Toxicity of boron to rainbow trout: A weight‐of‐the‐evidence assessment

From the large data set available on the toxicity of boron to aquatic organisms, the toxicity of boron to the early life stages of rainbow trout (Oncorhyncus mykiss) is the seminal issue relative to setting water quality criteria and effluent standards. Issues associated with the early life stage studies are the flat concentration-response curve, the low threshold of toxicity, and teratogenic effects observed. Recent laboratory and field studies offer new experimental data that make a weight-of-the-evidence assessment timely. In a re-examination of the effect of boron on the embryo-larval stage in rainbow trout and zebrafish, adverse effects due to boron deficiency are observed which decrease with increasing dose. It was found that low concentrations of boron stimulate embryonic growth in rainbow trout and increase the viability and survival of embryonic zebrafish. As boron concentration is further increased, the dose-response curve becomes flat as homeostatic processes are active; this is followed at higher doses by a new adverse response that increases with increasing dose. As a result, the dose-response relationship is U shaped, consistent with the characteristic shape of an essential micronutrient. Thus, effects originally reported to be toxicity at low exposures rather may be due to boron deficiency. Water analyses in trout hatcheries and field studies in wild trout streams add additional information on the toxicity of boron to trout. Of particular note is a controlled field study carried out in the Firehole River in Yellowstone Park (WY, USA), where trout populations survive and reproduction successfully occurs in natural water containing boron concentrations up to and in some cases greater than 1.0 mg B/L. Teratogenic effects due to boron exposure were not observed in any of these more recent studies.