contribute to impaired smolt development. 2. Are there critical thresholds associated with this adverse effect? Critical temperature thresholds for successful smolt development have been established. For Chinook salmon smolts, optimal water temperatures range from 10 to 17.5°C. Temperatures greater than 16°C cause impaired smolt patterns and decreased saltwater survival (Myrick and Cech 2001). 3. How important is this mechanism? The potential for the reversal of smolt transformation and population-level effects associated with warm water temperatures increases with the timing, severity, and duration of exposure to warm water temperatures and the number of fish exposed to such conditions. Downstream migration of fall-run Chinook salmon smolts typically occurs from April through June. The DWSC has water temperatures greater than 15°C beginning in March and the majority of days in April, May, and June. The effects of low DO concentrations on smolts have not been studied, but the effects of high water temperatures on smolt development have been studied. Water temperatures greater than 15°C can delay smolt development and reduce success in salt water. Smolt survival has been studied in the Delta looking at the link of survival with flow (Baker and Morhardt 2001). However, water temperature was not directly compared to survival. See Exposure to High Water Temperatures for a more detailed discussion. 4. How well is this mechanism understood? General relationships between water temperature and smolt development in Chinook salmon have been documented. However, no studies have investigated the effects of water temperature or DO concentrations on smolt development in Central Valley Chinook salmon. Jump to "Chinook Salmon" discussion under other adverse effects: Delta Smelt (Hypomesus transpacificus) Hypothesis: Delta smelt exposed to DO concentrations below the regulatory minimum experience impaired development. 1. What is the mechanism causing this adverse effect? Certain fish species experience developmental problems (e.g., underdeveloped or malformed organs) as a result of exposure to low DO concentrations during early life stages (General Effects), but no studies on the effect of low DO concentrations on delta smelt development have been published. 2. Are there critical thresholds associated with this adverse effect? The incipient limiting threshold for delta smelt has not been determined. 3. How important is this mechanism? The extent and importance of impaired delta smelt development caused by exposure to low DO concentrations are not known. 4. How well is this mechanism understood? The mechanism causing this adverse effect is not well understood because no studies of the effect of low DO concentrations on delta smelt development 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 experience impaired development. 1. What is the mechanism causing this adverse effect? Certain fish species experience developmental problems (e.g., underdeveloped or malformed organs) as a result of exposure to low DO concentrations during early life stages (General Effects). No studies on the effect of low DO concentrations on longfin smelt development have been published. 2. Are there critical thresholds associated with this adverse effect? The incipient limiting threshold for longfin smelt has not been determined. 3. How important is this mechanism? The extent and importance of impaired longfin smelt development attributable to exposure to low DO concentrations are not known because longfin smelt are not detected frequently in or near the DWSC; it is not known what developmental effects, if any, are caused by exposure to low DO concentrations; and it is not known whether life stages undergoing critical developmental processes (e.g., larvae) are present in the DWSC during periods of low DO. 4. How well is this mechanism understood? No studies on the effects of low DO concentrations on longfin smelt development have been published. Jump to "Longfin Smelt" discussion under other adverse effects: Sacramento Splittail (Pogonichthys macrolepidotus) Hypothesis: Sacramento splittail exposed to DO concentrations near the regulatory minimum do not experience impaired development. 1. What is the mechanism causing this adverse effect? Certain fish species experience developmental problems (e.g., underdeveloped or malformed organs) as a result of exposure to low DO concentrations during early life stages (General Effects). No studies on the effect of low DO concentrations on Sacramento splittail development have been published. 2. Are there critical thresholds associated with this adverse effect? 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 regulatory minimum. 3. How important is this mechanism? The extent and importance of impaired Sacramento splittail development as a result of exposure to low DO concentrations are not known. 4. How well is this mechanism understood? No studies of the effect of low DO concentrations on Sacramento splittail development 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 experience impaired development. 1. What is the mechanism causing this adverse effect? Certain fish species experience developmental problems (e.g., underdeveloped or malformed organs) as a result of exposure to low DO concentrations during early life stages (General Effects). For example, oxygen consumption rates in green sturgeon (A. medirostris) increased 500% between hatching and 31 days post-hatch (Gisbert et al. 2003 in Cech and Doroshov 2004). Presumably, this rapid increase in oxygen demand is related to organogenesis and commencement of organ function. If ambient oxygen levels cannot support this level of metabolism, it is likely that these important developments will be delayed or (worse) occur out of their proper synchrony. No studies on the effect of low DO on white sturgeon development have been published. 2. Are there critical thresholds associated with this adverse effect? The incipient limiting threshold for white sturgeon has not been determined. 3. How important is this mechanism? The extent and importance of impaired white sturgeon development attributable to exposure to low DO concentrations are not known because (a) it is not known what developmental effects, if any, are caused by exposure to low DO, and (b) it is not known whether life stages undergoing critical developmental processes (e.g., larvae) are present in the DWSC during periods of low DO. 4. How well is this mechanism understood? No studies on the effect of DO concentrations and white sturgeon development have been published. Jump to "White Sturgeon" discussion under other adverse effects: Green Sturgeon (Acipenser medirostris) Certain fish species experience developmental problems (e.g., underdeveloped or malformed organs) as a result of exposure to low DO concentrations during early life stages (General Effects). For example, oxygen consumption rates in green sturgeon (A. medirostris) increased 500% between hatching and 31 days post-hatch (Gisbert et al. 2003 in Cech and Doroshov 2004). Presumably, this rapid increase in oxygen demand is related to organogenesis and commencement of organ function. If ambient oxygen concentrations cannot support this level of metabolism, it is likely that these important developments will be delayed or (worse) occur out of their proper synchrony. No studies on the effect of low DO concentrations on green sturgeon development have been published. 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 experience impaired development. 1. What is the mechanism causing this adverse effect? Early life stages (eggs and larvae) of striped bass may have higher DO requirements than later stages and sensitivity to low DO concentrations because of a limited ability for behavioral avoidance (Breitburg 2002 in U.S. Environmental Protection Agency 2003). Without sufficient oxygen concentrations, growth and development may be impaired (Reduced Growth). 2. Are there critical thresholds associated with this adverse effect? There likely are critical thresholds related to the DO concentrations needed to maintain normal development, but these thresholds are difficult to delineate clearly. The following findings may serve as guidelines for determining the conditions under which adverse effects on development may be likely.
3. How important is this mechanism?
4. How well is this mechanism understood? The understanding of how low DO concentrations affect development of young striped bass is incomplete. However, some studies presented above under Are there critical thresholds associated with this adverse effect? provide useful findings regarding DO concentrations and the potential for impairment of striped bass development. Jump to "Striped Bass" discussion under other adverse effects: |
The extent and importance of impaired striped bass development rates caused by exposure to low DO concentrations are unknown. Given the current understanding of DO dynamics in the DWSC (see figure at right) and the fact that most striped bass spawn below the DWSC, this adverse effect may have little impact on overall striped bass populations. Until more information is known about spawning habitat use above the DWSC and the timing and abundance of larvae and juveniles in the DWSC, the potential impact of impaired development on striped bass populations in the DWSC will remain uncertain.