Selenium Stresses Green Sturgeon
In a CALFED Science Program-funded project, Dr. Dietmar Kueltz and his research team compared the effects of selenium-a highly toxic material at high concentrations-on two native species of sturgeon and discovered that threatened green sturgeon are much more sensitive to dietary selenium than are white sturgeon.
Kueltz and his team, including UC Davis professors Drs. Silas Hung, Joseph Cech, and Serge Doroshov, investigated how selenium introduced through the diet affects these two native sturgeon species.
After an eight week exposure to selenium in the diet, these data show significant (10 percent) mortality of green sturgeon (Acipenser medirostris) but not white sturgeon (Acipenser transmontanus). At this selenium concentration, growth of green sturgeon completely ceased while white sturgeon growth continued, said Dr. Kueltz. Green sturgeon are listed as threatened under the Endangered Species Act. Tissue samples were collected from both species during the selenium exposure experiments to clarify the molecular basis for the different capacities in each of the species. Green and white sturgeons provide a nice model system of two closely related species with clearly different tolerances to environmental stress. It’s likely that genetic differences between these two species cause the difference in selenium susceptibility, he said.
Selenium contamination is of great concern for Delta fish populations, impacting them often in combination with many other stressors. Although regulatory action has led to the reduction of selenium concentrations in the Delta in recent decades, the concentrations are still elevated and there is concern that significant disposal of selenium-rich agricultural drainage water into the Delta could occur again, i.e., if an extension of the San Luis Drain is approved to convey agricultural drainage from the western San Joaquin Valley to the North Bay.
“It’s likely that genetic differences between these two species cause the difference in selenium susceptibility.”
White sturgeon and green sturgeon are affected by bioaccumulation (the buildup of a substance in the tissues of a living organism) of toxic metal compounds because they are positioned at the top of aquatic food chains and are long-lived. Therefore, they are particularly susceptible to pollution of the Bay Delta system, Kueltz said. Both species of sturgeon are native to the San Francisco Bay and Delta system and are critical elements in the natural aquatic ecosystem. These endemic species represent the ancient lineage of modern ray-finned fishes and have exceptional biological value, Kueltz said. White sturgeon have commercial value for sport fishing and aquaculture, they also have an ecological value by controlling populations of invasive clams in the San Francisco Bay and Delta. Although green sturgeon have the same sport-fishing values, there are fewer of them. Additionally, white sturgeon are being farmed for caviar and meat, but green sturgeon are not because they’re difficult to reproduce in captivity. Each species occupies a distinct ecological niche: green sturgeon live mostly in the ocean-occasionally migrating back to brackish and freshwater habitats-while white sturgeon remain mostly in estuarine areas.
According to surveys conducted by the California Department of Fish and Game, white sturgeon populations are much more robust than green sturgeon populations. The number of green sturgeon is declining compared to that of white sturgeon-despite the fact that green sturgeon spend less time of their life cycle in the Bay Delta than white sturgeon.
Selenium is just one of the many toxicants, pollutants, and abiotic (non-living) parameters that impose stressful conditions to Delta fish populations, Kueltz said. Unless all of those parameters are taken into account it is impossible to single out one parameter such as selenium as being responsible for the decline of green sturgeon as compared to the more stable white sturgeon populations. Combinations of environmental stresses are likely responsible for many of the observed population-level effects.
Kueltz said that tangible water management solutions have to be based on knowledge about how changes in water regimes affect important environmental variables such as salinity, temperature and toxicant levels, but also how these environmental changes impact the key species that are critical elements of the ecosystem. White and green sturgeons are two of those key species whose populations contribute to ecosystem health in the Delta water system. His study suggests that white sturgeon should not be used as a substitute to study how best to conserve green sturgeon populations-as these data clearly show that even closely related species will be differently affected by environmental conditions and water operations.
If green sturgeon become more rare than they are now, then studies addressing their response to environmental stresses will become very difficult unless a captive breeding stock can be established, said Kueltz. Not having such animals available will make it even harder to predict future effects on this species. Water management decisions will always be based on human needs and the assessment of which type of human need-drinking water, personal hygiene, fisheries, agriculture, conservation, industries, etc.-has priority will always be subjective, Kueltz said. More studies are necessary to enable researchers to get a better grasp on how fish populations will evolve in the face of environmental stress.