It doesn't. The process is called bioaccumulation and is a function of how long the species live and as a predator, how much of food lower on the food chain they eat.

Sardines are safer than Mackarel because they don't live as long and predate less. Mackarel are better than Tuna because tuna is a longer lived predator that eats more, thus accumulates more mercury. Dolphins are top of the food chain, thus have higher mecury levels.