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The Problem
Aquatic environments have been highly altered in Californias Central Valley due to changes in hydrology caused by human development. With implementation of the Grassland Bypass Channel Project in September 1996, agricultural drainwater contaminated with selenium was removed from Salt Slough and from canals and ditches flowing through the interior of the Grasslands. Instead, the agricultural drainwater is now diverted into the Grassland Bypass Channel (formerly known as the San Luis Drain), which then discharges into the lowermost 10.6 km of Mud Slough and, eventually, into the San Joaquin River. Implementation of the Grassland Bypass Channel Project is believed to have benefitted aquatic biota in canals and ditches, their associated marsh ponds, Salt Slough, and the reach of Mud Slough lying upstream from the terminus of the Grassland Bypass Channel because these surface waters are no longer exposed to seleniferous tile drainage. However, aquatic biota in Mud Slough downstream from the Grassland Bypass Channel could be exposed to higher concentrations of selenium because canals, ditches, and marsh ponds in the interior of the
Objectives The purpose of this subtask is to better understand the extent to which the Grassland Bypass Project (GBP) in the Central Valley of California has altered environmental conditions for fishes in Salt and Mud sloughs and other surface waters within and downstream from the Grassland Water District (Grasslands), Merced County. Specifically, this subtask will attempt to determine if seleniferous discharge from the GBP has altered fish species assemblages and increased selenium body burdens in selected species. In addition, the subtask will attempt to determine if selenium-contaminated fish in the study area exhibit impaired reproductive success or other associated maladies. Results from this subtask are expected to help fine-tune the operation of the Grassland Bypass Channel so as to enhance the ecological value of the Grasslands and San Joaquin River for fish and wildlife resources. Methodology Fish Species Assemblages and Selenium Body Burdens:
During September-October 1997, fish were sampled with bag seines, beach seines, variable-mesh gill nets, and electrofishing from as many as 13 sites within the Grasslands and adjoining reaches of the San Joaquin River. Captured fish were identified and counted, then representative samples of green sunfish and bluegills were retained for selenium determinations. All other fish were released alive. In addition, water quality and other environmental variables were measured to characterize habitat conditions at the various sites. Fish samples were wrapped in plastic and frozen, then shipped to the BRD Columbia Environmental Research Center (CERC) for selenium determinations. Cluster analysis was used to determine if distinctive species assemblages were present within the sampling sites. Multiple discriminant analysis was used to identify the physicochemical variables that best explained the species assemblages. Analysis of variance was used to determine if selenium concentrations varied among the sample sites. Mosquitofish Reproductive Success: During May-July 2001, western mosquitofish were collected on at least two occasions from the San Luis Drain at Gun Club Road (selenium-contaminated site), North Mud Slough at Gun Club Road (reference site located upstream from the mouth of the Drain; this site has not received seleniferous drainwater since the Grassland Bypass Project became operational), North Mud Slough at State Highway 140 (selenium-contaminated site located downstream from the mouth of the Drain), and the San Joaquin River at Lander Avenue (reference site with no history of receiving seleniferous drainwater). About 20 pregnant females (identified by prominent dark-colored spots
Highlights and Key Findings Fish Species Assemblages and Selenium Body Burdens: During September-October 1997, a total of 9,795 fish representing 25 species was captured from 13 sampling sites in the Grassland Water District (Grasslands) and adjoining reaches of the San Joaquin River in Merced County, CA. Inland silverside (Menidia beryllina), green sunfish (Lepomis cyanellus), fathead minnow (Pimephales promelas), and western mosquitofish were most ubiquitous, found at all sampling sites. In addition to these four species, bluegill (L. macrochirus), redear sunfish (L. microlophus), largemouth bass (Micropterus salmoides), threadfin shad (Signalosa petenensis), goldfish (Carassisus auratus), red shiner (Cyprinella lutrensis), common carp (Cyprinus carpio), Sacramento blackfish (Orthodon microlepidotus), and black bullhead (Ictalurus melas) were abundant or very abundant at one or more sites.
Although several species exhibited restricted spatial distributions among sampling sites, association analysis and cluster analysis failed to identify more than one fish species assemblage within the study area. Comparisons with historical (pre-project) data suggested that the taxonomic composition of fishes within the Grasslands has not changed in response to the Grassland Bypass Project. On the other hand, discharge of seleniferous agricultural drainwater by the San Luis Drain is influencing body burdens of selenium in fish inhabiting North Mud Slough and the San Joaquin River. Individual green sunfish from the San Luis Drain contained as much as 23 ug Se/g (dry weight basis in whole fish), whereas composite samples of bluegill from North Mud Slough contained as much as 18 ug Se/g (bluegill were not captured in the San Luis Drain). Nevertheless, when compared to historical data from selected sites, selenium concentrations in bluegill have not generally varied over time in directions (upwards or downwards) predicted for the Grassland Bypass Project. Results from multiple regression analysis indicated that the concentration of waterborne selenium was the single most-important predictor of selenium body burdens in green sunfish and bluegill. Among bluegill, other environmental variables (stream channel width, discharge, and specific conductance values) were also important predictors of selenium body burdens. Mosquitofish Reproductive Success: In 2001, a new study was implemented to determine if western mosquitofish populations inhabiting the San Luis Drain, North Mud Slough, and the San Joaquin River suffered from impaired reproduction. This study was conducted because fishes from seleniferous habitats contained body burdens of selenium exceeding 4 ug/g dry weight, the proposed toxic threshold for adverse reproductive effects. However, preliminary results from field trials conducted during June-July 2001 did not detect evidence of reproductive impairment. Pregnant female mosquitofish collected from two seleniferous sites and two non-seleniferous sites produced fry that exhibited >96% survival at birth. Cursory inspection of the fry also did not suggest evidence of teratogenesis. Selenium determinations in samples of adult females and their fry from the reproductive trials verified that fish from seleniferous sites contained more selenium than did fish from non-seleniferous sites. In addition, newborn fry contained higher concentrations of selenium than did postpartum adult females.
Where Are We Headed In 2003 Field and laboratory work have been completed. During FY-03, statistical analysis and data interpretation will be conducted on data from the mosquitofish reproduction study. A final report will be prepared for FWS and BOR. Project Contact Michael Saiki Email: michael_saiki@usgs.gov Publications |
