One of the talks at the recent Salish Sea Ecosystem Conference showed how common household pesticides that are considered safe individually are lethal to juvenile coho when they are combined. The author found that the mixing of chemicals was either additive or synergistic meaning that the pesticides reacted with each other to become far more potent then if they were acting alone. Several mixtures that they looked at were found to be 100% lethal to coho.
Since 1996 the Environmental Protection Agency has mandated that chemical mixture testing be done for human health risks. However, in the aquatic environment the testing for chemicals is done only in isolation even though sampling of streams has shown that over 90% of the time multiple pesticides are found mixed together. Other studies have shown that coho essentially disappear from streams when the percent of impervious surface in the watershed reaches 10-15 percent. If legal and common household pesticides are part of this problem then recovering or protecting streams will be a much bigger challenge than previously thought.