Methyl mercury in bodies of water is absorbed by algae, the staple food of small fish that scavenge near shorelines. These small fishes are eaten by larger fishes, which in turn serve as prey for still larger fishes. Thus, the level of mercury accumulation among fishes gets higher as it goes up the food chain.

Gonzales, E. (2015, November 3). The problem with mercury. Manila Bulletin, p. C1.