"High mercury" refers to a situation where there is a high level or concentration of the element mercury in a particular context, such as in water, air, or food. Full definition
As when you were pregnant, you can include fish and seafood two to three times a week, but avoid fish with high mercury levels. (babycenter.com)
Eat fish and seafood 2 — 3 times a week, but avoid eating fish with high mercury levels. (acog.org)
It's an interesting fact that some people with high mercury exposure don't become toxic, yet others with relatively low exposure do. (articles.mercola.com)