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I I Boron I greater. The concentrations of boron eliciting chronic sublethal responses in C. decorus are dose to those reported in severely contami- nated systems in the Central Valley of California (15-29 mg B/L). I Fish I The boron toxicity database for fish is relatively extensive, and several compre- hensive summaries have been compiled recently (e.g., S]VDP 1990; Perry et a!. 1994). This literature, however, is mostly limited to evaluations of waterborne exposures to boron (Le., without dietary exposure) and also does not include any definitive data relating boron levels in fish tissues to toxic effects. Con- sequently, although the database is extensive, its interpretive value is hampered by the critical gaps in "field-relevant" toxicity data (i.e., dietary exposures and tissue-based toxicity thresholds). Another confounding feature is the fact that threshold-level effects are commonly seen at water concentrations of boron much lower than the EC50 (see Appendix II for definition of terms), but EC50s and LCSOs are the only standardized measures of toxicity consistently used in most bioassay-type toxicity studies. For sake of comparison, table 7 is largely restricted to summarizing EC50 and LC50 estimates of various studies. Toxicity measures based on various other endpoints are reported in SJVDP (1990) and Perry et a!. (1994). The general concentrations of boron associated with threshold-level (e.g., EC1 to EC10) measures of toxicity will, however, be briefly summarized in discussions to follow. I I I I I I I I I I I I I I The available literature indicates that boron levels of 0.001-0.1 mglL could reduce the reproductive potential of sensitive fish species, and concentrations exceeding 0.2 mglL could impair the survival of developmental stages for other species, under conditions providing continuous exposure from fertilization through 4 days posthatching (Birge and Black 1977). Birge and Black also found that boron compounds were more toxic to developmental and early posthatched stages than to adult fish. However, Hamilton and Buhl (1990) found no difference in the sensitivity of various life stages of fish exposed to boron for 96 hours. Both studies indicated that water hardness did not seem to affect boron toxicity (Birge and Black 1977; Hamilton and Buhl 1990). The early life stages of rainbow trout appear to be the most sensitive to boron, with a consistent dose-response-related lowest observable effect concentration (LOEC) of 0.1 mg BIL (Birge and Black 1977). High boron concentrations (25-200 mg/L) were required to consistently produce substantial impairment to trout embryos and alevins. High frequencies of both embryonic and postembryonic mortality in trout eggs were recorded only at boron concentrations of 50 mglL or more. Embryonic mortality and teratogenesis were the principal boron- induced responses at 50 mg/L or less. Percent hatchability of trout eggs generally was inversely proportional to exposure level from 1 to 200 mg BIL (Birge and Black 1977). Borax at or below 0.5 mg BIL did not reduce hatching frequency; at 200 mglL, hatchability dropped to zero. A high incidence of terato- genesis was observed over the range of exposure levels from 1.0 to 200 mg BIL. Borax and boric acid are unusual in that they exert low-level embryopathic effects on trout over a wide span of exposure levels (0.001-1.0 mg/L) (Birge and Black 1977). In channel catfish, at a concentration of 200 mg BIL, normal survival at 4 days posthatching was only 0-2 percent; at 300 mg BIL, many of the eggs did not hatch, and those that did produced deformed hatchlings. Normal survival was 100 percent at and below 1.0 mg BIL. In both channel catfish and rainbow trout, embryonic mortality and teratogenesis increased in hard water, and boric acid produced higher frequencies than borax (Birge and Black 1977). The low-level effects observed in reconstituted laboratory water, however, may not predict the much higher first effect levels under 0fJ