Do all fish have Mercury in them?

Primarily it's due to contaminents in bodies of water, whether in oceans, lakes, streams, etc. The majority of this pollution comes from manufacturing or agricultural industries either through dumping contaminents illegally or through the process of run-off, i.e. crop dusting plans spray crop fields with herbicides and the chemicals drain into lakes, streams, etc after it rains. From a manufacturing standpoint, the Mercury is released through smoke stacks, and when it rains, the rain carries the Mercury into the lakes, streams, oceans, etc. Bacteria convert the Mercury into a form that is easily absorbed by small insects and other organisms. When fish eat these, they absorb the Mercury. When a bigger fish eats a smaller fish, it absorbs it's Mercury as well and this is how it moves up the food chain to humans.