Using a critical body residue approach to assess the acute toxicity of a pesticide mixture to juvenile Chinook salmon (Oncorhynchus tshawytscha)

Abstract

Despite its better accuracy and applicability, the critical body residue (CBR) approach in toxicity assessments is not utilized as often as external concentrations, and even less often in mixture toxicity tests. In this project, we exposed juvenile Chinook salmon (Oncorhynchus tshawytscha) to two current-use pesticides (bifenthrin and fipronil) and a legacy pesticide degradation product (4,4'- dichlorodiphenyldichloroethylene, DDE) that are commonly found in migration waterways, and acute toxicity was related to pesticides residues in salmon tissue. An equipotent mixture of bifenthrin and fipronil was tested, and 50 % lethality (LR₅₀, determined as toxic units, TU) after 96 h of exposure was determined in the presence and absence of added DDE. The results suggested that bifenthrin and fipronil joint toxicity was likely additive though the LR₅₀ values were slightly greater than 1 (LR₅₀ = 1.52 (1.18-1.83)), and the addition of DDE did not significantly affect acute toxicity (LR₅₀ = 1.48 (1.25-1.75)). Residues that caused acute lethality were much higher than the maximum environmentally observed residues in field-caught salmon, which suggests little risk of a fish kill scenario at these exposure levels. This does not exclude the chance of sublethal effects occurring at these exposure levels, and the CBR approach could also be used to assess sublethal effects of pesticide mixtures. This work demonstrates the utility of the CBR approach in assessing mixture toxicity and the risk of exposure in field-caught salmon.

Keywords: 4,4′- dichlorodiphenyldichloroethylene; Bifenthrin; Fipronil; Fish tissue; Risk assessment; Whole-body analysis.