Development of a fipronil insecticide response spectrum model for juvenile Chinook salmon (Oncorhynchus tshawytscha)

A response spectrum model (RSM) was developed that relates sublethal effects to a whole-body residue dose metric to better assess the risk of fipronil exposure to outmigrating Chinook salmon. The use of whole-body residues obviates the need for bioavailability estimates of hydrophobic pesticides, it enables exposure estimates from all pesticide sources, and it is suitable for pre-existing fish monitoring programs. After 10-d of exposure to fipronil, sublethal endpoints were measured, which included swim performance, growth inhibition, routine swimming behavior, and response to light stimuli. Parent fipronil and fipronil metabolites (sulfone, sulfide, and desulfinyl) internal residues were analyzed. The RSM indicated that anxiety-like and shoaling behaviors were the most sensitive endpoints resulting from accumulated fipronil and fipronil metabolites, while swim performance was the least sensitive. The residues for the most sensitive endpoint on the RSM were a factor of 49-66 greater than the fipronil residues found in field caught juvenile Chinook salmon from the Yolo Bypass and Sacramento River (California, USA), in 2019 and 2020. This suggested that the risk of negative effects on outmigration due to fipronil exposure was low for those populations. The newly developed fipronil RSM can be applied to other salmon populations to help guide future management decisions regarding the effects of fipronil on sensitive Chinook salmon runs.