Critical review of tissue-based selenium toxicity thresholds for fish and birds

• Main Authors: Brix, K. V., Deforest, D. K., Fairbrother, Anne, Adams, William J.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/9549

Selenium-induced fish kills in Belews Lake, North Carolina in the late-1970s resulted in a substantial amount of research on selenium effects to aquatic life. Similarly, selenium-induced teratogenesis in aquatic birds at Kesterson Reservoir, California in the mid-1980s resulted in extensive research on selenium effects to aquatic birds. Unlike many other contaminants for which water exposure is the critical pathway for environmental effects, selenium ecotoxicology is driven by bioaccumulation in invertebrates and exposure to fish and birds via the diet. At sufficiently high levels, these exposures result in embryo teratogenesis and reduced survival of larval fish and bird chicks. Because of the dietary exposure pathway, assessment of selenium risks to the environment is best accomplished by evaluating concentrations in fish and bird tissues rather than the water. One of the end results of the intensive research efforts mentioned above has been the publication of tissue-based toxicity thresholds for both fish and birds by the U.S. Fish and Wildlife Service (USFWS). These thresholds are being used in the United States and Canada for environmental assessments and for establishing clean-up guidelines for remediation/reclamation projects. We critically evaluated these thresholds and the scientific literature upon which they are based. Specifically, we evaluated the USFWS proposed thresholds for selenium in fish whole body tissue (4 mg/kg dw), fish ovaries (10 mg/kg dw), fish diets (3 mg/kg dw), and bird eggs (6 mg/kg dw). We observed that the USFWS thresholds for fish tissues appear overly conservative and do not appear to be well supported by the scientific literature. In several cases, the USFWS interpretation of the studies on which the thresholds are based are contrary to our interpretation, that of U.S. EPA, and the authors that published the study. Similarly, for birds we found that the USFWS threshold is based on unpublished field studies using a non-selenium specific endpoint - chick mortality. Because the details of the study and raw data are not readily available, it is difficult to ascertain whether the study adequately controlled for confounding factors such as disease, predators, weather and other contaminants. This concern is heightened by the fact that the field data does not agree with results from controlled laboratory studies evaluating the same endpoint. Considering the above, we re-examined all of the existing data and established alternative thresholds that we believe are better supported by the scientific evidence available. Specifically, we proposed thresholds for fish whole body, ovary, and diet of 6-9, 17, and! 10-11 mg/kg dw and a bird egg threshold of 16 mg/kg dw. The details of our analyses and justification for the alternative thresholds are presented in this paper.