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National Irrigation Water Quality Program Guidelines I natural water conditions. Studies conducted for and by Procter and Gamble found that natural waters containing 0.75 mg B/L did not affect rainbow trout early life stages (Butterwick et al. 1989). Bingham (1982) was able to find at least some wild healthy trout in surface waters containing as much as 13 mg B/L, although it was not known how long those trout had been exposed nor whether they constituted a demographically healthy population. In demographically open populations, as was the case in Bingham's study, upstream and downstream movements can continually maintain the presence of fish even in a habitat where a closed population could not sustain itself. Thus, in such cases, data on the presence or absence of fish are of questionable value for delineating acceptable water quality characteristics. Based on a limited number of field surveys, Saiki and May (1988) suggested that whole freshwater fish typically contain <4 mg B/kg. Results from laboratory and field studies suggest that boron bioaccumulation is common in fish, but bioconcentration is not (Perry et al. 1994; Ohlendorf et al. 1986; Saiki and May 1988; Hamilton and Wiedmeyer 1990; and Thompson et al. 1976). Amphibians/Reptiles Birge and Black (1977) found that leopard frog embryos suffered 100 percent lethality or teratogenesis in water treated with borax or boric acid at exposure levels of 200 or 300 mg B/L, respectively. Boron compounds are more toxic to embryos and larvae than to adult amphibians, and amphibians are more tolerant of boron than fish, particularly at low concentrations (Birge and Black 1977). Birds Toxic effects of boron in birds, as reported in the literature, are summarized in table 7 at the end of this chapter. c:J In mallards, adverse reproductive effects have been reported at dietary concentrations of 1,000 mg B/kg; hatching success of fertile eggs, body weights of ducklings at hatch, and survival of ducklings from hatching to day 7 were all substantially reduced when breeding adults and their offspring were maintained on a diet supplemented with 1,000 mg B/kg. Although the mallards had markedly impaired embryo survival, the teratogenic effects described in boron egg- injection studies were not observed in this study. Mallard embryo mortality was greatest during the second half of incubation, when energy demands for embryonic growth were great. Because no adults died as the result of dietary boron treatment, it appears that embryos and hatchlings are the most sensitive mallard life stages to boron toxicosis (Smith and Anders 1989). I I Stanley et al. (1996) also found statistically significant adverse reproducti e effects in mallards fed 900 mg B (as bori acid) per kilogram of dry feed. Hatchin success was reduced to only 58 percent of c ntrols, suggesting that this level of di taryexposure is close to the EC50 value. At dietary exposure of 450 mg B/kg, hatc ing success was reduced to 88 percent of c ntrols, suggesting an approximate E 10 value. Concentrations of boron in ma lard eggs associated with these approxi ate EC50 and EClO dietary exposures of hens were, respectively, 38 and 22 mg/kg dw. I I I I I I I I I I I I I I I I Boric acid in the diet of ducklings hatched from untreated eggs proved to be less toxic than reported for ducklings hatched from boron-contaminated eggs. Hoffman et al. (1990) found 10 percent mortality at 10 weeks in ducklings from uncontaminated eggs that received 1,600 mg/kg dietary boron. Smith and Anders (1989) reported 21 percent mortality during the first week and 12 percent mortality during the second week in duck- lings that received 1,000 mg B/kg both from the adult hen mallard and in their own diet. In a natural setting, the ducklings would