Environmental Contaminants at the
colonial nesting bird surveys conducted at the Salton Sea from 1987-1991
showed a sharp decline in many nesting species including the Black Skimmer,
Great Egret, Snowy Egret and double-crested cormorants. A study
in 1991 indicated that these species were exposed to contaminants such
The hypothesis of this study was that this exposure may be contributing
to changes in reproduction. A particular concern in this study was
that the Brown Pelican (a federally listed endangered species) was beginning
to use the Salton Sea and may be exposed to the same contaminants.
In the first year of this study, tissue samples were collected for chemical
analysis and nests were monitored. The second year of the study
focussed on the Black Skimmer because it is believed to best represent
the exposure to contaminants experienced by the Brown Pelican.
Methods: In 1992 the differences in exposure to organic contaminants were studied. Black-crowned Night-herons were studied at three stages of development: 1) eggs collected early in the incubation period; 2) embryos near pipping and 3) approximately 10-day-old chicks that had consumed locally available prey. Eggs were analyzed for inorganics, and embryos and chicks were analyzed for organochlorine compounds. The eggs were collected at the Whitewater River delta colony, from Black Skinner nests at the Salton Sea, from white-faced Ibis nests at Finney Lake of the California Department of Fish and Game's Imperial Wildlife Area , and from the Great egrets at the Mallard Road colony at the Wister Unit of of the Imperial Wildlife Area. In 1993 the study focused on Black Skimmers. Eggs were collected and analyzed from four colonies. Sediment samples were also collected from each of three foraging areas used by Black Skimmers nesting at the four colonies. Two surface samples were collected from each area.
Results and Discussion: All but two of the egg selenium concentrations were above the toxicity threshold for water bird eggs provided by the National Irrigation Water Quality Program. Some birds at the Salton Sea may be experiencing low level reproductive impacts as a result of selenium exposure, but confirmation of this would require intensive nest monitoring. This study found no measurable reproductive impairment that could be attributed to selenium. Selenium is a concern at the Salton Sea because current plans for water conservation in the Imperial Valley include the possible use of tail water recovery systems. Tail water currently comprises a significant portion of the inflows to the Salton Sea and dilutes the more selenium rich discharges from subsurface drains. Water conservation in the Imperial Valley has the potential to increase surface water selenium concentrations and decrease the water volumes flowing into the Salton Sea. The concentrations of mercury found in this study were below those that had been identified as having a reproductive impact, so it was concluded that mercury contamination in birds is not likely to cause reproductive or other impacts in the Salton Sea watershed. PCB concentrations in this study were below those found to impact reproductive success of birds. This was not surprising as this is not a highly industrialized area, however, there are some industrial discharges crossing the international Border from Mexicali, so the situation warrants monitoring as long as sewage continues to enter the United States and flow into the Salton Sea. Of the organochlorines studied (DDE, DDT, DDD, chlordane, cis-nonachlor, HCB, Heptachlor epoxide, endrin, and dieldrin) p,p'DDE was detected most frequently and at the highest concentrations. The concentrations were high enough that some reproductive impacts would be anticipated. Forty percent of the eggs exceeded the threshold at which successful reproduction has been found to decline. The nest monitoring in this study did not provide an adequate sample size to indicate a statistical relationship between p,p'DDE concentrations in the egg and nesting success. A drop in concentrations of p,p'DDE, trans-nonachor, and PCB's was seen between eggs, pip-aged embryos, and juveniles. This suggests an impact of these compounds on embryo survival, but requires additional studies to confirm.
Summary and Conclusions: This study indicates that there are reasons to be concerned about contaminant exposure in piscivorous birds at the Salton Sea. Of greatest concern is p,p'DDE. Egg concentrations suggest that reproductive impacts such as eggshell thinning are likely at least in a portion of these populations. PCBs are of some concern and should be monitored until industrial effluent is no longer crossing the International Border in the New River and flowing to the Salton Sea. Selenium concentrations are at levels of concern and should continue to be monitored, especially if water conservation in Imperial Valley agriculture results in higher concentrations of selenium in the surface drains.
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