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Persistent, bioaccumulative, and toxic contaminants (PBTs) are known to co-occur in fish tissue, yet this covariance has not been explicitly incorporated into model-based risk assessments that inform fish consumption advisories. We utilize available US EPA datasets to statistically model the covariance among PBT concentrations in fish tissue and the dependence of this covariance on waterbody and watershed conditions. We find that most PBTs positively covary, whether fish were collected in rivers, lakes, or coastal waters. Mercury in lakes, and mercury, PFCs, and heptachlor in rivers, covary negatively with the other PBTs. While much of the variance and covariance in PBTs can be statistically related to fish characteristics and watershed and waterbody conditions, a large amount remains in model residuals. This implies that single contaminant models, even if highly precise, can misestimate total health risk by neglecting the substantial covariance with other PBTs that is left unmodelled.