What general rule applies to all wild-caught freshwater species concerning potential toxin levels?

The general rule suggests that larger, older fish accumulate more potential toxins than smaller, younger individuals.

A foundational principle in evaluating the safety of consuming wild-caught fish from freshwater ecosystems is the relationship between longevity and contaminant load. Because toxins such as mercury are not easily metabolized or excreted, they accumulate within the tissues of fish over their lifespan—a process known as bioaccumulation. Therefore, older fish, which have had more time to consume contaminated smaller organisms, inevitably carry higher concentrations of these substances. This biological reality leads to the actionable advice often given: when in doubt about water quality, focusing consumption efforts on smaller individuals helps mitigate the potential risk associated with toxin exposure.