An Ecological Risk Assessment of the Exposure and Effects of 2,4-D Acid to Rainbow Trout (Onchorhyncus mykiss)

Abstract

Numerous state and federal agencies are increasingly concerned with the rapid expansion of invasive, noxious weeds across the United States. Herbicides are frequently applied as weed control measures in forest and rangeland ecosystems that frequently overlap with critical habitats of threatened and endangered fish species. However, there is little published chronic toxicity data for herbicides and fish that can be used to assess ecological risk of herbicides in aquatic environments. We conducted 96-h flowthrough acute and 30-day chronic toxicity studies with swim-up larvae and juvenile rainbow trout (Onchorhyncus mykiss) exposed to the free acid form of 2,4-D. Juvenile rainbow trout were acutely sensitive to 2,4-D acid equivalent at 494 mg/L (95% confidence interval [CI] 334-668 mg/L; 96-h ALC(50)). Accelerated life-testing procedures, used to estimate chronic mortality from acute data, predicted that a 30-day exposure of juvenile rainbow trout to 2,4-D would result in 1% and 10% mortality at 260 and 343 mg/L, respectively. Swim-up larvae were chronically more sensitive than juveniles using growth as the measurement end point. The 30-day lowest observable effect concentration (LOEC) of 2,4-D on growth of swim-up larvae was 108 mg/L, whereas the 30-day no observable effect concentration (NOEC) was 54 mg/L. The 30-day maximum acceptable toxicant concentration (MATC) of 2,4-D for rainbow trout, determined as the geometric mean of the NOEC and the LOEC, was 76 mg/L. The acute:chronic ratio was 6.5 (i.e., 494/76). We observed no chronic effects on growth of juvenile rainbow trout at the highest concentration tested (108 mg/L). Worst-case aquatic exposures to 2,4-D (4 mg/L) occur when the herbicide is directly applied to aquatic ecosystems for aquatic weed control and resulted in a 30-day safety factor of 19 based on the MATC for growth (i.e., 76/4). Highest nontarget aquatic exposures to 2,4-D applied following terrestrial use is calculated at 0.136 mg/L and resulted in a 30-day safety factor of 559 (e.g., 76/0.163). Assessment of the exposure and response data presented herein indicates that use of 2,4-D acid for invasive weed control in aquatic and terrestrial habitats poses no substantial risk to growth or survival of rainbow trout or other salmonids, including the threatened bull trout (Salvelinus confluentus).

Exploring the Impacts of Two Separate Mixtures of Pesticide and Surfactants on Estrogenic Activity in Male Fathead Minnows and Rainbow Trout

“…Significant controversy surrounds the observed impacts of herbicides to aquatic organisms. For 2,4-D, ecological risk assessments have indicated no-observable effect concentration (NOEC) values for chronic growth or mortality in the mg/L range (Fairchild et al 2009), and recent evaluations using the United States Environmental Protection Agency (USEPA) tier 1 endocrine-disruptor screening assay for fish (fish shortterm reproduction assay) in FHM also indicated threshold concentrations for altered reproduction in the mg/L range (Coady et al 2013). In the study by Coady et al (2013), there were no significant differences between the control and 2,4-D-exposed fish regarding fertility, wet weight, length, gonadosomatic indices, tubercle scores, or blood plasma concentrations of VTG.…”

Effects of low, subchronic exposure of 2,4‐Dichlorophenoxyacetic acid (2,4‐D) and commercial 2,4‐D formulations on early life stages of fathead minnows (Pimephales promelas)

“…The period between fertilization to 14 dph is considered to be a critical developmental stage in fish (Laale and Lerner 1981;Bentivegna and Piatkowski 1998;Sarikaya and Selvi 2005), and toxicant exposure can affect early developmental stages more than mature life stages (Lotufo and Fleeger 1997;Sarikaya and Selvi 2005;M acov a et al 2008;Fairchild et al 2009;Mensah et al 2011). For example, swim-up rainbow trout, 10 to 14 d old, exposed to the 2,4-D herbicide amine formulation Weedar 64 showed significant adverse effects on growth parameters, whereas their juvenile counterparts were unaffected (Fairchild et al 2009).…”

“…Early developing stages in aquatic organisms are typically at higher risk from toxicants than their adult counterparts (Lotufo and Fleeger 1997;M acov a et al 2008;Fairchild et al 2009;Mensah et al 2011). Although early developmental stages often represent a more susceptible period for toxicant exposure (Laale and Lerner 1981;Von Westernhagen 1988;McKim 1995;Bentivegna and Piatkowski 1998;Oikari et al 2002;Mohammed 2013), most studies on the effects of fish exposed to 2,4-D used either adult or juvenile life stages.…”

Accuracy assessment of time–concentration–effect models in predicting chronic lethality from acute toxicity data

“…Their accuracy assessment, however, is invalid as well as that of Slaughter et al [18]. ACE models provide estimates of chronic lethality from acute test data that are protective of populations [14,15], as opposed to using arbitrary safety factors of 10 or 100. As we have demonstrated, the ACE models are very accurate and with low uncertainty, but users must be able to understand the basic concepts of toxicology, recognize typical and atypical exposure and dose responses at all observation times within acute tests, and use the data appropriately in the ACE software.…”

“…Therefore, ACE procedures use the entire acute toxicity data set, which greatly adds to the utility of the data available in regulatory, research, and risk assessment activities dealing with effects of chemicals on aquatic organisms. Acuteto-chronic models have been successfully applied [13][14][15] and have been found to be highly accurate in estimating low-level chronic mortality from acute toxicity data [10] for fish.…”