Potential Effects of Contaminants on Fraser River Sockeye Salmon

thresholds for fish tissues. Two types <strong>of</strong> toxicity thresholds were established from thescientific literature for each chemical <strong>of</strong> potential concern in each exposure medium (i.e.,surface water and sediments), including toxicity screening values and toxicity referencevalues. The toxicity screening values were intended to identify the concentrations <strong>of</strong>chemicals <strong>of</strong> potential concern below which adverse effects on aquatic organisms wereunlikely to be observed (i.e., toxicity screening values represented no observed adverseeffect concentrations). By comparison, toxicity reference values were intended to identifythe concentrations <strong>of</strong> chemicals <strong>of</strong> potential concern below which adverse effects onsockeye salmon were unlikely to be observed (i.e., toxicity reference values representedsalmonid-specific lowest observed adverse effect concentrations). As such, thebenchmarks are not subject to measurement errors.There are several sources <strong>of</strong> extrapolation errors in the effects assessment. The selectedtoxicity screening values are intended to represent no observed adverse effectconcentrations. However, as the toxicity screening value is a conservative value, theactual effects thresholds for sockeye salmon could be higher. This limitation wasmitigated by applying the toxicity screening values in a screening-level assessment only.By comparison, the toxicity reference values were intended to identify lowest observedadverse effect concentrations for sockeye salmon, or other salmonid species. Actualtoxicity thresholds for sockeye salmon could be higher or lower than the selected toxicityreference values. As it was not possible to evaluate the reliability <strong>of</strong> the selected toxicityreference values, the level <strong>of</strong> uncertainty associated with the assessments <strong>of</strong> risks tosockeye salmon associated with exposure to surface-water or sediment, and associatedwith the accumulation <strong>of</strong> contaminants in fish tissues, cannot be determined.Uncertainty in the effects assessments for aquatic receptors is also increased by gaps in theavailable data. Specific data gaps in the effects assessment are presented in Section 7.4.Such data gaps in the effects assessment (i.e., absence <strong>of</strong> toxicity screening values ortoxicity reference values) generally results in under-estimating risks to sockeye salmonutilizing spawning, rearing, and migration habitats within the Fraser River Basin. That is,the effects <strong>of</strong> many chemicals <strong>of</strong> potential concern on the survival, growth, andreproduction <strong>of</strong> sockeye salmon could not be evaluated.7.3 Uncertainties in the Exposure AssessmentThe exposure assessment is intended to describe the actual or potential co-occurrence <strong>of</strong>stressors and receptors. As such, the exposure assessment identifies the exposure123