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The Problem Several ongoing USGS studies are assessing the status and dynamics of mercury contamination associated with past hydraulic mining operations in the Sierra Nevada. By comparison, little work is being conducted on mercury contamination in historically dredged environments in urbanized areas (e.g., the cities of Folsom, Gold River, and Rancho Cordova) within the American River watershed. Lake Natoma, an 8,760 acre-feet reservoir on the American River, is managed by the U.S. Bureau of Reclamation (USBR) for multiple uses that include fishing and other water-based recreation. Preliminary results from 22 fish samples collected in Lake Natoma during August 2000 indicate that the mercury content may approach or exceed guidelines for human consumption. Thus, the USBR has requested that USGS conduct a survey of mercury contamination in edible portions (muscle tissue or fillets) of selected sport fishes from Lake Natoma. Objectives Specific objectives of this subtask are as follows: (i) to determine if mercury concentrations in fillets vary spatially among selected fishes, and (ii) to determine if mercury concentrations in fillets vary according to size or age of the fishes. These results will be used to confirm and extend findings from the preliminary sampling and provide additional data to public health agencies so that regulators and land managers can make informed decisions. Methodology 1. Sample sites. Fish were sampled from Lake Natoma at four general locations as follows (roughly from upstream to downstream): the Negro Bar vicinity, the Mississippi Bar vicinity (north shore), the Alder Creek and Willow Creek vicinities (south shore), and the Nimbus Dam vicinity. Sampling efforts focused on shoreline areas of the reservoir where boat-mounted electroshocking was typically most effective. All fish were collected during September-October 2002. 2. Fish collections. Largemouth bass (Micropterus salmoides), bluegill (Lepomis macrochirus) or green sunfish (L. cyanellus), channel catfish (Ictalurus punctatus) or white catfish (Ameiurus catus), and rainbow trout (Oncorhynchus mykiss) were captured with a boat-mounted electroshocker or gill nets. At each sampling site, as many as 15 catchable-size individuals of each species were assigned unique identification codes and retained for mercury determinations. Excess fish and all non-target species were counted and released alive. Although the designation of a minimum size for catchable fish is often arbitrary, this study adopted either the minimum legal size limit established by the State of California or the minimum useful (for human consumption) sizes suggested by various authors. In Lake Natoma, the largemouth bass fishery is regulated by a minimum size restriction of 12 inches (305 mm) whereas other fishes have no legal size restrictions. However, anglers generally keep fish of the following minimum sizes: for bluegills and other sunfishes, 6 inches (152 mm); for channel catfish, 12 inches (305 mm); and for bullheads (close relatives of the white catfish), 7 inches (178 mm). The California Department of Fish and Game suggests that a "harvestable trout" is a fish between 7 to 10 inches (178-254 mm) or 2.5 to the pound (about 182 g). Immediately after capture, each fish retained for mercury determinations was measured for standard and total lengths, then placed into a clean plastic ziplock bag (along with its unique code identifier) and chilled on wet ice. Within 24 hrs, the fish was weighed, then scale or pectoral spine (catfish only) samples were taken. Scale and spine samples ware being stored in coin envelopes marked with the unique identification code of the individual fish. Skinless fillets from both sides of each fish were obtained by dissection, then the two fillets were weighed and wrapped in clean plastic sheets, double-bagged in clean plastic ziplock bags, and frozen (-10°C). The remaining carcass of each fish was examined for sex determination (presence of ovaries or testes), then archived by wrapping and bagging in plastic before freezing. (Note: Archived samples may be used to obtain otoliths if scales and pectoral spines are not suitable for age determinations.) 3. Mercury determinations. A maximum of 300 samples of fish fillets will be analyzed for moisture content and total mercury concentration. Frozen samples packaged at the Field Station will be shipped overnight to the Columbia Environmental Research Center (CERC), Columbia, Missouri, for mercury measurements. Fillet samples will be freeze-dried and homogenized at CERC. Determinations of moisture content and mercury concentrations, and quality control procedures will follow standard CERC protocols. Highlights and Key Findings Fish collections occurred in September-October 2002. Where Are We Headed In 2003 Fish samples will be shipped to CERC in January 2003 for mercury determinations. Fish scales and pectoral spines will be aged. When available, data from this investigation will be statistically analyzed and interpreted. A final report will be prepared for BOR. Project Contact Michael Saiki Email: michael_saiki@usgs.gov Publications |
