Life Stage, chorion, and hatchery origin differences in the toxicity of 6PPDQ to coho salmon

Several salmonid species die from acute exposure to 2-((4-methylpentan-2-yl)amino)-5-(phenylamino)cyclohexa-2,5-diene-1,4-dione (6PPDQ) – a ubiquitous pollutant that enters waterways via roadway runoff. Coho salmon (Oncorhynchus kisutch) is the most sensitive species. Toxicity has been characterized for some coho life stages, but differences in study design and source populations between studies diminish comparability of life stage sensitivity. Coho were exposed to 6PPDQ at five distinct life stages (embryos, alevins, fry, parr, and adults) and as alevins from two different hatcheries; results were characterized by measures of sublethal morphological and behavioral effects, survival analysis, and concentration response analysis. Exposures during embryo development significantly altered morphology and caused post-hatch mortality and differences in phototaxis. Dechorionated embryos were vulnerable to 6PPDQ, with increased mortality relative to intact embryos. Embryos did not die in acute exposures, but 24-h LC50s were 784 ng/L for alevin, 239 ng/L for fry, and 71 ng/L for parr. Adults were as sensitive as parr. Alevins from different hatcheries had similar sensitivity. These findings reshape understanding of 6PPDQ toxicity to early life stage coho and reveal novel sublethal effects.

Methods
Six replicate aquaria containing ~100 coho salmon embryos each were episodically exposed to 6PPDQ on 13, 24-hour intervals over the first 55 days of development, alongside six replicate control aquaria. Mortality rates were recorded daily for all replicates, and on two occasions five individuals from each replicate were subsampled and their yolk sizes, eye sizes, and lengths were measured. After hatching, 20 individuals from each replicate were placed in light/dark decision aquaria and the number of fish observed in the lighted half of each aquarium was recorded each minute in two, 10-minute tests.

In another experiment, the survival of three replicates containing 10, 27-day-post-fertilization coho salmon embryos of each of four treatments was monitored for 17 days. The dechorionated exposure treatment exposed embryos that had been removed from their chorion to 6PPDQ for 24 hours followed by 16 days of clean water. The intact exposure treatment exposed embryos with intact chorions to the same exposure regime as the dechorionated exposure treatment. The dechorionated control was embryos removed from their chorions in clean water, and the intact control was embryos with intact chorions clean water. 

In acute exposures, coho salmon were exposed in groups of 4 to 10 individuals per aquarium to a series of 6PPDQ concentrations and their survival was assessed after 24 hours. Acute exposures were conducted with coho salmon alevin 11 days after hatch, fry 31 days after hatch, parr one year after hatch, and adults three years after hatch. All the above were from Voights Creek except for the adults, which came from nearby Clark Creek. Additionally, coho alevin from a different creek of origin, Bingham Creek, were exposed acutely for comparison with the Voights Creek alevin.