Temperature determines the rate at which retene affects trout embryos, not the concentration that is toxic

The toxicity of waterborne retene (7-isopropyl-1-methyl phenanthrene) to post-hatch embryos of rainbow trout (Oncorhynchus mykiss) was assessed at 5 and 11 °C. Survival times of retene-exposed embryos were 70 % longer at 5 °C than at 11 °C, but survival times and LC50 s did not vary when time was expressed as degree-days (thermal units), i.e., at a common stage of development. The size of survivors decreased with increasing retene concentrations, but not with temperature. Retene did not bioconcentrate to any extent (bioconcentration factors < 2) at either temperature, indicating effective biotransformation by embryos. However, concentrations of retene metabolites were slightly higher at 5 °C, suggesting slower excretion rates than at 11 °C. The relative expression of cytochrome P450 proteins (CYP1A) did not vary with temperature but increased with retene concentration, as indicated by cyp1a mRNA concentrations. The induction of CYP1A protein by retene exposure was evident in the vasculature of eye, brain, heart, kidney, liver, gill, mouth, intestine, muscle, and yolk-sac. However, immunohistochemical staining was greater at 5 than at 11 °C for all tissues except liver and muscle. Overall, temperature effects on retene toxicity disappeared when the duration of embryo development and retene exposure were expressed as thermal units (degree-days). Temperature controlled the rate of embryo development and the rate of toxicity (time to a toxic endpoint), but not the concentrations that were toxic.