Evaluating the Effects of Forestry Herbicides on Fish Development Using Rapid Phenotypic Screens

Herbicides are used to control invasive or noxious plants on public lands throughout the western United States. These chemicals are often applied in the upper reaches of watersheds that provide spawning and rearing habitat for anadromous species of Pacific salmon Oncorhynchus spp., steelhead O. mykiss, and bull trout Salvelinus confluentus. As a consequence, natural resource managers must often weigh the ecological benefits of using herbicides to control nonnative plants against the potential for unintended toxicological impacts on salmonids, particularly during early life stages. However, the effects of chemical control agents on fish development have not been widely investigated. Their use in the vicinity of aquatic habitats, including areas that support threatened and endangered species, is therefore a concern. To address this uncertainty, we used the zebrafish Dania rerio as a model experimental system for investigating developmental toxicity, which involved conducting rapid and sensitive phenotypic screens for potential developmental defects resulting from exposure to six herbicides (picloram, clopyralid, imazapic, glyphosate, imazapyr, and triclopyr) and several technical formulations (Tordon K, Transline, Habitat, Plateau, Garlon 3A, and Renovate). Zebrafish embryos were exposed continuously through 5 d of development at nominal concentrations ranging from 3 mu g/L to 10 mg/L. Detailed screens were used to examine aspects of ontogeny from early development (gastrulation and segmentation) through organogenesis, hatching, and morphology as free-swimming larvae. Growth was measured at the end of each exposure interval. To detect defects in neural development (sensorimotor integration), the escape reflex of larvae was monitored at 3, 4, and 5 d after fertilization. No developmental toxicity was observed in response to the six individual herbicides or the different technical formulations. The absence of toxicity at relatively high exposure concentrations suggests that noxious weed control activities are not likely to pose a direct threat to the health of salmonids at early life stages.