Primary Driver: Ability of Fish to Avoid
and Tolerate Low DO Concentrations

This primary driver is affected by the two secondary drivers shown above.

• To read about the primary driver, scroll down this page.
• To read about a secondary driver, click on that driver in the diagram above. The discussion of the secondary driver describes both the general effects and the species-specific effects of the driver.

Ability of Fish to Avoid and Tolerate Low DO Concentrations

Fish exposed to low DO concentrations may avoid adverse effects through two mechanisms—either by migrating to other locations to avoid the effects or through physiological mechanisms that allow the fish to tolerate these conditions. Fish live in an environment where oxygen is much less available than in terrestrial systems. As a result, fish have developed certain physiological adaptations to these environmental conditions that allow them to efficiently extract what little DO is available. Fish possess a suite of mechanisms to compensate for reduced oxygen availability when DO concentrations decline. The threshold oxygen concentration at which a species begins to seek alternative habitat because of lethal oxygen concentrations depends on the energy costs and tradeoffs among a number of risk factors, such as:

• mortality from low oxygen concentrations;
• mortality from predation; and
• loss of reproductive and feeding opportunities (Breitburg 2002).

The DWSC may provide different fish habitat types for the species and life stages that use it. For some fish, the DWSC functions mainly as a migratory pathway to upstream spawning locations. Alternately, this same area may constitute important rearing habitat for other fish during their journey to the lower Delta. Tolerance of low DO concentrations varies greatly among life stages and species. Therefore, the presence and accessibility of alternate habitats to the DWSC must be determined in the context of the life history and habitat requirements of each species.